Ecological associations where one species enhances habitat for another nearby species (facilitations) shape fundamental community dynamics and can promote niche expansion. However, it is unclear how strong such associations may be, and how they influence how and where species persist and coexist. In numerous examples around the world, birds may nest near more formidable species in order to gain protection from nest predators. Here, we summarize a series of studies quantifying the consequences of colonially nesting long-legged wading birds (Pelecaniformes and Ciconiiformes) nesting above crocodilians.
1) Nest success and colony persistence by wading birds is strongly limited by access to breeding colonies by predatory mammals. 2) Alligators (Alligator mississippiensis) appear to offer protection from mammals (<3% nest predation rates) when water levels allow alligator mobility; when alligators were confined to pools by low water, herons abandoned colonies due to raccoon (Procyon lotor) predation. 3) Herons at > 40 randomly chosen colonies were never found nesting without alligators below their nests despite the availability of unused islands. Herons were attracted to novel sites in larger numbers at locations with bird + alligator decoys than at either control sites or sites with bird or alligator decoys alone, suggesting that the birds are attracted to nest over crocodilians. 4) In the form of falling chicks, wading bird colonies typically supply large amounts (cf 25 GJ yr-1) of correctly sized alligator food items. Alligators living within active colonies were in significantly better body condition at the end of the dry season than those not associated with colonies. Alligators within colonies were also able to tolerate water conditions outside the range of those preferred by their non-colony counterparts. 5) the ranges of breeding wading birds is very strongly overlapping with that of the alligator, despite the broad availability of wetlands outside the range of the alligator. This information collectively suggests a strong two-way ecological facilitation in which presence of alligators serves to protect nesting wading birds from mammalian predators, and brood reduction by wading birds contributes significantly to the energy budgets of alligators. Colonially nesting birds and crocodilians co-occur in many tropical and subtropical wetlands worldwide, and we suggest that the relationship we describe may be a keystone process for both of these ecologically important species-groups.