Variation in egg size in Great Tits.

Great Tit (Parus major) Science Article 17

abstract

Egg-laying in many bird species is believed to be energetically limited. We then expect egg-size to be strongly affected by environmental conditions during egg formation. When the ambient temperature is low or food availability is poor the females might not have enough resources for both thermoregulation and the formation of a large egg and hence lay smaller eggs. We measured egg-volumes of almost all first clutches in two populations of Great Tits Parus major during the breeding seasons 1989 to 1992. We also measured ambient temperature during the egg formation period and for some individuals food abundance at the locality where they bred. There were no significant differences in egg-volume among years and study areas. Egg-volume did not change seasonally and did not differ between young and old females. Egg-volume increased with female body weight but heavier females laid proportionally smaller eggs. At low temperatures Great Tits laid smaller eggs and early in the season young females were affected more strongly by temperature than older birds. Females only laid smaller eggs at low food abundances when the temperature was also low. Therefore, environmental conditions only explained a small proportion of the observed variation in egg-volume. Clearly, other not yet understood factors contribute to variation in egg-size of Great Tits.

Barba E., Garcia D.M., Gil-Delgado J.A. & Lopez G.M., ARDEA 82 (2): 315-328

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