The redistribution of non-breeding dunlins in response to the post-DDT recovery of falcons

Large-scale changes in predator populations are occurring worldwide due to (re-)introductions, over-exploitation, or recovery after decimation by pesticides and persecution. These widespread changes may affect the distribution of their prey. We studied the continental-scale distributions of non-breeding Calidris alpina pacifica and C. a. hudsonia (Pacific and Atlantic dunlins, respectively), as numbers of their major predators-peregrines (Falco peregrinus) and merlins (F. columbarius; together 'falcons')-increased after DDT was banned in 1973. For the period 1975-2010 we compiled the number of dunlins and falcons in each of 244 Christmas Bird Count circles, which cover most of the dunlins' non-breeding ranges. Over the study period, falcons increased by 6.5- (Pacific) and 3.1- (Atlantic) fold, spread to more count circles, and the number of dunlins per falcon fell. The annual total count of the two dunlin sub-species fluctuated strongly and independently. We measured annual aggregation as the expected proportion of a subspecies total found on the same count circle as a randomly selected dunlin. The average aggregation of Pacific dunlins (0.117) was about double that of Atlantic dunlins (0.059), with annual variation largely attributable to changes in a few large count circles. The slope of the aggregative response to year-on-year changes in dunlin numbers was identical on the two coasts. The response to the ongoing falcon increase was positive and significant on the Pacific and slightly negative on the Atlantic. We interpret these results using a version of the ideal free distribution that includes predation danger.