INTRASPECIFIC COMPETITION INFLUENCES FOOD RETURN-PREDATION RISK TRADE-OFF BY WHITE-CROWNED SPARROWS

The trade-off of food return against predation risk was quantified for winter flocking White-crowned Sparrows (Zonotrichia leucophrys), and the effect of intraspecific aggression on this trade-off was tested. F eeding bowls, containing a 1-L sand/seed mixture, were placed at three distances from cover. Control treatments (equal seed densities at eac h bowl) were compared with experimental treatments (higher seed densit ies farther from cover). More birds fed farther from cover when associ ated food return was higher, but age-classes responded differently to treatments. On average, dominant adults fed closer to cover than subor dinate immatures, even when higher reward was available farther from c over. As predicted if risk of social interaction influences the food r eturn-predation risk trade-off: (1) immatures switched their feeding l ocation more readily than adults and (2) when the reward differential among bowls was especially large, adults shifted to feed farther from cover, and displaced immatures towards the bowl closer to cover. White -crowned Sparrows traded-off food return against predation risk, and t his trade-off was influenced by the risk of social interaction such th at subordinates were willing to risk higher predation if the risk of s ocial interaction was thereby reduced.