PREY SWITCHING IN RED-NECKED PHALAROPES PHALAROPUS-LOBATUS - FEEDING LIMITATIONS, THE FUNCTIONAL-RESPONSE AND WATER MANAGEMENT AT MONO-LAKE, CALIFORNIA, USA

The ability of red-necked phalaropes Phalaropus lobatus to switch prey under conditions of changing prey abundance at Mono Lake, California, was tested in order to predict the potential effects of continued wat er diversions there on migratory waterbird populations. Red-necked pha laropes cannot switch prey because they are incapable of surviving on a diet of just the hardier of two invertebrates threatened by salinity increases at Mono Lake. Individuals experimentally limited to brine s hrimp Artemia monica as a primary prey source lost mass rapidly until death ensued, or until they were offered other prey. These data show t hat a marked preference for one prey may indicate important physiologi cal limitations in a predator. Such limitations may partly explain Mur doch's empirically supported prediction (Ecol. Mongr. (196), 39, 335-5 4) that prey switching will not occur where strong prey preferences ex ist. These results also indicate that strong prey preferences of migra tory birds can, and probably should, help determine water management p olicy at wetlands of importance to shorebirds, especially in cases whe re initial prey diversity is low.