Competition for early arrival in migratory birds

1. It is widely accepted that the arrival order of migratory birds is corre lated with the condition of the birds, which leads to high quality individu als occupying prime sites. However, the theoretical backgrounds for this ar gument have been lacking. A simple game-theoretic model of arrival timing i s provided which investigates the evolutionary stability of condition-depen dent arrival order in territorial migrant birds. 2. Competition for territories or other priority-dependent benefits can lea d to arrival dates far preceding the cost-minimizing date (the optimum date in the absence of competition) for all but the weakest individuals. Increa sing the number of competitors can generate a 'cascading' competition for e arly arrival, which advances arrival dates further apart from the individua l optimum dates for the onset of breeding. 3. At equilibrium, arrival order corresponds strictly to condition order on ly if marginal costs of advancing arrival are always larger for individuals in lower condition. If spring mortality vacates territories for later-arri ving birds, the criterion for 'honest' arrival order becomes still stricter : differential survival costs should exist, but survival differences among individuals (or, alternatively, territory quality differences) should not b e very large. 4. If the habitat is saturated so that there is a risk of not obtaining a t erritory at all, or if worst territories are of much lower value than the r est, competition may lead to the majority of the population arriving within a fairly short interval, followed by a much later floating fraction. This synchrony in the arrival of breeders imposes an increasing cost for the les ser fit breeding birds. Thus, arrival costs paid are not necessarily highes t for earliest arriving individuals, but for these who have the most to los e if they drop a few steps in the arrival order. 5. Competition for high quality territories can also lead to partial migrat ion, in which case birds in good condition are expected to be most likely t o remain resident.