Flocking behaviour of passerines: a dynamic model for the non-reproductive season

Summary The behaviour of a small male passerine bird over a typical winter day is studied by a dynamic programming model. The bird can be either unpaired or paired; an unpaired bird can forage in a flock, forage alone or sing to attract a mate. Foraging increases his reserves, while singing reduces them. The optimal policy and the expected behaviour of birds depend both on time and reserves. The model predicts that birds will flock, especially in the morning, if flocking birds find more food (foraging efficiency), and also more flocking can be expected when the predation risk is lower in a flock (antipredator benefit). Where flocking gives lower variance in food intake, with the same mean (reduced variance benefit), birds with low reserves at the end of the day choose to forage alone (high variance option), while otherwise they are risk-averse and forage in a flock. The cost of flocking increases with time in a day and with the probability of mate attraction through singing. Decisions inevitably involve trade-offs. Where flocking results in antipredator benefit, but also lower foraging gain, birds with low reserves forage alone, but birds with high reserves flock.Offprint requests to: A.I. Houston