Ranging behavior of female and male Shiny and Screaming Cowbirds while searching for host nests

SCARDAMAGLIA, R.; REBOREDA, J.C.

AUK

AMER ORNITHOLOGISTS UNION

Año: 2014 vol. 131 p. 610 - 618

0004-8038

Brood parasitic cowbirds are hypothesized to search for and locate host nests within a relatively constant area, as this would facilitate the monitoring of nests over time and the synchronization of parasitism with host laying. We tested this hypothesis in Shiny Cowbirds (Molothrus bonariensis) and Screaming Cowbirds (M. rufoaxillaris) by radio-tracking females and males for 3-6 consecutive days and determining individual daily morning ranges and cumulative morning ranges. In Shiny Cowbirds, the mean size of morning daily ranges and cumulative morning ranges was larger for males than for females, but we did not find sexual differences in Screaming Cowbirds. In both species, there was an extensive overlap in the morning ranges of individual females between consecutive days and the addition of new area to their ranges decreased over time. For both Shiny and Screaming Cowbirds, morning ranges of conspecific females tracked the same day overlapped, indicating lack of territoriality. Male and female Screaming Cowbirds that were trapped together were also spatially associated during radio-tracking, indicating joint nest searching and social monogamy. Most tracked Shiny and Screaming Cowbirds used mainly one roost, relatively close to their morning ranges, which was maintained throughout the breeding season. Our results show that Shiny and Screaming Cowbird females use relatively constant areas for nest searching and that female and male Screaming Cowbirds search for nests together.