18. Morning Nest Arrivals in Cowbird Hosts: Their Role in Aggression, Cowbird Recognition, and Host Response to Parasitism

Indigo Buntings (Passerina cyanea) nesting in old-field habitats in central Missouri are parasitized at least four times as often as Field Sparrows. I used model cowbirds placed near nests to test if host aggression explained this difference. Although both Field Sparrows and Indigo Buntings responded to Brown-headed Cowbird models with significantly more chips than to a Fox Sparrow (Passerella iliaca) control, only one female Field Sparrow and one Indigo Bunting pair attacked the cowbird model. Because the utility of aggression and cowbird recognition may depend on a host's actually encountering laying cowbirds, I also observed arrival times of hosts and cowbirds at nests near dawn during egg laying. Laying female Indigo Buntings rarely encountered laying cowbirds; the mean arrival time for Brown-headed Cowbirds at host nests was 11.4 min before sunrise (N =8), and female Indigo Buntings arrived 16.7 min after sunrise (N = 6). Laying Field Sparrows arrived at nests before or at about the same time as cowbirds (mean arrival 17.4 min before sunrise, N =16). Most parasitized Field Sparrow nests were abandoned in 1992 and 1993, suggesting that detection of female cowbirds at the nest causes Field Sparrows to desert their nests. These results suggest that aggression rarely prevents cowbirds from laying in nests of these hosts, but that recognition of brood parasites during laying elicits host nest desertion by Field Sparrows. Additionally, nest desertion by hosts may lead to underestimates of parasitism frequencies, because deserted nests are less likely to be found. Nest arrival times may limit the efficacy of host defenses against cowbirds, especially if cowbirds must commute to breeding areas from distant communal roosts.