Dominance and feeding interference in small groups of blackbirds

Dominance and/or interference parameters play a pivotal role in most ideal free distribution models, but there has been scant empirical study of the e xact manner in which the)jointly operate. We;e investigate how foraging eff ort and success varied amongst individuals of different dominance rankings in groups of 1-3 wild blackbirds (Turdus merula) attracted to patches of hi dden food. Foraging effort (number of feeding movements per unit time), as opposed to vigilance tradeoffs, was greater when an individual fed with a s ubordinate conspecific than when it fed alone, but tended to be less when i t fed with a dominant individual. within dr;ads, changes in foraging effort were associated with the direction of the dominance relationship, but not the relative difference in dominance rank between the two individuals. Simi larly amongst threesomes, top-ranked birds (but not the lowest-ranked indiv idual) showed higher foraging effort compared to when foraging alone. Top-r anked birds also profited from a greater increase in foraging success (food items per unit effort) than bottom-ranked birds when feeding ill threesome s than when feeding alone. Dominant birds showed increased foraging success , but not effort, after displacing a subordinate. Our results suggest that an individual's foraging effort is determined by the interplay of group vig ilance benefits and interference costs, the latter being more expensive for subordinate individuals. The foraging success of dominant birds ma) furthe r increase if they use subordinates as food-finders. We discuss the implica tions of our findings for interference parameters in current Ideal Free Dis tribution models.