North Atlantic Oscillation (NAO) effects of climate on the relative importance of first and second clutches in a migratory passerine bird

1. Climatic variables are known to affect reproduction and survival in birds. 2. Barn swallows Hirundo rustica L. are long-distance migratory passerines that produce two broods in much of the European range. The relative importance of first and second broods for annual reproductive success in relation to a measure of general climatic conditions - the North Atlantic Oscillation (NAO) - was assessed during a long-term study from 1970 to 2000. 3. The size of first clutches increased with increasing NAO values, and clutch size increased during the study period. Brood size and hatching, fledging or breeding success did not show a significant temporal change, nor were they related significantly to NAO. However, in years with high NAO index values first broods were relatively larger than second broods compared to years with low NAO values. Similarly, the breeding success of first relative to second broods was larger in years with high NAO index values compared to years with low NAO values. 4. The quality of offspring measured in terms of T cell-mediated immune response at age 12 days was related to NAO, with offspring of first broods having stronger immune responses than offspring from second broods during years with high NAO values. 5. Individual females showed patterns of change in brood size in response to change in NAO that paralleled the patterns observed at the population level. This provides evidence of phenotypic plasticity. 6. Immigration to the study population increased following a year when annual productivity was caused mainly by fledglings from the first brood. This implies that increases in NAO increases the size of the barn swallow population through effects on the relative productivity of first broods.