Dominance and feeding interference in small groups of blackbirds

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Abstract
Dominance and/or interference parameters play a pivotal role in most ideal free distribution models, but there has been scant empirical study of the exact manner in which they jointly operate. We investigate how foraging effort and success varied amongst individuals of different dominance rankings in groups of 1-3 wild blackbirds (Turdus merula) attracted to patches of hidden food. Foraging effort (number of feeding movements per unit time), as opposed to vigilance tradeoffs, was greater when an individual fed with a subordinate conspecific than when it fed alone, but tended to be less when it fed with a dominant individual. Within dyads, 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. Similarly, amongst threesomes, top-ranked birds (but not the lowest-ranked individual) showed higher foraging effort compared to when foraging alone. Top-ranked birds also profited from a greater increase in foraging success (food items per unit effort) than bottom-ranked birds when feeding in threesomes 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 vigilance benefits and interference costs, the latter being more expensive for subordinate individuals. The foraging success of dominant birds may further increase if they use subordinates as food-finders. We discuss the implications of our findings for interference parameters in current Ideal Free Distribution models.

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