Timing of Laying by Swallows (Hirundo rustica) and Sand Martins (Riparia riparia)

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Abstract
Constraints in the timing of breeding of swalllows and sand martins were assessed using an energetics model based on time budgets and food collection rates. Energy budgets were constructed for egglaying and incubating female swallows and sand martins from time budgets and D2O18 measured flight costs. Food boluses were obtained by collaring nestlings, and the rate of food collection was determined. The availability and the body composition of the insect prey were measured. The diets of the swallow, sand martin and house martin during egglaying are compared. The swallow is more reliant on large items than are the other hirundines. During egglaying sufficient energy and protein are quickly collected by both species for their daily energy requirements and for egg production: the limiting nutrient is Ca, but this may be obtained from grit. At critically low temperatures the foraging rate is depressed to such a low level that the females are unable to meet their daily energy requirements. The female swallow probably defers laying until the risk of encountering bad weather during incubation is low since she incubates alone. The sand martin lays earlier because it can risk incubating in bad weather since the male shares in the incubation duties.