Floating bird nests provide resources for wildlife: ecological roles of vertebrates using Aechmophorus grebe nests at Clear Lake, California

Abstract

Floating bird nests occur ephemerally in many wetlands, providing resources for wildlife, but the species using such nests and their ecological roles are poorly studied. Grebes (Podicipedidae) construct floating nests anchored to vegetation. During 2010–2017 we studied the vertebrates using nests of the colonially-nesting western grebe (Aechmophorus occidentalis) and Clark’s grebe (A. clarkii) at Clear Lake, California. In addition to both grebe species using each other’s nests for copulation and egg dumping, we identified 47 other species of vertebrates using Aechmophorus grebe nests, including one fish, one amphibian, two turtles, one snake, 39 birds, and three mammals. Ecological roles of vertebrates using grebe nests included: egg dumping (one bird); predation on grebe eggs, chicks, or adults (four birds and three mammals); scavenging on broken grebe eggs (five birds); resting (one amphibian, three reptiles, and 15 birds); foraging for non-avian prey on nests (13 birds) or in adjacent water (five birds); and incidental destruction of grebe eggs (one fish and two reptiles) and nests (many species). Floating grebe nests undergo a process of ecological succession. Egg dumping, predation, and scavenging occur during early stages of nest development and maintenance. Once abandoned, older nests are used for resting or foraging for prey in nearby water. Decomposing nests are colonized by arthropods which are preyed upon by birds. Because floating bird nests increase food web complexity by attracting aquatic, semiaquatic, and terrestrial organisms at all trophic levels, protecting the engineers and their nests should be considered a conservation priority by wetland managers.