A Test of the Prey Size Hypothesis as an Explanation for Decreased Wading Bird
Nesting in the Southern Everglades
Dale E. Gawlik
Everglades Division, South Florida Water Management District, West Palm
Beach, FL.
One hypothesis of why wading bird nesting in the southern Everglades has declined is that shortened hydroperiods due to drainage have reduced the size structure of the fish community. If wading birds need large fish for profitable foraging, then any reduction in the availability of that size class would decrease wading bird body condition and the ability of birds to successfully produce young. This hypothesis was first proposed for Wood Storks but has since been extended to wading birds in general. This hypothesis is attractive because it posits a solid link between wading bird populations and more fresh water reaching the southern Everglades estuaries (i.e., increased hydroperiod), a major restoration goal. The implications of increased size structure of the fish community are currently unknown for wading bird species other than the Wood Stork. Such information is critical if we are to restore and sustain the entire suite of species that comprise North America's most diverse wading bird community.
Previous studies have documented the importance of water depth in determining foraging site quality for wading birds. Therefore, any test of fish size also should account for water depth. In this study I tested the effects of fish size and water depth on the numerical response of free-ranging wading birds. For each bird species, the null hypothesis was that neither water depth nor fish size had an effect on site selection. Treatments were fish size (golden shiners 5 cm or 9 cm in total length) and water depth (19 cm, 28 cm, or 37 cm). Initial fish density in all ponds was about 5 fish/m 2 , which would be about average for density of preysized fish in the Everglades. Data were analyzed in a mixed-model framework with the response variable transformed as ln( y + 0.5).
The experiment was conducted from 24 Jan - 11 Feb 1997 in 12 0.2-ha ponds adjacent to the northern Everglades of Florida. The ponds were built within a
1545-ha constructed wetland. This experiment was one of a series of experiments that examined foraging site quality for wading birds.
Bird abundance (all species pooled) at the site increased from day zero, when ponds were initially stocked with fish, until day seven, when abundance peaked at about 240 birds (Fig. 1). Thereafter bird abundance decreased to about 30 birds at

day 15, and then increased abruptly for two days. This spike in abundance was probably due to an influx of migrants following the passage of a cold front, something that is assumed to occur in South Florida but has rarely been documented. The general pattern of bird use prior to the cold front was very similar to an experiment with fish density in 1996. d u e l c p e a F e
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Also, similar to the previous experiment were the relative abundance of species and the timing of their peak densities. The Great Egret was the most abundant species overall but it didn't reach maximum density until the abundance of White
Ibis and Snowy Egret had started to decline. This suggests that Great Egrets found the site attractive even after White Ibis and Snowy Egrets were abandoning the site because of reduced prey availability.
Analysis of the treatments showed that increased water depth reduced the density of Great Egrets, Little Blue Herons, Snowy Egrets, Tricolored Herons, Glossy
Ibises, White Ibises, (P < 0.05, all tests) and Wood Storks (P = 0.07), although the relationship was weak for Wood Storks. Fish size was strongly (P = 0.003) and positively related only to density of Great Egrets. The relationship was weakly (P

= 0.06) positive for Wood Storks and weakly (P = 0.07) negative for Glossy
Ibises. The latter species experienced high rates of kleptoparasitism by Boattailed Grackles when handling large prey. Neither treatment variable affected the density of Great Blue Herons (P > 0.16, both tests).
This study supports the notion that Great Egrets and Wood Storks prefer foraging sites with larger fish than what are commonly found in the Everglades today. The stronger response to prey size by the larger bird species is consistent with the greater energetic requirements of these birds. The smaller bird species showed no preference for larger fish, and Glossy Ibis, the smallest species, actually avoided sites with large fish. Although two of the three largest wading bird species preferred foraging sites with large fish, these birds have very different population trends, suggesting that fish size is not likely to be a mechanism controlling populations of both species. Wood Storks would be most likely to be affected by a reduction in fish size because they prefer sites with larger fish and previous experiments showed that these birds are more constrained in their choice of foraging sites than are other Everglades wading birds. There is no evidence that a reduction in the size structure of Everglades fishes would have had an impact on foraging site quality of the other species examined here.
Dale E. Gawlik, Everglades Division, South Florida Water Management District,
3301 Gun Club Road, West Palm Beach, FL 33406, phone: 561-686-8800 x
4539, Fax: 561-681-6310, dale.gawlik@sfwmd.gov