Prior residence influences contest outcome in flocks of non-breeding birds

We sought to determine whether early arrival was a determinant of contest outcome in loosely organized, non-breeding flocks of birds. In White-throated Sparrows (Zonotrichia albicollis) arrival date during autumn migration, i.e. within-year prior residence, was a significant determinant of contest outcome for those birds that were not present on the study site in previous years. To determine whether the advantage of early arrival was due to prior residence per se, as opposed to some correlate of arrival date (e.g. condition), we experimentally delayed the arrival of 60 migrants. We found a significant effect of the delay: the outcome of contests between naturally arriving (control) birds and experimentally delayed birds was significantly related to the difference between the control bird's natural arrival date and the experimental bird's delayed arrival date. Thus, prior residence per se, and not some correlate of arrival date, had a significant effect on a naïve individual's ability to win contests. Interestingly, arrival date had no effect on contest outcome among birds that had wintered on the site in previous years. Because a prior residence advantage accumulates in a time-dependent manner, our results suggest that fighting ability or perceived resource value increases with site familiarity. Thus, there may be selection on arrival date and site-faithfulness as behavioral strategies to increase access to resources.