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Colonial Nesting and Social Feeding as Strategies for Exploiting Food Resources in the Great Blue Heron (Ardea Herodias)

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In this paper I present data collected to test certain predictions arising from the hypothesis that colonial nesting and social feeding in birds are adaptations that enhance the efficiency of exploitation of unpredictable food supplies. The hypothesis suggests that individuals benefit from nesting in colonies because they have the opportunity to learn about good feeding areas by following other birds from the nesting colony to the feeding grounds. If a bird is unsuccessful on one foraging trip, it will observe and follow more successful birds on subsequent trips; in this way the colony acts as an 'information centre'. I collected two types of data to test this idea as applied to the Great Blue Heron (Ardea herodias). (i) I recorded flights of parent birds. (ii) I measured the rate of food intake of birds hunting for food in flocks of different sizes. The analysis of foraging flights from the colony showed the following: (a) The birds used different feeding grounds on different days; this suggests that the food supply is ephemeral. (b) The birds tended to leave the colony in groups, which would be expected if they were following each other. (c) Birds from neighbouring nests tended to use the same feeding grounds and tended to leave the colony in groups (within the overall grouping of the colony as a whole). This would be expected if birds mainly copy their near neighbours. All these results support the idea of an information centre. I did not collect any data on whether successful birds were being followed by unsuccessful ones, but the data do seem to show that birds follow one another. On the feeding grounds, I showed by use of models that herons are attracted to feed in areas where there are other birds, and that they are more attracted by a group than by a single bird. If flock feeding helps in locating good feeding places, we can expect that flock birds would do better in terms of food intake than solitary individuals. A step-wise multiple regression showed that rate of food intake of a bird (grams of fish caught per minute) is an asymptotic function of flock size. A bird in a flock of 20 gets about 5 times as much food per minute as a solitary bird. Further, the percent success (i.e. strikes resulting in a capture) is higher in flocks, and the coefficient of variation of feeding rate is lower. Thus flock birds get more food for relatively less effort and stand a smaller chance of doing very badly in terms of food intake. I discuss various possible explanations of the advantage of the flock birds, and conclude that it is not a result of searching harder, less 'nervousness' of predators or stirring up the fish to make them easy to catch. It is a consequence of the fact that flocks only build up where feeding conditions are good. I present a simple graphical model to show how this happens. In conclusion, my results support the idea that colonial nesting and social feeding are adaptations concerned with finding food, but there are also other factors involved in the evolution of sociality in ardeids and other birds.

Affiliations: 1: Institute of Animal Resource Ecology, University of British Columbia, Vancouver, B. C., Canada

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