Estimation of Crayfish Abundance and Size-Structure in Diets of White Ibis Chicks

We investigated the use of crayfish gastroliths and exoskeleton pieces for quantifying crayfish abundance and size-structure in diets of White Ibis (Eudocimus albus) chicks in the Everglades. We then quantified cray fish and fish abundance from various small hard parts and intact fish heads in 23 boluses, taken from two nesting colonies in Water Conservation Area 3 (WCA 3) and Arthur R. Marshall Loxahatchee National Wildlife Refuge (Lox) in the Florida Everglades during May 2006. We determined that using gastroliths to estimate crayfish numbers in boluses had significant drawbacks; only a small fraction of field-caught crayfish bear gastroliths and the two crayfish species in the Everglades differ in percentage bearing gastroliths. In contrast, counts of crayfish rostrums and chelae pairs gave simple and similar estimates of crayfish in the boluses. The two colonies had strikingly different diets in May 2006; New Colony 3 (Lox) boluses were dominated by crayfish while birds from the Alley North (WCA 3) boluses were fish-dominated and had few crayfish. Using measurements of the crayfish rostrums we deter mined the size-structure of crayfish found in the diets of the New Colony 3 birds, and determined that the crayfish in the diet were relatively large (mean = 19 mm carapace length) when compared to the available crayfish in the marsh. These crayfish were also large relative to previous reports of crayfish found in White Ibis diets in the Everglades.