Reproductive Costs in the House Martin (Delichon urbica)

(1) The aim of the study was to identify costs of reproduction in house martins and to make an assessment of the value of life-history theories and qualitative differences between individuals for interpreting intraspecific variation in reproductive output. (2) Breeding statistics for house martins in central Scotland were similar to southeastern England (except laying was 8 days later and II Y. fewer had a second clutch). There was a correlation between arrival date at the colony and the start of breeding for each individual. Females invariably arrived after their mates. (3) Older males were heavier, tended to pair with older, double-brooded females, laid earlier and reared more young. (4) Older females bred earlier and reared more young but were not heavier. (5) Adults lost weight when food was scarce and also during the most demanding (middle) phase of nesting growth. Laying anomalies (a stop to laying due to food shortages) probably led to significant weight losses in females. (6) Laying interruptions wasted time, reduced the chance of a second brood and marginally lowered the size of the first. (7) First brood fledglings survived equally over winter, irrespective of brood size and (less convincingly) parent age. (8) Older parents tended to be more successful in bringing clutches to independence. (9) Mortality of adult house martins occurred mainly outside the nesting period and averaged 57%. The most important finding of the study was that double-brooded females survived less well (especially if they bred early) than single-brooded ones. This did not apply to males and there was no age-specific change in mortality for double brooded birds of either sex. (10) Food supply limits the start of breeding and can account for some of the changes in output through the season. For a full explanation of intraspecific differences in output within first and second broods however, it is useful to invoke qualitative differences between individuals. They may be considered as proximate mechanisms whereby the attributes of individuals (females in particular) (i.e. weight, size and ability) interact with resources leading to a reproductive pattern which is optimal for each individual. The advantage of early breeding for males was much greater than for females (as indicated by the reproductive value of different breeding patterns) in which the benefits of early laying and double broodedness were largely offset by the mortality costs experienced by female parents.