Geographical Trends in Breeding Parameters of Swallows Hirundo rustica and House Martins Delichon urbica

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Abstract
Geographical trends in demographic traits were studied in swallow H. rustica and house martin D. urbica from populations in the western Palearctic. Clutch size increased with latitude in swallow and clutches 2nd with longitude in house martin 2nd clutches. The number of broods per season decreased latitudinally and for the house martin longitudinally as well. The number of eggs per pair per breeding season thus decreased with latitude in both species and with longitude in the house martin. The difference in the longitudinal response between the 2 species may be due to the higher and thus more weather-influenced foraging niche of the house martin. Hatchability of house martin eggs increased with latitude. Between year population size variations increased with latitude in the swallow. Ashmole''s (1961) hypothesis did not account for geographical trends in clutch size. Geographical variation in nest predation could not explain variance in clutch size. Royama''s (1969) model on clutch size trends taking food availability, time available for feeding and the temperature-dependent energy requirements of young into consideration explained a large fraction of the variance in clutch size among both species. The absolute metabolic level of the nestlings decreased from north to south.

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