Breeding ecology and behaviour of a colonial hirundine : a study of the sand martin (Riparia riparia) using DNA fingerprinting
Some potential benefits and costs of coloniality in birds were investigated, including ectoparasitism, antipredator behaviour, and social competition with particular emphasis on social-sexual behaviour associated with genetic parentage. The species studied was the sand martin (Riparia riparia) , which was abundant and highly colonial in the study area in central Scotland.
Population decreases from 1990-1992, including a particularly notable decrease in the number of pairs in 1991 (46% lower than 1990), were associated with a decrease in mean adult body-size, apparently due to natural selection for smaller sizes related to adverse conditions in the wintering areas. During breeding, most losses occurred before hatching mainly due to desertion or predation of eggs.
Anti-predator behaviour, specifically the time until a predator was detected and mobbing started, tended to decrease with subcolony size, and was interpreted as a benefit of coloniality. Nests treated with a pesticide to kill arthropod ectoparasites had heavier chicks than control nests, suggesting that ectoparasit ism may be a cost for sand martins. The cost was possibly dependent on colony size since the effects of ectoparasites on nestling growth tended to be more marked in a larger colony.
Artificial eggs placed in nests at the pre-laying and laying stages were more often rejected prior laying (46%) than after the start of laying. Amongst 46 broods (excluding 6 used for male removal experiments) and 170 offspring analyzed by multilocus DNA fingerprinting, 41.3% of the broods, and 20.6% of the offspring were derived from extra-pair fertilizations (EPFs), quasi-parasitism (QP) and intraspecific brood parasitism (IBP) . Amongst these, EPFs were most frequent, with 34.8% of broods containing at least one offspring fathered by an extra-pair male, accounting for 14.7% of the offspring. The proportion of nestlings resulting from QP was 4.1%, while IBP accounted for 1.8% of chicks.
The occurrence of extra-pair paternity in broods was unrelated to the body size of the attendant male parent, clutch or brood size, or age of the male (estimated from ringing date). Late broods tended to have a greater incidence of EPFs, however, and males were cuckolded more frequently in second broods than in first broods.
Male removal experiments and observations of mate- guarding behaviour gave some indication that the risk of cuckoldry for males increased as the intensity of mate- guarding decreased, but other factors (such as female choice) might also have been involved in regulating the frequency of EPFs. Cuckolded males (as determined by DNA fingerprinting) did not reduce their level of parental effort, measured as rate of brood provisioning, suggesting that EPFs represented a reproductive cost for the cuckolded individuals, while benefiting the individuals which achieved EPFs.
The potential costs and benefits of coloniality, and the implications of mixed-reproductive strategies (MRS) in avian parental care and mating systems are discussed.