Analysis of Double-Crested Cormorant Nest Spatial Patterns in Single and Mixed -Species Colonies and their Effect on Cormorant Behaviour and on Black-Crowned Night Heron Spatial Patterns

Double-crested cormorants (Phlacrocorax auritus) are managed because they eat fish, transform habitat and can affect other waterbirds. Conversely, cormorants have been highly successful in North America since the 1970s due to increased food supply and their adaptability, although other factors may be important. I researched cormorants at Tommy Thompson Park, Ontario, and tested 1) whether tree-nesting cormorants nesting with black-crowned night herons (Nycticorax nycticorax) decreased heron population through direct or indirect effects and 2) whether ground-nesting cormorants changed their behaviours due to nest density or position and whether there was a behavioural trade-off that affected fledging success. Cormorant variables were correlated with heron declines due to indirect effects in some areas. Increased density was related to lower aggression, but there were no trade-offs between the behaviours. My research illustrates the different behaviour and spatial patterns for cormorants in single and mixed-species colonies and in ground versus tree nesting colonies.