Radio‐Tracking Confirms a Unique Diurnal Pattern of Spatial Occurrence in the Parasitic Brown‐Headed Cowbird

Abstract

Brood—parasitic Brown—headed Cowbirds (Molothrus ater) in the eastern Sierra Nevada of California, breed and feed in almost totally disjunct areas that reflect local optima for finding host nests and food, respectively. Radio—tracking showed that five females and four of eight males spent mornings in host—rich habitats such as forests and then commuted 2.1—6.7 km to one or more prime feeding sites such as horse corrals and bird feeders for the rest of the day. The four noncommuting males, which were all yearlings and possibly socially subordinate, also showed high mobility bud did not occupy the same area each morning. Since cowbirds lay eggs in the morning and were rarely seen feeding then, the disjunct areas visited by commuters can be characterized as morning—breeding (egg—laying) and afternoon—feeding areas. We found little evidence of territoriality on morning ranges, nor did we find evidence of prolonged pair bonds. The morning ranges of commuters averaged 68 ha, and their total home ranges, including afternoon—feeding areas, averaged 442 ha. These are among the largest breeding home ranges described for passerines, and they equal those of certain raptors. Raptors require large areas to provide a sufficient prey base, and cowbirds require similarly large areas to provide a sufficient number of host nests. The cowbird's commuting pattern, which is unique among passerines, involves a shift from largely asocial behavior in the morning to extreme sociality in the afternoon. Unlike related nonparasitic icterids that disperse from central breeding sites to feed, cowbirds do the reverse, dispersing from centralized feeding sites to breed. The cummuting behavior of cowbirds is yet another example of the flexibility of a species' behavioral ecology in response to the dispersion of resources essential for maintenance and reproduction.

Sex differences in the effects of captivity on hippocampus size in brown-headed cowbirds (Molothrus ater obscurus).

“…Obviously, more research is needed to isolate why we found sexspecific effects of captivity on Hp volume in M. a. obscurus despite a lack of sex-specific Hp size in wild birds of the population we studied. Future studies should consider subspecies and within-subspecies population differences as cowbird space use appears to depend on a number of habitat, habitat use, and population size dimensions (Darley, 1982(Darley, , 1983Dufty, 1982;Lowther, 1993;Rothstein, Verner, & Steven, 1984;Rothstein et al, 1986). Precedents for such within-species population differences are found in black-capped chickadees (P. atricapilla).…”

Egg laying in inappropriate nests by the Brown-headed Cowbird (<em>Molothrus ater</em>): acts of parasitism or emergency egg dumping?

“…Females then generally search for and apparently select the next nest for parasitism, although nothing is known about whether females "line up" nests for parasitism. By late morning, females have generally left the host nesting area and travelled to areas to forage, eventually flying to a roost site in the evening (e.g., Rothstein et al 1984;Curson et al 2000). Before sunrise the next morning, females fly directly to the selected nest, approach it "stealthily," often in the dark (Scott 1991;Sealy et al 2000;Sealy and McMaster 2004), and parasitize it within a few seconds (Sealy et al 1995).…”

The brown-headed cowbird and its riparian-dependent hosts in New Mexico

“…Female cowbirds are active but generally silent in breeding ranges in the morning (Rothstein et al 1984). They are highly mobile and perch or fly high above the ground (Rothstein et al 1984), presumably searching for hosts' nests.…”

“…They are highly mobile and perch or fly high above the ground (Rothstein et al 1984), presumably searching for hosts' nests. Breeding ranges are rarely used for foraging (Rothstein et al 1984), but Yokel (1986) found that cowbirds feed opportunistically on cicadas in breeding ranges in a California riparian habitat. Morning ranges of males frequently overlap female breeding ranges (Yokel 1986), and males often travel with females (Rothstein et al 1984).…”

“…Breeding ranges are rarely used for foraging (Rothstein et al 1984), but Yokel (1986) found that cowbirds feed opportunistically on cicadas in breeding ranges in a California riparian habitat. Morning ranges of males frequently overlap female breeding ranges (Yokel 1986), and males often travel with females (Rothstein et al 1984). Males are active and vocal in the morning and in feeding ranges.…”