Differential reproductive success of Brown-headed Cowbirds with Northern Cardinals and three other hosts

To understand low production of Brown-headed Cowbirds (Molothrus ater ater) by a large host, Northern Cardinal (Cardinalis cardinalis), we compared 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 broods fledged by cardinals but in 62 of 93 broods fledged by other hosts (P < 0.001). Notably, neither the frequency of parasitized nests nor 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 were about 50% larger than cowbird eggs. Many cowbird eggs, as well as cardinal eggs, disappeared from nests, which remained active. Second, the incubation period of cardinals was short, only about 10 h longer than for cowbird eggs. Third, cardinal clutches were small (mode = 3 eggs). Many cowbird eggs were laid after incubation had begun. These features combined to produce great differences in body masses of young cardinals and cowbirds, even when cowbirds hatched first. Differences were accentuated when cowbirds hatched after cardinals. Competition in mixed broods often resulted in underweight cowbirds, which usually died before or soon after hedging. Cowbirds reared without cardinal nestmates grew well and usually fledged and survived well. Finally, inter-brood intervals were much longer for cardinals than for Song and Chipping Sparrows. Cardinals had the lowest number of successful broods per host-pair. Cowbirds thrived when reared by cardinals in broods with only one or no cardinal nestmates. We suggest that host brood-reduction enables cowbirds to exploit large hosts, particularly when the host-incubation period is short.