Guano Rain and Growing Pain: Co-Nesting Dynamics of Double-Crested Cormorants and Black-Crowned Night-Herons at Tommy Thompson Park

Abstract

This study investigates the co-nesting dynamics between double-crested cormorants (Nannopterum auritum) and black-crowned night-herons (Nycticorax nycticorax) in Tommy Thompson Park, Toronto, Canada, from 1992 to 2023. Linear mixed models with Bayesian inference were used to examine the impacts of cormorant abundance, nest density, management practices, and environmental factors on night-heron population growth. The strongest and only statistically significant relationship was a positive association between cormorant and night-heron growth indices. Results showed substantial uncertainty in the effects of most variables on night-heron growth indices, with wide credible intervals for nest densities, night-heron road proximity, and management activities. Nest densities of both species and proximity to roads had minimal effects on night-heron colony growth, with posterior means near zero. Management activities showed a slight positive but non-significant effect on night-herons. The study revealed that while cormorant population growth generally benefited night-herons, there was high uncertainty in parameter estimates, potentially due to small sample sizes and ecosystem complexity.