Foraging habitat partitioning in Roseate and Common terns

-I examined foraging by sympatrically breeding Roseate (Sterna dougallii) and Common (S. hirundo) terns to investigate whether habitat was partitioned, and whether use of foraging habitat was related to disparate population sizes between the species. Foraging Roseate Terns were associated with physical features such as shoals and drift lines more frequently than Common Terns were. Common Terns foraged under a wider range of habitat conditions than Roseate Terns did, and Common Terns were associated with predatory bluefish (Pomatomus saltatrix) more often. Flocks in which proportions of Roseate Terns were higher than expected (based on the relative sizes of the breeding populations) were smaller and less dense than other foraging flocks, and they formed at sites which were shallower, closer to shore, and windier. Roseate Terns were more successful in flocks in which proportions of Roseate Terns were higher than expected than when in other flocks. Common Terns' foraging efficiency was lower in flocks in which proportions of Roseate Terns were higher than expected than in other flocks. Roseate Terns have a smaller area of preferred foraging habitat available than do Common Terns, and this may be reflected in their population sizes. Received 11 May 1989, accepted 1 December 1989. THE QUESTION of how species coexist has long been of interest, because to understand the differing mechanisms that allow coexistence is to comprehend much of the natural control of biological diversity (Schoener 1974a). When two sympatric, closely related species appear to have very similar needs, we may ask whether mechanisms exist that enable them to avoid direct competition. Implicit in this question is the presumption that two species with identical requirements cannot coexist (Gause 1934). Roseate (Sterna dougallii) and Common (S. hirundo) terns are partially sympatric, closely related congeners, which occasionally hybridize (Hays 1975). Their North Atlantic range has largely overlapped throughout historical time, yet Roseate Terns appear always to have been far less numerous than Common Terns (Nisbet 1980). To address this question, the factors that limit these species must be identified. The Roseate Tern's northwest Atlantic population has recently been listed as "endangered" (Federal Register 52 FR 42064), which makes identification of limiting factors a matter of practical as well as academic interest. The recovery plan (Andrews et al. 1988) calls for quantitative investigations of their foraging ecology. It has been observed that Roseate Terns sometimes feed separately from Common Terns (Nisbet 1981), that foraging Roseates are attracted to certain physical features, and that they follow bluefish (Pomatomus saltatrix) less frequently than do Common Terns (Safina 1985, Kirkham and Nisbet 1987). However, Roseate and Common terns are frequently found in mixed flocks (Nisbet 1981, Duffy 1986), and at these times they take the same prey. Competition occurs between the species (Safina in press). When species compete, the inferior competitor should be either driven to extinction (Gause 1934), forced to coevolve a habitat shift (Schoener 1974b), or limited to a "realized niche" (Hutchinson 1958) by competition. Because Roseate Terns in the northwest North Atlantic have long coexisted with Common Terns in relatively low numbers, it seems unlikely that Roseate Terns are being forced to extinction through competition with Common Terns (it is concern over human predation and colony-site loss that has resulted in this population's "endangered" designation). We might expect, then, to find evidence of partitioning mechanisms. The mechanisms by which species partition their habitat, not merely the presence of differences in habitat use, are of principal interest (Schoener 1974a). In this study I sought to establish what quantitative differences exist in the physical and biotic characteristics of foraging habitat used by these two species, whether differential foraging efficiencies exist in different situations, and whether differences in foraging habitat use can 351 The Auk 107: 351-358. April 1990 This content downloaded from 157.55.39.59 on Sat, 15 Oct 2016 04:44:39 UTC All use subject to http://about.jstor.org/terms 352 CARL SAFINA [Auk, Vol. 107 help explain the low relative numbers of Roseate Terns as well as their long coexistence with the numerically dominant Common Terns in this region. I made three predictions: first, that foraging-site characteristics would differ between species; second, that each species' foraging efficiency would be greater in their principal foraging situations; and third, that Roseate Terns' use of foraging habitat would be more specialized (they would forage in a smaller range of conditions), resulting in smaller exploitable foraging area.