A spatial and genetic analysis of Cowbird host selection

Abstract

Molecular genetics makes it possible to measure basic but long elusive parameters of the breeding biology of the Brown-headed Cowbird (Molothrus ater).We examined cowbird fecundity and host selection behavior using a combination of molecular genetic techniques to link female cowbirds to the eggs they lay, radio-telemetry techniques to track female cowbirds' daily movements, and geographic information systems (GIS) to integrate these genetic and spatial data.Our study site lay within a forested 1300.halandscape in New York composed primarily of mature forest with adjacent old fields.We found that female cowbirds used their home ranges as principal egg-laying areas.Individual females used characteristic individual home ranges throughout the breeding season, and they returned to the same home range every breeding season.Over one-half (54%) of females laid all their eggs in host nests inside or close to their home range.Proximity to a female' s home range was the only significant ecological or biological feature affecting a cowbird' s host selection.Neither host species identity, nest height, adult mass, egg size, incubation period, nor host taxonomic classification predicted which nests would be parasitized.Eggs laid outside the home range were frequently found in multiply-parasitized nests located along common flyways or in conspicuous sites that a cowbird could discover opportunistically.We also found that female cowbirds avoided laying more than one egg in a particular host nest, even though multiple parasitism characterized over one-third of parasitized nests in the study.Finally, we estimated that effective cowbird fecundity lies between a minimum of 1.72 eggs per female and an upper bound of 8.16 eggs per female.Effective cowbird fecundity is defined as the actual number of cowbird eggs laid in appropriate host nests and not ejected; it is lower than raw fecundity or the physiological egg production capacity of cowbirds.We suggest that the female cowbird' s use of home range is a critical element in its breeding behavior, enabling cowbirds to use a known-host selection strategy.Experienced female cowbirds selectively parasitize the host pairs that nested in their home ranges in previous breeding seasons and were most successful.The three elements of cowbird breeding behavior reported here challenge the stereotype of the Brownheaded Cowbird as an r-selected species that produces a large number of young and invests no parental care.Instead, these results suggest that cowbirds lay fewer eggs in host nests than has been speculated and that they do invest parental care.Two examples of parental care we discuss are observing a host' s parental behavior and nest success before parasitizing it, and laying each egg in a different host nest, even though that requires females to search longer and to find a larger number of host nests.Current cowbird trapping programs should be evaluated for their effect on age structure of cowbird populations and resulting parasitism patterns.Yearling females may be associated with higher rates of multiple parasitism and higher rates of parasitism on more conspicuous hosts.Conspicuous hosts such as the Black-capped (Vireo atricapillus) and Least Bell' s (Vireo bellii pusillus) vireos are probably most at risk from cowbird populations with disproportionately high numbers of immigrant yearling female cowbirds such as those created by trapping programs.