Laying interruptions, or even a several-day hiatus during egg laying, have been documented in response to a variety of environmental and internal factors. These interruptions alter the laying schedule of the affected female and have been documented to impact final clutch size in some, but not all, species. The Yellow Warbler (Setophaga petechia), a migratory songbird species, is a determinate layer that typically lays one egg every 24 h until clutches are complete—usually 4 or 5 eggs at Delta Marsh, Manitoba. Nesting observations were recorded from 1974 to 2010 at 961 nests in which laying was occasionally interrupted. Laying interruptions influenced the final clutch size in one of 2 ways. (1) Complete clutches were still laid after a possible delay in ovulation past the ‘optimal period.' Five-egg clutches in nests that experienced a laying interruption provide support for this scenario because 5 eggs is accepted as the largest clutch size generally recorded in this population. (2) Warblers interrupted laying for at least a day but did not lay another egg to compensate for the lost days during the interruption, resulting in smaller clutches on average (i.e., more 3- and 4-egg clutches), possibly because females were incapable of laying due to environmentally or nutrient deficiency induced atresia. The increase in 3-egg clutches and timing of interruptions in the laying cycle support the latter scenario. Date in the breeding season, order in the laying sequence, and, to some extent, higher temperatures preceding clutch initiation promoted the occurrence of laying interruptions. Laying interruptions significantly influenced final clutch sizes, which in turn could affect the reproductive output of the population.

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