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Differential Food Distribution by Male and Female Green-Backed Tits (Parus monticolus) in Relation to Nestling Size

ABSTRACT. 
 In altricial birds with biparental care, the pattern of food distribution within a brood is a consequence of conflicts between family members. To optimize their investment, parents may feed nestlings according to at least two different types of cues: begging behavior of nestlings and nestling characteristics such as size or sex. We investigated the begging displays of nestlings and the feeding response of parents in 12 broods of the Green-backed Tit (Parus monticolus) by video recording. Male and female parents provisioned nestlings at similar rates and with similar prey loads and often used highly predictable, consistent feeding locations across trips and on different days. Furthermore, both sexes chose separate locations in 7 of the 12 broods. Nestlings that begged more intensely and those that occupied positions closer to the provisioning parent had a higher probability of receiving food from both parents. Begging intensity and positioning did not differ with respect to nestling size. Moreover, no significant sexual dimorphism was detected among the nestlings. When controlling statistically for nestling behavior, we found that male and female parents differed in their distribution of food to different-sized nestlings: males preferentially provided food for the largest and medium-sized nestlings, whereas females did not prefer nestlings of a particular size. These results suggest that the food-distribution pattern in Green-backed Tits is determined not only by nestling behavior but also by parentally biased favoritism toward nestling size.