Status and reproductive success of the Caspian Tern (Sterna caspia) in the U.S. Great Lakes

Caspian Tern (Sterna caspia) breeding colonies exhibit a disjunct distribution on all continents except South America where they apparently do not nest. On the North American continent, breeding colonies are located in at least 6 geographical regions which are separated by more than 500 km (see Voous 1960, Martin 1978). Current evidence for Caspian Terns (Ludwig 1968, in prep.) and other larids (Austin 1940, Mills 1973, Harrington 1974, Ludwig 1968, Chabrzyk and Coulson 1976, Southern 1977) is that offspring tend to return to the region of their natal colony to breed. This tendency, and the geographical separation between Caspian Tern breeding areas in North America suggests there is little mixing between areas, and that birds in each region comprise a population that maintains itself primarily through reproduction. It is conceivable, therefore, that one population could decline dramatically because of local perturbations while other populations were unchanged. The possibility of a local decline would be greatest for small, discrete populations particularly if the birds nested at only a few colonies.