Number of Pairs, Timing of Egg-Laying and Clutch Size in a Subalpine Sand Martin Riparia riparia Colony, 1968-1985

Abstract

A Sand Martin colony in the subalpine (subarctic) birch belt at Ammarnis, Swedish Lapland (65'N, 590 m a.s.l.), was studied from 1968 through 1985 (excl. 1970-1971). Colony size varied between 8 (1985) and 80 (1975) pairs, and the colony was deserted completely during egg-laying in 1979. Start of egg-laying varied from 3 June (1984) to 28 June (1982), and most often took place between 10 and 20 June. The start of laying was closely correlated with the date when 50% of the ground was snow-free. This date was about one month before start of egg-laying and before arrival of the Sand Martins. There was a much weaker correlation between start of egg-laying and temperature in late May and early June, indicating that the length of the snow-free period before arrival was.more important. Temperature after arrival, however, modified the effects of the general advancement of spring. There was a critical temperature range (mean temperature of 5-70C or maximum temperature of 10-12'C) below which temperatures deterred onset of laying. A temperature below this range after some pairs had started laying deterred new pairs from starting. Abundance of flying insects rather than temperature as such is likely to be the critical factor determining the start of laying, but there are no data on insect abundance during the early stages of breeding. The seasonal decline of clutch size was 0.2 eggs wk-' within years, but less between years with different median dates of laying.