Rasmussen, J. L., University of Manitoba, Winnipeg, Canada, rasmussen912@yahoo.com
Underwood, T. J., Kutztown University, Kutztown, USA, underwoo@kutztown.edu
Sealy, S. G., University of Manitoba, Winnipeg, Canada, sgsealy@cc.umanitoba.ca

LIMITS OF GRAPS-EJECTION IN HOSTS OF THE PARASITIC BROWN-HEADED COWBIRD

It is not known why small hosts of the Brown-headed Cowbird (Molothrus ater) accept parasitic eggs. Acceptance may be due to the inability to recognize cowbird eggs or because hosts cannot grasp-eject them efficiently, i.e., without damaging their own eggs. Alternative rejection methods, such as puncture-ejection, incur fitness tradeoffs. Grasp-ejection has a negligible cost to hosts, but likely requires a certain bill length to grasp-eject a cowbird egg, a length that is not known. To estimate the minimum bill length required to grasp-eject a cowbird egg, we tested the ability of known grasp-ejectors, Gray Catbirds (Dumetella carolinensis) and American Robins (Turdus migratorius), to grasp-eject widths larger than a cowbird egg by adding sequentially larger objects to their nests. Using the ratio of the maximum width of grasp-ejected objects to bill length, determined from video-recorded ejections (N=105), we estimated that a host with a tomial length as short as 16 mm can grasp-eject a cowbird egg efficiently. This estimate allows for a prediction of which cowbird hosts are physically constrained to accept parasitic eggs.

Oral presentation

Session #:G03
Date: Tuesday, 8/5/08
Time: 3:15 PM

Presentation is given by student: No