Birds generally begin incubation either before laying the last egg of a clutch or when the last egg is laid. We report a third incubation tactic, in which incubation onset is delayed beyond the laying of the last egg. In an Illinois population of American Robins (Turdus migratorius), ∼20% of females delayed the onset of incubation ≤4 days beyond clutch completion. Females that delayed incubation nested earlier and took longer to hatch their young than those that initiated incubation before or on the clutch completion date. Using a cost-beneft approach, we propose that predictably low levels of predation at the study site may have reduced the costs associated with delaying incubation. Females do not appear to pay any fitness costs associated with delaying incubation and may benefit by replenishing food reserves early in the season, when the energetic demands of incubation are higher. Alternatively, females may benefit by delaying incubation as a means of timing the hatching of young with availability of food resources. Although this delayed incubation may be attributable to the unique conditions of the study site, we suggest that this behavior may be found more widely, particularly in populations experiencing similar reductions in predation risk, such as populations in urban areas.
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