Age-Related Patterns and Body Condition in Wintering-Site Use in the European Robin (Erithacus rubecula)

Partial migration systems offer valuable insights into how avian populations adapt to environmental shifting. While breeding ecology is widely documented, the factors governing winter site fidelity remain understudied. Using long-term ringing data from Constant Effort Ringing Stations (CESs) in central Iberia, we assessed repeated wintering-site use based on the number of wintering seasons in which individuals were recorded. Then, we examined its association with body condition and age class in the European Robin (Erithacus rubecula). Our results show that body condition significantly increases throughout the wintering period (F1,1750 = 140.325, p < 0.001), though this effect exhibits interannual variation (F1,844 = 5.858, p = 0.016), possibly reflecting yearly environmental fluctuations. Furthermore, adults who presented higher body condition the previous winter exhibited higher records than juveniles (F1,598 = 8.278, p = 0.004). When accounting for unequal recapture opportunities, this association lost statistical significance, whereas age remained a significant predictor throughout the analyses (p < 0.001). These findings highlight the importance of considering age-related processes and temporal variations in body condition when assessing wintering strategies. However, the observational nature of the study and variation in sampling efforts limit our ability to distinguish true site fidelity from detection and survival.