Long Term Study of an Undisturbed Cormorant Colony

Abstract

Arboreal nesting by double-crested cormorants (Phalacrocorax auritus) has not been studied as intensively as ground-nesting populations, and there are little published data on tree colony initiation and development, as well as long-term use of trees for nesting. Cormorants established nesting in an existing great blue heron rookery at Ninepipe National Wildlife Refuge (NNWR) in Western Montana in 1974. The primary goal of this study was to record the initiation and growth of an arboreal cormorant colony under undisturbed natural circumstances. The colony has been intensively studied since its inception in 1974, and data on nest success, population dynamics, foraging sites and behavior were collected over the next 18 years, until 1992. Foraging sites and resource partitioning developed with the growing colony. Nest stratification changed over time in response to substrate condition and nest re-use. Areas where nests were concentrated moved within the colony, and nest success was correlated with weather patterns and natural events. Anti-predator response varied with the avian predator involved. Display behavior and fledging of young varied in some aspects from these behaviors in ground nesting populations.

Keywords: Double-crested cormorant, Ninepipe National Wildlife Refuge, Montana, arboreal nesting, great blue heron, rookery, colonization, nest success, avian predators, foraging, migration