Session V: Bird Studies

Abstract #: 97502

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A TEST OF ENVIRONMENTAL FACTORS CONSTRAINING THE USE OF FORAGING SITES BY WADING BIRDS (CICONIIFORMES) IN THE EVERGLADES
Gawlik, Dale E.
Everglades Systems Research Division, South Florida Water Management District, West Palm Beach, FL

ABSTRACT
It is reported that food availability (i.e., abundance and vulnerability to capture) is the single most important factor limiting populations of wading birds in the Everglades. Constraints on the acquisition of food by wading birds are therefore the primary barriers to restoring sustainable populations to this degraded ecosystem. I manipulated two potential constraints (prey abundance and water depth) on the use of foraging sites to test the hypotheses that each component limits foraging-site use by free-ranging wading birds. I conducted the experiment in 12 0.2-ha ponds using water depth treatments of 10 cm, 19 cm, and 28 cm, and fish (Notemigonus crysoleucas) density treatments of 3 fish/m2 and 10 fish/m2. The temporal dynamics of site use by birds indicated species-specific differences in the ability to find food patches as well as to exploit a wide range of water depths. For example, white ibis (Eudocimus albus) and wood storks (Mycteria americana) found food patches quickly but did not utilize patches at a wide range of depths. In contrast, great egrets (Casmerodius albus) increased in abundance more slowly but occupied the entire range of depth treatments. Water depth affected the use of sites by 6 of the 8 species examined whereas fish density affected only the white-plumage social-feeding species. The degree to which a species was limited by either prey abundance or water depth was a function of both their morphological characteristics and behavioral plasticity. These results suggest that foraging opportunities in the Everglades are most limiting for white ibis, wood storks, snowy egrets (Egretta tricolor) and tricolored herons.