Class AVES Order PASSERIFORMES Suborder OSCINES Family REGULIDAE (KINGLETS AND GOLDCRESTS)
- Very small birds with medium-long wings and tail, thin, straight and pointed bill, longish legs; general grey-green plumage coloration offset by pale wingbars, variably contrasting head markings, one species plainer-headed.
- 8–11 cm.
- Nearctic, Palearctic and, marginally, Oriental Regions.
- Coniferous forest, also mixed and deciduous woodland, locally laurel forest with tree-heath.
- 1 genus, 6 species, 28 taxa.
- No species threatened; none extinct since 1600.
The position of the genus Regulus in the avian system has a chequered history, which is due to the fact that the kinglets and goldcrests exhibit external morphological similarities to several passerine families. Thus, they have been placed with or close to the tits and chickadees (Paridae) or the Old World warblers (Sylviidae). Attempts to demonstrate a supposed phylogenetic relatedness with the Yellow-browed Tit (Sylviparus modestus), a member of the Paridae, or with the White-browed Tit-warbler (Leptopoecile sophiae), which has been placed variously with the long-tailed tits (Aegithalidae) or the Sylviidae, were rejected on the grounds of genetic data. In various recent molecular phylogenies of passerines, the regulids appear as an isolated group at the base of a sylvioid/parid branch, and results of studies of skull morphology by H. Morioka, and of DNA–DNA hybridization by C. G. Sibley and J. E. Ahlquist, suggested family rank for Regulus. This treatment is widely accepted today. Interestingly, a very recent in-depth analysis by P. Alström and colleagues of the superfamily Sylvioidea, one of three such superfamilies within Sibley and Ahlquist’s “parvorder” Passerida, indicated that the position of Regulus was uncertain. Nevertheless, these authors, whose analysis was based on sequence data involving nuclear myoglobin intron II and the mitochondrial cytochrome b gene, concluded that it was one of eight genera for which there appeared to be little support for continued inclusion within that superfamily. Although the family Regulidae consists of only six currently accepted species in a single genus, Regulus, the taxonomic status of certain populations has for long been disputed, and debate continues. Discussion has concentrated on the Atlantic populations on the Canary Islands and Madeira, but it is only the more recent acoustic and molecular-genetic studies that have allowed a deeper understanding of the true taxonomic position of these isolated populations. This applies not only to insular populations, but also to continental ones, such as those of the Goldcrest (Regulus regulus) in Asia. Within the genus, but with the exception of one species, there is remarkable morphological homogeneity with regard to size, coloration and colour pattern. The sole exception is the North American Ruby-crowned Kinglet (Regulus calendula). This species, the largest member of the genus, differs further from its congeners in having an intensely red crest and in lacking lateral black crownstripes, as well as in all kinds of vocalizations. As a consequence, it has even been assigned to a separate genus, Corthylio. According to the results of several genetic studies, it is only distantly related to its Nearctic sibling, the Golden-crowned Kinglet (Regulus satrapa), and to all four Old World species in the family.
Bioacoustic and molecular research has helped to end the dispute about the phylogenetic relationships of the Golden-crowned Kinglet, which was sometimes regarded as a New World vicariant of the Common Firecrest (Regulus ignicapilla). On the basis of morphology, it was first included in an artificial group of “eyestripe kinglets”. Along with the Golden-crowned Kinglet, this group encompasses all species with a black eyestripe, namely the Common Firecrest, which at the time also incorporated the Madeira Firecrest (Regulus madeirensis), and, from Taiwan, the Flamecrest (Regulus goodfellowi), although, strictly speaking, the facial pattern of the last-mentioned differs from that of the typical “eyestripe kinglets”. This view was challenged by the findings of studies of comparative bioacoustics, which demonstrated that the Golden-crowned Kinglet shares a common bipartite song pattern with the Goldcrest. Molecular genetics supported this bioacoustic-based prediction, placing this kinglet as a sister-species to the entire Old World Goldcrest subspecies complex. Although the position of the poorly investigated Flamecrest in the regulid tree is still an open question, there is strong evidence that the four “eyestripe kinglet” species do not form a monophyletic group and that the black eyestripe is a plesiomorphic character within the regulids. Among the Goldcrest assemblage of forms, the populations of the Canary Islands have been considered by some authorities to be affiliated with the Common Firecrest, while they have been treated by others as a separate species, the “Tenerife Goldcrest” or “Tenerife Kinglet”. Ethological, vocal and molecular studies undertaken by H. Löhrl and E. Thaler, P. H. Becker, und J. Martens and co-workers clearly indicated, however, that all Canarian populations are true members of the Goldcrest subspecies complex, which ranges from the Azores and the Canaries eastwards over the entire Palearctic Region to Japan. According to mitochondrial molecular markers and to territorial song pattern, teneriffae is close to west European and Azorean populations but is distinct at the subspecies level. The focus on the Canarian populations has so far neglected all the disjunct subspecific populations in Asia, namely the races tristis and japonensis and the “ himalayensis group”. Bearing in mind the remarkable morphological, vocal and genetic diversification of the Goldcrest’s many populations, it can be foreseen that discussion on their taxonomic rank will become intense anew. There is still a widely accepted view among taxonomists that “ R. teneriffae” forms a sister-species pair with R. regulus, but this would make the latter a paraphyletic clade and does not properly reflect phylogenetic relationships. A new taxonomy of the Goldcrest assemblage of forms does seem desirable, but this should be based on a thorough revision of the genus. In the meantime, it is probably better to follow the alternative approach of treating the Goldcrest as a single polytypic species, including the “Tenerife Goldcrest” as a Canarian subspecies. This accords with recent treatments by E. C. Dickinson and by A. Martín and J. Lorenzo. It is worth mentioning here that the Goldcrests of the Canary Islands, hitherto all placed in the single subspecies teneriffae, have very recently been found to consist of two subspecies; the western populations, those on La Palma and El Hierro, were described as a separate race, ellenthalerae, in 2006. In the case of another Atlantic-island endemic, the Madeira Firecrest, morphological differences and also, as recently established, vocal and genetic differences from continental populations of Common Firecrests are remarkable. Differences in molecular markers and voice were found to be at a level that clearly suggests species rank for this highly restricted population. Madeira Firecrests have calls and a territorial song that are easily distinguishable from the homogeneous calls and song types of all subspecies of the Common Firecrest. On the molecular level, cytochrome b distances between Madeira Firecrests and Common Firecrests from Europe are remarkably high, at 8·5%. This distance value compares with the divergence level between other long-established regulid species, such as the 9% between the Goldcrest and the Golden-crowned Kinglet, and equates to a separate evolutionary history of roughly 4 million years.
The six members of this family are among the smallest of all passerines, having a size that is equalled in its tininess only by a few east Asian Phylloscopus warblers. A number of hummingbirds (Trochilidae), a New World non-passerine group, are smaller, although numerous hummingbird species exceed regulids in both size and weight. The total length ranges from about 9 cm to slightly beyond, but it is very homogeneous among all regulid species. All weigh approximately 6 g to about 8 g, but variation in weight during the course of the day, a result of accelerated metabolism, is remarkable. The sexes do not differ appreciably in dimensions. Regulids do not stand out as having any striking body proportions. They appear as tiny birds with medium-sized wings and tail, generally looking rather slender in the summer months and more compact during the winter period. The small, needle-like bill is straight and pointed, and the oval nostrils are basal, indistinctly operculated and each covered by a single stiff feather. This peculiar nostril feather is lacking in the Ruby-crowned Kinglet, in which it is replaced by several short and stiff bristles. The tip of the tail is incised. The general plumage coloration is grey-green, and is similar in both sexes. This is offset by pale wingbars and variably contrasting head markings. The most conspicuous head pattern is a bright yellow to golden or even reddish crownstripe in combination with blackish lateral crownstripes, a pale supercilium and a dark eyestripe. On females, the median stripe is always less bright and tends towards yellowish. Male Ruby-crowned Kinglets have a bright red central crownstripe, but otherwise both sexes have a much plainer head, lacking stripes, while the lateral crownstripes are reduced in Goldcrests of the subspecies tristis. The feathers forming the median crownstripe are longish and erectile, and they play a major role in courtship and antagonistic behaviour. When the bird is in an unaroused state, the stripe is narrow and nearly completely covered by parts of the lateral stripes, thus being reduced to a small unobtrusive line. When it is excited, on the other hand, these bright feathers are erected and are then very conspicuous, and are able to indicate very precisely various kinds of motivation. Although the two regulid species found in Europe are very similar to each other in external morphology, minute differences in foot morphology and slight differences in food preferences allow them to occur together without noticeable interspecific competition. The two American species also have widely overlapping ranges. Within the global range of the family, nowhere do more than two of its six members occur sympatrically. Regulids possess well-developed toe pads with deeply incised furrows, enabling them to move within small-sized parts of tree foliage. Consequently, the strictly conifer-dwelling species have toe-pad fissures that are especially deeply cut and narrow. The Eurasian Goldcrest and the North American Golden-crowned Kinglet are true conifer specialists (see Habitat), and their markedly grooved soles and large toe pads are perfectly adapted for perching on thin conifer twigs, allowing these species even to clasp individual needles. The preferred foraging strategy of clinging and hanging vertically from twigs is optimized by the relatively long hind toe and hind claw. The ecologically more generalist Ruby-crowned Kinglet and Common Firecrest are less bound to conifers and make use of different feeding strategies from those employed by the typical conifer specialists. As may be expected, the toe-pad furrows of these two species are less developed and the surface of the pads is smoother. Predominant feeding movements of the two generalists are connected to flight, such as frequent and intense hovering, and, especially in the case of the Ruby-crowned Kinglet, to the capture of flying insects. The different feeding strategies are linked with subtle differentiation of the flight apparatus. This is apparent in the fact that the Ruby-crowned Kinglet has a longer humerus, a longer ulna and longer secondaries compared with the Golden-crowned Kinglet, while the Common Firecrest, in comparison with the Goldcrest, has a longer humerus and a shorter primary P9, the latter forming a more rounded wingtip. Both skeletal and feather characters of Ruby-crowned Kinglets and Common Firecrests allow for longer and more sustained flights. In contrast, Golden-crowned Kinglets and Goldcrests are comparatively slow in foraging behaviour, covering shorter distances interrupted by sudden stops. When attacking prey, however, these two species are capable of conspicuous zigzagging. The great manoeuvrability of flight is ensured by the greater graduation of the tail of both species and, in the case of the Goldcrest, the longer alula compared with that of the Common Firecrest. Recent molecular data provide highly persuasive evidence for morphological-character evolution, and hold against a parallel ecological differentiation of a conifer specialist and a more generalist species in the Nearctic and the Palearctic. According to the molecular regulid phylogeny, all adaptive characters with regard to boreal conifer habitats are derived features which evolved only once in the common ancestor of the Golden-crowned Kinglet and the Goldcrest.
In behavioural and morphological aspects, all members of the family are notably adapted to a life in coniferous forest. With the exception of the two island species, confined to Madeira and Taiwan, respectively, the breeding areas of all Regulus species cover large parts of the boreal forests of the Palearctic and Nearctic Regions, but also extend southwards to several nearly subtropical areas. For example, Common Firecrests breed in north Africa and Goldcrests in the Canary Islands, while the Ruby-crowned Kinglet nests in the south-western parts of the United States and the Golden-crowned Kinglet breeds as far south as south Mexico and Guatemala. The subspecies obscurus of the Ruby-crowned Kinglet extends that species’ range farther south, to the island of Guadalupe, off west Mexico, but it may now be extinct there (see Status and Conservation). The breeding area of the Goldcrest is largely congruent with the distribution of the Norway spruce (Picea abies) and other spruce species in Asia, and extends between the July isotherms of 14°C and 23°C. In Europe, Goldcrests reach cooler climate zones than do Common Firecrests, and they breed farther northwards, to the northern limit of the conifer zone. They inhabit conifer forests of the low mountain ranges and alpine mountains up to the timber-line, at maximum elevations of about 2200 m in Switzerland, 2000 m in the Caucasus, 2600 m in Japan and 4000 m in the Himalayas. Goldcrest habitats are dominated by Norway spruce, silver fir (Abies alba) and mountain pine (Pinus mugo), while larch (Larix), Scots pine (Pinus sylvestris) and other conifers, mainly long-leaved species, are typically avoided for feeding and nesting. In some parts of the range, such as in western Europe, Goldcrests not uncommonly nest in isolated spruces or firs in mixed forest or in gardens. Generally, however, it is only in the non-breeding season, and especially during migration, that this species also invades shrub and bush vegetation, mixed and deciduous forests, gardens and parks. At such times it commonly occurs in loose flocks. The more generalist Common Firecrest is less tied to conifers, as it also inhabits mixed and deciduous forests, having a preference for oak (Quercus). Its almost purely European breeding range extends between the July isotherms of 16°C and 24°C. In mountain regions Common Firecrests are usually found no higher than 1000 m, thus at lower altitudes than those reached by Goldcrests. The North African populations of the Common Firecrest, however, ascend somewhat higher than do those in Europe, breeding in open forests of cork oak (Quercus suber) and holly oak (Quercus ilex) at up to 1600 m, and in the Middle Atlas also in pure Atlantic cedar (Cedrus atlantica) forest above 1600 m. In the laurel (Lauraceae) forest vegetation of the east Atlantic islands of the Canaries, the Azores and Madeira, the ecological niche occupied by this avian family consists of local endemic species of tree-heath (Erica), a habitat to which the local populations of the two regulids concerned are strongly adapted. The Madeira Firecrest inhabits mountain areas at elevations between 600 m and 1550 m, and is a common breeder in shrub vegetation of tree-heath and broom (Genista), in relict laurel forest, in deciduous forest dominated by oak, and even in stands of the introduced Japanese cedar (Cryptomeria japonica). In alien eucalyptus (Eucalyptus) and acacia (Acacia) vegetation, which nowadays replaces large parts of the former Madeiran laurel forest, this regulid is absent. On the Azores, likewise, the indigenous laurel forest has been almost eradicated, but Goldcrests are well adapted to the introduced Japanese cedar. In the semi-open or, indeed, open agricultural habitats now dominating large parts of most of these islands, Goldcrests are frequently found inhabiting even small groups of introduced conifers and tree-heath in areas of pasture. They are common also in stands of the local short-leaved juniper (Juniperus brevifolia) in the central plains of Flores and Pico. Only on the Canary Islands have Goldcrests obviously adapted also to pine forest, here composed of the endemic Canary pine (Pinus canariensis) and the introduced North American Monterey pine (Pinus radiata). As on other Atlantic islands, however, habitat selection is dependent on the presence of a suitable amount of tree-heath, rather than on the conifers themselves. In open pine forest lacking a well-developed tree-heath canopy, Goldcrests are nearly absent, whereas they are frequent in pine stands with a well-developed heath understorey. They are abundant also in relict laurel forest on La Palma, Tenerife, La Gomera and El Hierro, the four islands of this archipelago which the Goldcrest is known to inhabit. Because of their adaptation to shrub and bush stands on the Canary Islands, Goldcrests are among the first and the most common bird species to colonize successive mountain vegetation after fires. Similarly, at middle elevations in the Rocky Mountains of North America, Golden-crowned Kinglets are among the first invaders of the primary successive aspen (Populus) vegetation following burns. On Taiwan, the Flamecrest is restricted to the middle and upper levels of the central mountain areas, where it ascends to 3700 m in altitude. Its typical breeding habitat is, again, coniferous forest, here dominated by fir, hemlock (Tsuga) and spruce. It is also common in mixed coniferous and broadleaf forests at lower elevations. Like their two counterparts in the continental Palearctic, the North American species exhibit an ecological differentiation. The Golden-crowned Kinglet is bound primarily to spruce–fir and cedar–hemlock forests. It is fairly common in black spruce (Picea mariana) and white spruce (Picea glauca), hemlock and balsam fir (Abies balsamea). In the Rocky Mountains it inhabits an altitudinal belt up to 2740 m, and in western Arizona its breeding grounds reach up to 3450 m. The habitat preferences of Ruby-crowned Kinglets encompass black spruce in the eastern parts of the breeding range, and spruce–fir forests, lodgepole pine (Pinus contorta) and Douglas fir (Pseudotsuga menziesii) in Alaska and the west of Canada. This species, however, is less tied to conifers than is the Golden-crowned Kinglet. It also inhabits mixed and deciduous forests in the southern parts of its range, where it accepts mature and old aspen stands, as well as scrub, and it breeds from middle elevations up to 3000 m. Moreover, outside the breeding season, the Ruby-crowned Kinglet is fairly common in second growth, shrublands and human settlements, such as farms and gardens.
As intimated earlier (see Morphological Aspects), the members of this family are among the world’s smallest birds. As a consequence of the high rate of metabolism, they are almost continuously foraging, even during song display and nest construction. A Goldcrest that is prevented from feeding can lose a third of its weight in only twenty minutes, and a captured individual may starve to death within less than an hour. Occasionally, dead regulids have been found entangled in spider webs, or entrapped in large thistles such as burdocks (Arctium). All regulids exhibit continuous rapid wing-flicking while foraging. This is sometimes performed asymmetrically; indeed, Ruby-crowned Kinglets in general flick the wings asynchronously. This trait becomes more conspicuous, and the flicking more synchronous, when the bird is more aggressive. In threat display, it presents the paler underside of the wings. Aggressiveness is reduced outside the breeding season, and during the winter months the birds spend their time almost exclusively in foraging and feeding. At this time of the year they are less agitated, and there is a notable decrease in the incidence of energy-consuming flight activities and even in the otherwise frequent wing-flicking. During cold periods, both the Goldcrest and the Common Firecrest prevent heat loss by sleeping in close contact with members of their own species, a habit known also for other small passerines overwintering in cold climates, such as Northern Wrens (Troglodytes troglodytes), Long-tailed Tits (Aegithalos caudatus) and treecreepers (Certhia). When initiating “contact sleep” at the roost, Goldcrests and Common Firecrests utter what has been termed the “assembly call”, a delicate drawn-out trill which, interestingly, is interspecifically understood. The two North American species were until recently thought not to indulge in this roosting behaviour, which could explain why they appear to lack this call type. In November 2002, however, B. Heinrich, while studying the winter roosting behaviour of birds at dusk in western Maine, in the north-east United States, watched a group of three Golden-crowned Kinglets as they pressed closely together in a huddle on a spruce branch. In the following month, he found four individuals sleeping in a similar manner. The winter temperature in this part of the United States is extremely low, and often drops well below zero during the night-time. Observations there indicate that, at that time of the year, Golden-crowned Kinglets always travel in groups of 2–4 individuals, which forage until dusk or later and presumably then roost while huddled together as a means of preventing heat loss. So far as is known, similar behaviour has not been reported for the Ruby-crowned Kinglet, but this species spends the winter season farther south, in warmer temperatures (see Movements). In the non-breeding season, Goldcrests, Flamecrests and Golden-crowned Kinglets form loose troops and associate with small flocks of other passerines. Interspecific communication between members of such mixed flocks by the use of similar contact or alarm calls is possible to a certain degree. For example, Goldcrests in the non-breeding season maintain vocal contact with, in particular, Crested Tits (Parus cristatus) and Eurasian Treecreepers (Certhia familiaris). With regard to adaptive behaviour among the Regulidae, it is notable that the vocal repertoires of only the Goldcrest and the Golden-crowned Kinglet include a variable and highly modifiable component, the subsong, with imitations of other species (see Voice). The ability to learn and imitate other species’ vocalizations may be seen as a pre-adaptation for withstanding winter in cold climates, where co-operation in mixed flocks increases the individual’s fitness. Both Ruby-crowned Kinglets and Common Firecrests occasionally join mixed-species non-breeding flocks, but usually in low numbers. Accordingly, the subsong of each of these two is invariable, without vocal imitations of other species. Antagonistic and courtship behaviour encompasses rather conservative features, as well as behavioural sequences which serve as isolating mechanisms between sympatric species. Throughout the genus, the brightly coloured crest is a highly effective signal which is displayed in both intrasexual and intersexual contexts. Territorial song display is often accompanied by crest-raising. It is notable, however, that there is a strong difference between sympatric species in antagonistic display postures. The “Forward display” of male Goldcrests comprises rapid and stiff up-and-down movements and head-bowing with a fully raised crest, whereas aggressive Common Firecrest males do not bow the head but, instead, point the bill towards the opponent, thereby exhibiting the orange crest and the black-and-white face pattern. Similar interspecific differences are evident in the courtship displays. In antagonistic encounters between male Ruby-crowned Kinglets, each individual presents the maximally raised red crest while adopting an upright position, with the neck stretched and the tail feathers spread. A long-term aviary study by Thaler of a mixed pair consisting of a male Common Firecrest and a female Goldcrest finally revealed that such behavioural differentiation provides an effective pre-mating barrier between the two species. In six years of observation, communication between the captives was common outside the breeding season and included intense vocal contact, as well as body contact during sleep. With the onset of the breeding cycle, however, the differences between the two species in courtship and antagonistic behaviour continually led to rough aggression and for years prevented successful mating. Interestingly, pre-mating isolation through vocalizations appears to be of minor importance for these two regulids, since nesting and other calls were mutually and correctly understood. In fact, the main isolating mechanisms seemed to be based on constantly repeated misinterpretations of the congener’s specific courtship and threat display postures. Furthermore, the different facial patterns of the two species, one having a white supercilium and a black eyestripe and the other having a broad white eyering, turned out to be an effective signal for species recognition and thus prevented mating. Among other manipulations, an artificial eyestripe given to the Goldcrest female finally helped to initiate copulation, although the differences between the two species in nesting behaviour caused additional problems. During seven years of constant aviary effort, a total of 14 hybrids hatched from 13 successive broods, but the offspring was never raised by the mixed pair because of the even larger behavioural differences between the heterospecific parents during the brooding stage. Two hybrids from a later brood of another mixed pair exhibited intermediate behaviour with regard to vocalizations and to foraging strategy and prey choice. These hybrids preferred to feed on springtails (Collembola), as do Goldcrests, but they captured them in hovering flight and attack-flight, as is typical of Common Firecrests. In conclusion, a similarly high energetic investment in prey capture and feeding, together with further behavioural maladaptations, would presumably result in highly reduced fitness of Goldcrest × Common Firecrest hybrids under natural conditions.
In general, Regulus vocalizations are thin and low and are easily missed by the human ear. They are uttered predominantly from high up in the canopy of mainly coniferous trees, and they easily escape attention, this being especially true of the short calls. Calls are widely used, and are important signals in the often dark canopy of coniferous trees, where visual contact is greatly reduced. Maybe as a consequence of this, the call repertoire is large and highly varied. The repertoires of the two mainland European species have been relatively well studied. Each embraces up to 16 different call types, which are used in various social contexts. The contact and flight calls are a thin “seeh” or “sreeh”, and the distance call, for maintaining individual distance, is a harsh “zick” in the case of the Goldcrest and a weaker “zee ze” in the Common Firecrest, while additional call types include, among others, excitement calls, appeasement calls of females and alarm calls. Both sexes give soft trilling nesting calls at the nest-site and sharp “bundling calls” when collecting and transporting nest material. Common Firecrest males also utter a specific courtship-feeding call (see Breeding), a shrill “seesee”. Antagonistic behaviour, very marked and distinct in this family, affords still different call types, such as persecution calls, “song of rage” and aggressive calls, which are frequently emitted during encounters between highly aggressive males. The high-pitched contact and alarm notes, up to 10 kHz in frequency, are very similar for most regulids, except for the low-pitched call series of the Ruby-crowned Kinglet. The regulid subsong is a continuous chatter given by both sexes, mainly during the post-breeding period in autumn, and in winter and early spring. It is typically delivered for up to several minutes without a break, and comprises different sections separated by short pauses: call series, rapid trills, and so-called “subsong motifs”. Unlike any other regulid vocalization, these last-mentioned motifs, and even a single note, may cover a broad frequency range of between 2 and 10 kHz combining elements of different types, often of a typical squeaking sound. Individual variation in subsong is least marked in the Common and Madeira Firecrests, which may incorporate only a single motif, sometimes described as the “ei-weija” sound, the name referring to its tonal quality. At the other extreme, highly variable subsong repertoires are a common vocal characteristic of Golden-crowned Kinglets and Goldcrests, that of the former comprising six or seven different motifs per individual and that of the latter more than 20 motifs per individual. Vocal variation among European populations of the Goldcrest increases with geographical distance, but subsong is also subject to temporal change, since single motifs may emerge within or disappear from an individual or a population-wide repertoire during successive years. As with other passerines, Goldcrest subsong develops from the song of juveniles during a sensitive phase up to approximately the 47th day after hatching. As aviary experiments suggest, subsong and all call types are innate vocalizations, but subsong nevertheless often incorporates imitations of the calls of other species, indicating that learning processes have at least a certain impact on its ontogeny. Playback experiments confirmed a social function of regulid subsong in both intrasexual and intersexual encounters. In captivity, dominant individuals show a higher rate of subsong display than do low-ranked individuals, and male Goldcrests react more aggressively towards unknown motifs than towards those from their neighbours’ repertoires. Subsong display between the sexes is paralleled by sexual excitement, and often precedes copulation. This fact, at least, contradicts a hypothesis by W. H. Thorpe, who suggested that subsong would indicate low sexual motivation because it is given mainly outside the breeding season. In contrast to most calls and subsong, full song is generally given exclusively by males during the breeding season, in a territorial context and in order to attract mates. Females usually do not sing. The male delivers his song while foraging in the upper third of conifer trees, but not from a regularly frequented songperch. Among the whistled and high-pitched songs of most Regulus species, only the loud and melodious warbling of the Ruby-crowned Kinglet stands out. Indeed, this species’ lively song has led to its being ranked as one of the most brilliant songsters among the North American passerines. Song development in the Regulidae, as with that of many other songbirds, is highly influenced by learning processes. In aviary experiments, both the Goldcrest and the Common Firecrest were able to learn each other’s song from playback of tape recordings, and in the wild individuals giving mixed songs have been observed in fragmented coniferous forest in northern Spain, where both species are rare breeders, and also in southern parts of Britain, where the Common Firecrest is rare and local. The predisposition for learning the song type of the congener in these regions seems to be due to the low abundance of vocal tutors belonging to the individual’s own species. All regulid songs share a certain common pattern, namely a partition into only a few subunits. This fact, once recognized, made the song a valuable tool for phylogenetic studies within Regulus, one that is hardly paralleled by other passerines. Becker was the first to describe the common two-part song of Goldcrests and Golden-crowned Kinglets, which consists of a stereotype main part and a short and highly variable terminal flourish, the latter being incorporated from the innate subsong-motif repertoire. Common Firecrest song usually does not include such a terminal flourish, but in the case of high aggression males may end the song with the typical subsong motif “ei-weija”. Nevertheless, captive Common Firecrests were obviously able to learn the final parts of Goldcrest repertoires from playback of recordings. Intrageneric comparison of Regulus song pattern by M. Päckert and colleagues revealed five acoustically distinct subunits, each of them defined by a characteristic frequency range, and element and time parameters, and thus representing a basic module of regulid territorial song. Some of these song modules are acoustic autapomorphies of single species; others are genus-wide features or are acoustic synapomorphies of species-pairs, such as the “final part” of Goldcrest and Golden-crowned Kinglet songs. Within a species, song syntax, the specific sequence of subunits in one song, is widely homogeneous and is not influenced by geographical or subspecific variation. The most simple song patterns are found on Madeira, where the song of the Madeira Firecrest is composed merely of innate subsong and rage calls, and on Taiwan, where the Flamecrest’s song is no more than a high-pitched series of “seeh” notes on a constant frequency. In a cladistic approach, various acoustic characters mapped on a molecular regulid phylogeny were found to be carriers of reliable phylogenetic signals, predominantly innate syntax structures, while others were obviously prone to homoplasy, defined as parallel evolution on different branches of the tree. Moreover, the degree of vocal differentiation among Regulus species was quantifiable and directly proportional to genetic distances: in other words, songs of closely related taxa are measurably more similar in syntax pattern, element composition and other factors than are those of distant relatives. Although this correlation may appear rather trivial, the regulids represent the first and the most thorough quantitative case study of its kind, and one which, furthermore, lent persuasive support for the well-discussed value of certain acoustic characters in the solving of taxonomic questions (see Systematics).
Food and Feeding
All members of the family are almost exclusively insectivorous. They prey on small-sized arthropods having a soft cuticle, especially springtails, aphids (Aphidoidea) and spiders (Araneae), among others. They also feed on cocoons and eggs of spiders and insects, and occasionally take pollen. In Europe, the Goldcrest and the Common Firecrest are ecologically differentiated with regard to prey size, foraging strategy and micro-habitat choice. In coniferous habitats, Goldcrests prefer to forage in dense branches, moving up and down the vertical twigs, and they exploit the underside of branches more often and more intensely than do Common Firecrests. In contrast, the latter preferentially exploit the upper surface of conifer branches, and of leaves in deciduous trees, and tend to avoid dense foliage. The Madeira Firecrest, too, frequently feeds on the upperside of leaves and twigs, but it also forages in moss and lichens covering the branches and trunks of laurel and oak trees. Studies in wintering habitats have revealed that flocks of Common Firecrests cover a given distance about three times faster than do Goldcrests. All regulid species also collect flying insects by performing hovering flights in front of the outer branches of spruce and fir trees and of tree-heath. Common Firecrests and Ruby-crowned Kinglets, however, hover more often and for more protracted periods of time compared with their respective sympatric congeners. During autumn migration, both European species occasionally hover at large spider webs, from which they pick trapped insects. While Goldcrests prefer to take small aphids, psyllids (Psyllidae) and other plant-sucking insects from among the entrapped invertebrates, Common Firecrests will even snatch and eat the large orb-web spiders themselves. Although both species carefully avoid making direct contact with the web, these small birds have, on rare occasions, been found stuck in a spider web, either unable to move or dead. Goldcrests are highly specialized on small prey, with a strong preference for springtails. During the winter months, springtails represent the most important part of their diet, accounting for more than 60% of this regulid’s intake. In particular, bark-living species of the genus Entomobrya are a regularly available and highly efficacious food source. Discrimination by Goldcrests against large prey species was verified also in aviary experiments undertaken by Thaler. When offered a wide range of spiders, Goldcrests first selected the smallest-sized ones, those of about 20–40 mg, whereas Common Firecrests always discriminated against small prey and preferred large nursery-web spiders of the genus Pisaura, which weighed up to 200 mg. Furthermore, captive Common Firecrests frequently ignored small-sized prey such as springtails. Under natural conditions, they also prey on large caterpillars, moths and crab spiders (Thomisidae). Goldcrests in the Canary Islands are less selective in terms of prey size than are those in mainland Europe, presumably a result of adaptations to the local arthropod fauna. They exhibit no marked preference for springtails, but they commonly feed on long-legged arthropods such as orthopterans and, in captivity, they readily took phasmids, which are generally shunned by European Goldcrests. Both Goldcrests and Common Firecrests usually deal with large prey items by beating them repeatedly against a branch in a stereotyped fashion. Feeding and foraging strategies of the North American kinglets are comparable to those of their European counterparts. Both of the Nearctic species are mainly insectivorous; their food spectrum comprises various insects, including adults, larval instars and eggs, as well as spiders and mites (Acarina). As with the two widespread Palearctic species, they exhibit a degree of differentiation with regard to prey and habitat choice and to foraging strategy. Ruby-crowned Kinglets may spend about half of their foraging time in deciduous trees, and are highly specialized on the capturing of aerial prey. In aviary experiments by Thaler, they showed particular interest in winged insects, mostly moths, and preferred these to other food resources such as caterpillars or spiders. They also consume ants (of the genus Lasius), preferably winged individuals, a particular dietary adaptation for which the Ruby-crowned Kinglet stands out among its congeners. Captives, however, when given the choice, were found to prefer aphids over ants. This North American species is possibly the most generalist of the genus in terms of habitat choice and feeding substrate. The Golden-crowned Kinglet, in contrast, forages mainly by gleaning while hanging upside-down from twigs or small branches. It is far more of a conifer specialist, although it will forage also in mixed woodland. The diets of regulids outside the breeding season are less specialized. They cover a broader range of food resources than is consumed during the nesting season, and include vegetable matter, as well as arthropods. The Ruby-crowned Kinglet, for example, occasionally feeds on small amounts of weed seeds and on berries of wax myrtle (Myrica cerifera), of poison ivy (Rhus) and of red cedar (Thuja plicata) in the autumn and winter months. With decreasing food availability during migration, prey choice of Goldcrests becomes less selective and larger prey species, such as adult winter moths (Operophtera brumata), are also taken. During the first days of life, young Goldcrests and Common Firecrests are fed almost exclusively with springtails; larger food items are not accepted, and occasionally spiders are regurgitated. From the fifth day onwards, the nestling diet includes aphids and a high amount of snail shells, the latter being fundamental for osteogenesis. During later stages of the nestling period, adults offer a larger variety of food to older chicks. From the 15th day the spectrum includes larger moths and caterpillars, as well as various arthropods typically avoided by adults, such as harvestmen (Opiliones), earwigs (Dermaptera) or centipedes (Chilopoda). At Golden-crowned Kinglet nests, harvestmen (of the family Phalangiidae) and moths can constitute between 50% and 60% of the diet of older chicks. Other prey species, including caddis flies (Trichoptera), woodlice (Isopoda) and dwarf spiders (Erigonidae), are obviously inedible to regulids and are always rejected by nestlings.
Relationship with Man
Seemingly, the rather inconspicuous members of this family have not attracted the attention of human beings in any particular way. Owing to their small body size and their preferred habitat, in which they often live at the very top of conifers, they tend to escape our eyes and, because of the faintness of their voices, they often elude our ears, too. The rather cryptic way of life of the Regulidae may be one reason why these little birds are not among the best-known species within the European and the North American indigenous avifaunas. One possible exception is the Ruby-crowned Kinglet, the most noticeable regulid species because of its loud and melodious song; indeed, it has a reputation as one of the most brilliant songsters among North American birds. It is, however, the superlatives of nature that interest us and that we keep in mind as worthy of remembering. In the avian hall of fame, the regulids hold at least one record, namely that of being the smallest of the indigenous birds, at least in the Palearctic Region. As a further peculiarity, the scientific name Regulus means “little king” and refers to the brightly coloured crownstripe and to the associated image of a small king, or “kinglet”, among songbirds. This is reflected also in some other vernacular names for the members of this family, such as the French roitelet. In fact, in the few cases in which these birds have attracted the attention of poets, writers and composers, they appear as the sympathetic “little kings” of the woods, as illustrated by two examples from European literature of the late nineteenth century. In his poem Bei Goldhähnchen, the German writer Hans Seidel, who lived from 1842 until 1906, recounts a flying visit to a Goldcrest’s home. His verses fondly describe the typical regulid habitat in a “ green palace of fir”, the diet of “midge salad and roast beetle”, the territorial song “like finely spun glass”, and the clutch of eggs as “very twee, the ten of them”. Seidel may have had a soft spot for these tiny birds and had obviously observed the life of the Goldcrest, although he misinterpreted some dietary details. For instance, Goldcrests rarely feed on beetles (Coleoptera), even if these were roasted. The literary folk tale Goldhähnchen, written by Ludwig Bechstein in 1857, introduces the Goldcrest with a reference to its most noticeable character, its tininess: “Once there was an old man in a little forest hut who had several children and also a Goldcrest, which is the smallest among the European birds and belongs to the genus of the wrens…”. Regardless of the equivocal taxonomic assignment, the caged Goldcrest of Bechstein is a guarantor of luck and prosperity to the family; it lays a golden egg each day, and before he passes away the father advises his children never to give it away to anybody. Later, however, the oldest son, against his father’s will, sells the Goldcrest to a crooked merchant, who knows about the bird’s secret, and misfortune soon befalls the family. Its members end up in poverty, and the brothers and sisters separate. Unlike the Goldcrest in the fairytale, all regulids are unsuitable birds for keeping in a cage or an aviary. The detailed ethological studies and aviary experiments carried out over a protracted period of time by Thaler were made possible only by her many years of experience of regulid biology and behaviour, and thanks to her thorough and dedicated care for the captive birds, their broods and their vulnerable offspring. These long-term aviary studies, which have provided a substantial insight into the ethology and ecology of Regulus species, are unlikely ever to be reproduced in such elaborateness. Scientists dealing with Goldcrests, firecrests and kinglets will for the most part, therefore, have to content themselves with their role as observers from a distance and with the rare honour of catching a very occasional glimpse of the private life of regulids.
Status and Conservation
Presently, there is no concern about the status of any of the six members of this family. The four continental species occupy large areas, and their habitat, mainly coniferous forest, is not endangered. Furthermore, individual subspecies, at least those of the Goldcrest in Asia, which are highly differentiated in terms of morphology, voice and genetics and therefore need special attention from the conservationist’s point of view, have reasonably large ranges. The situation of the Madeira Firecrest and the Flamecrest, each confined to a single island, is somewhat different. On Madeira, suitable habitat is limited because of human settlements and landscape exploitation, but the Madeira Firecrest has shown itself to be well capable of utilizing even disturbed remnants of the original habitat. On Taiwan, the Flamecrest is found chiefly in the coniferous forests of the mountainous parts of the island and seems to be fairly widespread, at least in national parks. It appears currently to be common, but forest reduction may represent a potential future threat for this endemic species. Consideration should perhaps be given to the conservation status of the Goldcrest populations on the east Atlantic islands, which comprise several well-differentiated subspecies. Indeed, the Goldcrests on the Canary Islands have often been regarded as representing a separate species (see Systematics). Both these and the three races on the Azores are well adapted to bushy heath vegetation (see Habitat), and, notwithstanding their tiny ranges, they appear thus far to be thriving. Nevertheless, the extreme vulnerability of small-island bird populations may be emphasized by the case of the Guadalupe subspecies, obscurus, of the Ruby-crowned Kinglet. This isolated race, found only on the Mexican island of Guadalupe, off the west coast of north Baja California, is a threatened taxon, listed as being in danger of extinction in several official treatments on protected flora and fauna by Mexican governmental institutions from 1994 until 2005. Since there are no reliable records of this taxon in recent years, the Guadalupe kinglet population is presumed by some ornithologists to be already extinct. As indicated at the beginning of this section, the four widespread regulids are not considered to be faced with any serious threats. Even so, they do occasionally suffer large-scale mortality. During particularly harsh winters, huge numbers of these tiny birds can perish over wide areas, and the population of, for example, the Goldcrest in western Europe may be reduced to a fraction of its previous level by protracted periods of cold and icy weather. Fortunately, regulids are able to counter such losses because of their large clutch sizes and double-brooding habit, coupled with generally high rates of breeding success. Populations generally recover from massive losses within two or three years. Mortality resulting from migration and its attendant dangers can also be high (see Movements). Nevertheless, such losses appear not to have any significant adverse impact on the populations of these delightful birds.
Becker (1978), Blecker (1985), Corti (1927), Cramp (1992), Desfayes (1965), Dickinson (2003), García (1985), Glutz von Blotzheim & Bauer (1992), Haftorn (1986b), Löhrl & Thaler (1992), Londot (1980), Martens et al. (1998), Mayr & Amadon (1951), Mayr & Cottrell (1986), Päckert et al. (2003), Sibley & Monroe (1990, 1993), Spicer & Dunipace (2004), Stepanyan (1966), Sturmbauer et al. (1997), Thaler (1980, 1988, 1989, 1990a), Vaurie (1957), Voous (1962).