Body morphometrics, age structure, and partial migration of urban Merlins

ABSTRACr.-Saskatoon, Saskatchewan, has both a dense urban breeding population of Merlins (Falco columbarius) and a large wintering population. We censused Merlins in the city to determine population size. Individuals were trapped to identify banded birds and to obtain sex, age, and body-measurement data. We collected data during the breeding seasons and winters of 1985 to 1988. The wintering population, which varied from 22 to 38 birds, consisted almost entirely of locally hatched birds; only 4 of 303 Merlins seen during the four winters of this study were confirmed to be unbanded. Males predominated in the winter-trapped sample (32 of 55) but not significantly so. There was a significant difference in the age distribution of the sexes. Most males (29 of 32) were 3 years old or younger, and most females (12 of 23) were 4 years old or older. Body mass of adults trapped in winter was significantly higher (10.0% for males, 5.3% for females) than those of nonmigrants trapped during the breeding season. Yearling males were 7.6% heavier than breeding yearlings. Yearling female masses were significantly lower (8.7%) in winter than during the breeding season. Of 45 birds trapped in winter, 20 subsequently bred in the city. These birds occupied preferred nesting sites (those showing more consistent occupancy) more often than would be expected by chance. Nests involving a nonmigratory male tended to be more productive than those with a presumed migratory male. We suggest that increased familiarity with the breeding area may convey some advantage to nonmigratory males. Recruitment into the breeding population did not vary at a statistically significant level with migrant/nonmigrant behavior. Only 1 of 15 yearling males, and 0 of 9 yearling females, caught in winter obtained a breeding territory the next spring, even though yearlings made up 21% of the breeding population during our study. Wintering Merlins had a much greater tendency to have offspring that also wintered; individuals presumed to be migratory did not. This implies a genetic component to the development of a resident nonmigratory population. Received 10 January 1989, accepted 3 July 1989.