We investigated the amount of time Black-tailed Gulls (Larus crassirostris) incubated and its effects on hatching interval within two-egg clutches. Incubation patterns were classified into three categories (rising, steady, or pulsed) related to daily change of incubation rate until clutch completion. Hatching intervals (mean ± SD, 0.95 ± 0.76 days) were significantly shorter than laying intervals (2.56 ± 0.75 days). There was a significant positive relationship between incubation rate on the day when the first egg was laid (day 1) and hatching interval (Spearman's rs = 0.677, P = 0.016). The incubation rate on day 1 also increased (Spearman's rs = 0.521, P = 0.039) as the breeding season progressed, and hatching interval expanded (Kruskal-Wallis test: χ22 = 8.3, P = 0.016, range = 0–2 day). Thus, the amount of time gulls spent incubating on day 1 was important in affecting hatching intervals and suggested that parents partially controlled hatching intervals with seasonal change by timing of the onset of incubation in this species.
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