|Mist netting - David Norman|
This long run of data with large sample sizes, and annual assessments of the breeding population from counting nest-holes, allowed a detailed analysis, jointly with Will Peach, which has recently been published in Ibis. The details are in the paper (click here for a copy or e-mail firstname.lastname@example.org) but in outline the work shows …
1. The annual survival of adult Sand Martins averages around 35%, varying from as low as 10% to a high over 60%, mostly determined by the rainfall in their Sahel wintering quarters. There are fewer insects in the dry years and more birds starve. This is already well-known for several trans-Saharan migrants but this study also showed that the effect is non-linear: above a certain level of African rainfall the birds’ survival flattens off, limited by mortality elsewhere in the life cycle.
2. In this study, there was no effect of summer weather (temperature or rainfall) on adult survival.
3. The size of the breeding population is mostly determined by the survival and return of adults, and much less by recruitment of new birds (one-year-old first-time breeders and immigrants from elsewhere).
|Sand Martin - Lawrence G Baxter|
4. This is the first study to show that overwinter survival in the Sahelian winter quarters is density-dependent. Thus, if the population is high, there is more competition for insect food and more Sand Martins die; if the population is low, even in a dry year there is more food to go round and more martins survive and return to the breeding grounds. 'Population' here means the winter population of all western European Sand Martins, which mix in the Sahel during winter.
5. The recruitment rate of first-time breeding adults was also density-dependent. More un ringed ‘new’ adults were captured during summers when local colony size in Cheshire was relatively small, and vice versa. Competition amongst breeding pairs for nesting sites or food might have caused this pattern.
|Sand Martin - John Harding|
David and Will comment that such density dependence was suspected from previous studies on other species but required a vast amount of fieldwork and statistical analysis to prove it. Such competition for insect prey may apply to other insectivorous migrants that rely on the seasonal flooding of wetlands across the Sahel zone. Better rains in recent years have allowed breeding populations of several Sahel-dependent species to increase in recent years, but the threat of drought continues to hang over the people and the wildlife of the region.
Thanks to David Norman for letting us know.