Great lakes double‐crested cormorant management affects co‐nester colony growth

Abstract

The population of double-crested cormorants (Phalacorocorax auritus; cormorants) in the North American Great Lakes has increased substantially since the 1970s, sparking economic, social, and biological concerns that have led to widespread management of the species within United States waters. Previous studies have quantified behavioral impacts of cormorants on other waterbird species that share breeding colony sites with cormorants. However, no study has yet examined how these impacts might scale to entire colonies, nor have potential effects of cormorant management on co-nesters been examined. Our objective was to estimate effects of cormorant abundance and management on colony growth indices of 4 species that commonly co-nest with cormorants in the North American Great Lakes; 3 of these species are conservation or stewardship priorities for the region. We estimated colony growth using the Great Lakes Colonial Waterbird Survey and comparable Canadian surveys, conducted between 1976 and 2010. We then applied linear mixed models to determine association of co-nester colony growth indices with cormorant abundance and management presence and intensity while controlling for other factors that likely influenced growth rates. According to the fitted models, black-crowned night-heron (Nycticorax nycticorax) colony growth was negatively related to cormorant abundance and management, whereas great blue herons (Ardea herodias) had little response to cormorant abundance, and herring gulls (Larus argentatus) and ring-billed gulls (Larus delawarensis) responded positively to cormorant abundance and management. These results suggest that cormorant management may not be as neutral to co-nesters as is often assumed. Responsible management plans for cormorants should take into account the likely effects on co-nesters present so that conservation and management goals for co-nesters can also be met. Ó 2017 The Wildlife Society.

Double-Crested Cormorant Disturbance: Restoration Potential for a Forested Urban Park

“…Resource Depredation Order was established in 2003 by the US Fish and Wildlife Service and the US Agriculture/Wildlife Service, permitting the culling of DCCs in 24 eastern states including those adjacent to the Great Lakes (Mercer et al, 2013;Strickland et al, 2011;Weseloh et al, 2012;Wires and Cuthbert, 2006;Wyman et al, 2018). DCCs in the western states are not actively managed, and in British Columbia, Canada, they are listed as 'threatened' on the provincial Red List (Mercer et al, 2013).…”

“…These depositions reduce vegetation growth, prevent seed germination, create toxic soil environments for plant and tree roots, and can lead to the ultimate demise of vegetation in the nesting areas (Boutin et al, 2011;Ishida, 1996;Lafferty et al, 2016;McGrath et al, 2012;Natusch et al, 2017;Taylor et al, 2011). Habitat alteration at this extent can severely increase interspecific competition for nesting habitat between DCCs and other colonial waterbirds (Quinn et al, 1996;Somers et al, 2007;Somers et al, 2011;Wyman et al, 2018). It is estimated that 25% of the forest canopy at TTP has been lost as a result of DCCs (Taylor et al, 2011).…”

Full annual cycle tracking of Black‐crowned Night‐Herons suggests wintering areas do not explain differences in colony population trends

“…Conversely, populations of Black‐crowned Night‐Herons are stable or increasing in other regions (Hothem et al 2020). In the Great Lakes state of Ohio, the species is listed as state‐threatened, with continuing declines possibly due to changes in land use, particularly large‐scale conversion of wetlands to agriculture (Dahl and Johnson 1991, Rodewald et al 2016), over‐maturation of nesting trees, competition for nest sites with Double‐crested Cormorants ( Phalacrocorax auritus; USDA‐APHIS 2013, but see Wyman et al 2017), and/or the resulting decline in suitable nest‐site vegetation due to excessive cormorant droppings (Shieldcastle and Martin 1997). However, potential threats during migration and winter cannot be ruled out because little is known about the migratory behavior and connectivity of Black‐crowned Night‐Heron populations to specific non‐breeding areas (but see Henny and Blus 1986, Mikuska et al 1998) and no birds from any population have been tracked throughout the full annual cycle.…”