Ecology of Fear in a Colonial Breeder: Colony Structure in Ground-Nesting Great Cormorants Phalacrocorax carbo Reflects Presence of Predators

Ground-nesting Great Cormorants were monitored in three neighbouring colonies at Lake IJsselmeer, The Netherlands. Using aerial photographs taken during peak breeding time, nest density and nearest neighbour distance determined for four sequential years. In addition, species number of predators determined. total, five mammalian nine avian predatory associated with the Cormorant colonies. Spatial distribution nests mostly showed dispersed random patterns rather than a contagious pattern. latter distribution, less between expected both from equal pattern, was found colony De Ven 2013 last year its existence. predator Red Fox Vulpes vulpes arrived 2010. all colonies, highest shortest predators. At low to moderate pressure, ground-nesting left free space that used by adult birds take-off landing. During years existence shrinking an almost circular shape, extreme lowest edge-to-surface area ratio. But Foxes present, fringe still caused greater losses due direct predation. Breeding success fluctuated synchronously but lower where higher. arrival young over resulted strong decline breeders, eventually leading complete abandoning site 2014. Large gulls formed another important group did not cause abandon site. Vooroever colony, bush tree cover supplied shelter allowed breed without causing decrease, as case when no available. Greater reduced distances are considered be pro-active response individual presence When numbers increased, within-colony open spaces normally exist under circumstances filled up nests, leaving little or room landing departure. This leads edge effects shape thereby potentially limiting predation risk. As consequence high densities, interference other Cormorants. study is first show structure waterbirds affected forces attraction repulsion founding driven.