DIFFERENTIAL REPRODUCTIVE SUCCESS OF BROWN-HEADED COWBIRDS WITH NORTHERN CARDINALS AND 3 OTHER HOSTS

To understand low production of Brown-headed Cowbirds (Molothrus ater ater) by a large host, Northern Cardinal (Cardinalis cardinalis), we c ompared that production with that of three small hosts: Song Sparrow ( Melospiza melodia), Chipping Sparrow (Spizella passerina), and Yellow Warbler (Dendroica petechia). Cowbirds were present in only 11 of 63 b roods fledged by cardinals but in 62 of 93 broods fledged by other hos ts (P < 0.001). Notably, neither the frequency of parasitized nests no r the number of cowbird eggs per parasitized nest varied significantly among the hosts. Unusual features of cardinals as hosts accounted for the low production of cowbirds by cardinals. First, cardinal eggs wer e about 50% larger than cowbird eggs. Many cowbird eggs, as well as ca rdinal eggs, disappeared from nests, which remained active. Second, th e incubation period of cardinals was short, only about 10 h longer tha n for cowbird eggs. Third, cardinal clutches were small (mode = 3 eggs ). Many cowbird eggs were laid after incubation had begun. These featu res combined to produce great differences in body masses of young card inals and cowbirds, even when cowbirds hatched first. Differences were accentuated when cowbirds hatched after cardinals. Competition in mix ed broods often resulted in underweight cowbirds, which usually died b efore or soon after hedging. Cowbirds reared without cardinal nestmate s grew well and usually fledged and survived well. Finally, inter-broo d intervals were much longer for cardinals than for Song and Chipping Sparrows. Cardinals had the lowest number of successful broods per hos t-pair. Cowbirds thrived when reared by cardinals in broods with only one or no cardinal nestmates. We suggest that host brood-reduction ena bles cowbirds to exploit large hosts, particularly when the host-incub ation period is short.