Spring Arrival Times and Assortative Mating of Kestrels in South Scotland

Abstract

(1) In a partially migrant population of kestrels studied for 4 years in south Scotland, migrants settled in spring from late February until late April. Mean arrival time was earlier following a mild winter, but was not related to local food supply, so that the mean length of courtship (pairing to egg-laying) was longer in a poor vole year (when laying was late) than in good times (when laying was early). (2) Within years, migrants that arrived early tended to pair earlier, and lay earlier, than those that arrived late. Later pairing birds partly compensated for this by having shorter courtship than those that paired early. (3) Males and females arrived over roughly the same period and there was no difference in their mean arrival dates. However, within pairs, the males arrived about 4 days, on average before their females. The sex ratio of winter residents was biased towards males, so more early arriving females than males were able to pair with residents on arrival. This resulted in a surplus of unpaired males that persisted until the end of the settling period when all breeding birds were paired. (4) In the 2 years when they bred in substantial numbers, first-year birds arrived later than adults. In males, but not females, first-year birds resident over winter waited longer than adults before finding a mate. (5) Selective mating of first-year birds and adults occurred in 1978 but not 1979. The observed preponderance of yearling-yearling and adult-adult pairs in 1978 seemed largely due to the different arrival times of the two age groups, though this did not entirely explain the infrequency of yearling-male x adult-female pairs compared with adult-male x yearling-female. The data were consistent with the idea that females, but not males, preferred adult partners when these were available.