UNSUITABILITY OF TREE SWALLOWS AS HOSTS TO BROWN-HEADED COWBIRDS

Brown-headed Cowbird (Molothrus ater) Science Article 21

abstract

Swallows are parasitized rarely by Brown-headed Cowbirds (Molothrus ater). I added cowbird eggs to 15 Tree Swallow (Tachycineta bicolor) nests. One egg disappeared and seven failed to hatch whereas 63 of 66 host eggs hatched. The seven cowbird hatchlings died in 3-12 (mean = 6.3) d, whereas six of the seven swallow pairs each fledged the cowbird’s four swallow nest-mates. The cowbirds grew more slowly than cowbird nestlings in naturallyparasitized nests of typical hosts. Tree Swallows appear to be unsuitable cowbird hostsduring both incubation and nestling periods.

ALEXANDER M. MILLS, J. Field Ornithol., 59(4):331-333

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