Filling gaps in the full annual cycle of the Black-crowned Night-Heron (Nycticorax nycticorax)
Kristie A. Stein, MS
Advisor: Christopher M. Tonra
Migratory birds carry out different stages of their life cycle in geographically disparate locations complicating our ability to track individuals over time. However, the importance of connecting these stages is underscored by evidence that processes occurring in one stage can influence performance in subsequent stages. Over half of migratory species in North America are declining, and it follows that understanding the factors limiting population growth is a major focus of current avian conservation. Globally, Black-crowned Night-Herons (Nycticorax nycticorax) are common and widespread, but populations across the Great Lakes region in long-term decline. Within Ohio, Black-crowned Night-Herons historically nested at 19 colonies but currently occupy only five of those sites. The largest colony, West Sister Island, represents an important breeding area for many species of wading birds and currently hosts the majority of the night-heron breeding population in Ohio. The number of nesting pairs at West Sister Island has been monitored since the 1970s, but little is known about the population outside of the breeding season. My research is the first to examine multiple stages of the full annual cycle of Black-crowned Night-Herons. My overall objectives were twofold: (1) to examine how conditions experienced in the nest carry over to influence survival during the post-fledging period and (2) to describe the migratory behavior and nonbreeding ecology of Black-crowned Night-Herons. To examine rates of reproductive success, I monitored Black-crowned Night-Heron nests at two mixed-species breeding colonies in Lake Erie. I found that nest success rates were high in relation to breeding colonies in other parts of the range and predation events were uncommon. Additionally, I identified potential limiting factors to individual nestlings by confirming that youngest nestlings within a brood are at a disadvantage in terms of growth rates, as those with slow growth were also less likely to survive to fledging. Black-crowned Night-Herons that occupied nests higher in the canopy produced more offspring to fledging. I utilized an automated telemetry array in Ohio and Michigan to track post-fledging movements and survival. I did not find evidence that carry over effects were present from pre- to post-fledging, but female birds experienced higher mortality rates compared to males during the six months following fledging. Aside from a small number of recoveries of banded birds, little information exists on the migratory or stationary nonbreeding portions of the annual cycle of Black-crowned Night-Herons. To measure movements throughout the annual cycle, I deployed satellite transmitters on adult night-herons during the post-breeding season. All Black-crowned Night-Herons exhibited migratory movements and two strategies were evident, where the total duration of migration was greatly influenced by the time spent at stopover locations. These different strategies indicated that stopover habitat requirements vary between individuals during both autumn and spring migration. Migratory routes followed the same general patterns in autumn and spring. The majority of Black-crowned Night-Herons spent the nonbreeding season in the southeastern United States, but the spread of locations spanned five countries. Unlike many migratory species, where the nonbreeding period is characterized by stationary behavior, Black-crowned Night-Herons made large scale movements during the nonbreeding period, relocating up to 902 km from one home range to the next. The information that our research has provided fills large gaps in our knowledge of the full annual cycle and can be used to inform conservation planning both within Ohio and in other declining populations.