Food Supply in Early Spring and Stability of Pair Bonds in the Varied Tit Parus varius

The findings from previous food-addition experiments designed to examine the effects of food availability on reproductive parameters could have led to false conclusions because most experimental designs fail to take account of population parameters during periods preceding the experiments and because the studied effects of supplemental food could have been confounded by effects on age composition and pair bonds. To test this possibility, we provided a population of the Varied Tit Parus varius with extra food (sunflower seed) from February to April and compared density, survival rate and mate change of the fed population with those of an unfed population. A similar comparison is made between these populations before and after the addition of food. In the fed area, population density was stable and pair bonds from the winter were maintained. By contrast, the survival rate in the unfed area was low resulting in lower density, and many members of winter units changed their partners. Since food supply in early spring promoted the stability of pair bonds and may make age composition more stable, we propose that food-addition experiments should include not only comparisons of reproductive parameters of fed and unfed populations, but also comparisons of both age composition and pair-bond stability before and after the addition of food.