Data from: Wintering areas predict age-related breeding phenology in a migratory passerine bird

Understanding connections between breeding, stopover and wintering grounds
for long-distance migratory birds can provide important insight into
factors influencing demography and the strength of carry-over effects
among various periods of the annual cycle. Using previously described,
multi-isotope (δ13C, δ15N, δ2H) feather isoscapes for Africa, we
identified the most probable wintering areas for house martins (Delichon
urbica) breeding at Badajoz in southwestern Spain. We identified two
most-probable wintering areas differing in latitude in West Africa. We
found that the probability to winter in the northern region was related to
age and sex of individuals. Specifically, experienced males (i.e., two
years or older) winter in the northern area with a greater probability
than experienced females, whereas first-year females winter in the
northern area with a greater probability than first-year males. In
addition, wintering area was correlated with breeding phenology, with
individuals wintering in the northern area initiating their clutches
earlier than those wintering in the southern area. For birds wintering in
the northern area, there was no relationship between age and clutch
initiation date. In contrast, young birds wintering in the southern area
initiated their clutches earlier than experienced birds wintering in this
area. There was no significant correlation between wintering area and
clutch size or the number of fledglings produced. We hypothesize that the
relationship among social status, population density and winter habitat
quality should be the most important driver of the carry-over effect we
found for this population