. The Canadian field-naturalist. 256 The Canadian Field-Naturalist Vol. 103 20 c CD e 10 CL Breeding pairs before egg-laying Nonbreeding pairs Failed pairs Pairs with egg or chick All years. Ql lua&fl II 16 21 26 June II 16 21 26 31 July 10 15 20 25 30 August Figure 6. Occurrence of pairs in relation to breeding status (percentage of occupied sites containing pairs). Bars represent mean attendance in five-day intervals beginning on the dates indicated. less than 1% could be attributed to wind speed). The effects were detectable mainly because of the large sample sizes available for analysi
. The Canadian field-naturalist. 256 The Canadian Field-Naturalist Vol. 103 20 c CD e 10 CL Breeding pairs before egg-laying Nonbreeding pairs Failed pairs Pairs with egg or chick All years. Ql lua&fl II 16 21 26 June II 16 21 26 31 July 10 15 20 25 30 August Figure 6. Occurrence of pairs in relation to breeding status (percentage of occupied sites containing pairs). Bars represent mean attendance in five-day intervals beginning on the dates indicated. less than 1% could be attributed to wind speed). The effects were detectable mainly because of the large sample sizes available for analysis (n = 492 days over six years). However, the most notable responses to weather appeared to be nonlinear, since the mass departures and arrivals in the pre-laying period were often associated with a marked change in weather conditions. To illustrate, 1 identified 16 obvious "spikes" in the attendance patterns observed before 1 June (Figure 1). The first day of attendance after an absence of one or more days I treated as day 0 and calculated average weather conditions on the two previous days, on the day of return, and on the following two days. The conspicuous transition in weather conditions associated with the return to land was a shift from northerly to southerly winds, increasing clouds, and rain (Table 1). Throughout the breeding season, fulmars made foraging trips lasting several days. Therefore, the effect on attendance counts of a change in weather conditions would be expected to last for several days. To test that possibility, I compared means of attendance on day i resulting from different conditions of wind speed or direction on each of the five preceding days. Attendance was still significantly depressed up to three days after a strong wind (>I0 knots), and two days after a north wind (Table 2). Finally, 1 considered the possibility that cyclic changes in weather conditions could account for cyclic patterns of colony attendance by fulmars. Among four we
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