Egg Weights and Brood Reduction in the European Swift (Apus apus)

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Abstract
In southeast England [UK], the timing of egg laying in the European swift is related to ambient temperature, rain and strong winds. Individual females lay at dates constant with respect to the population mean for the year and tend to lay eggs of constant size for the weather conditions prevailing at the time. Eggs are normally laid at 2 day intervals but in adverse weather egg formation is prolonged to 3 or even 4 days and clutch size is reduced. In such conditions, clutch initiation is postponed by females who are otherwise about to lay. Egg weights are small relative to the size of the bird but contain more yolk than in comparable nidicolous passerines. Egg weights vary annually except for eggs laid late in the season. Clutches in which at least 1 egg fails to hatch tend to be lighter than those in which all eggs hatch, but successful and unsuccessful eggs differ little in weight. Heavy eggs are large in linear dimensions and produce large chicks which more frequently survive to fledging. The egg-chick size correlations hold for eggs laid at each position in the clutch; the greater mortality of small eggs is due mainly to 3rd-hatched chicks who fail in food competition with older, larger siblings. Chicks from large eggs fledge more quickly than those from small eggs but this is due almost entirely to the larger hatching size of these chicks and not to egg size itself. Female swifts may normally have difficulty in accumulating enough energy reserve to form eggs, thus sensitizing egg formation to weather-influenced fluctuations in insect abundance. Within these constraints the relative sizes of the eggs within the clutch are adjusted to optimize sibling competition and brood reduction, as an adaptation to unpredictable food supplies for the nestlings.