RADIO-TRACKING CONFIRMS A UNIQUE DIURNAL PATTERN OF SPATIAL OCCURRENCE IN THE PARASITIC BROWN-HEADED COWBIRD'

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 but 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 commuting 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.