Costs and Benefits of Foraging Alone or in Mixed-Species Aggregations for Forster's Terns

Forster's Terns (Sterna forsteri) often forage alone or within mixed- species foraging aggregations. We investigated the relative costs and benefits of these two foraging strategies. Metrics used to assess differences in strategies included the number of attempts for prey (both successful and missed), capture efficiency, and capture rate. Forster's Terns foraging socially cue on presence and behavior of other aggregating species, especially Snowy Egrets (Egretta thula). Joining aggregations significantly increased their capture rate, but not their capture efficiency, compared to solitarily foraging terns. However, total aggregation size, influenced primarily by the two most abundant species, Laughing Gulls (Larus atricilla) and Snowy Egrets, affected tern behavior. For example, diving terns often collided with Laughing Gulls, causing frequent missed attempts for socially foraging individuals. Thus, large aggregation sizes had a negative effect on foraging success, perhaps because of interference and/or prey depletion. In contrast, solitarily foraging terns dove less often for prey but also missed less often. This may be why terns were observed foraging solitarily as aggregation sizes and congestion increased.