Breeding ecology of urban Merlins (Falco columbarius)

Abstract

We studied the breeding ecology of Merlins (Falco columbarius) in Saskatoon, Saskatchewan, Canada, from 1971 to 1990. The nesting population increased from 1 pair in 1971 to a maximum of 31 pairs in 1989. At 25.4 pairs/100 km2, the 1989 nesting density is the highest recorded for this species. The population increased at a rate of 0.165 ± 0.015, or 17.9%, per year. The growth rate of the population declined, although not significantly, with increasing density. Merlins spaced their nests regularly, with nearest-neighbour distances ranging from 161 to 4669 m. Clutches were initiated between 19 April and 29 May. Earlier clutches had more eggs and produced more young. Weather did not appear to influence the breeding phenology, but fledgling House Sparrows (Passer domesticus, the primary prey) were maximally available when Merlins had their chicks. The reproductive success of this population was one of the highest recorded for this species. In only 4% of the nests did any eggs fail to hatch. The mean and modal clutch sizes were 4.2 ± 0.04 (SE) and 5, respectively, and mean and modal brood sizes were 3.8 ± 0.03 and 5, respectively. We did not observe any significant effect of nesting density on either clutch or brood size. Males delivered most of the food and were most active in nest defence against corvids, while females primarily incubated, brooded, and fed the nestlings. This expanding Merlin population had higher and more consistent reproductive rates than other populations.