Full Annual Cycle Ecology
Natural selection acts on individuals throughout their annual cycle and full life cycle. To understand how the ecology and evolution of birds has evolved, we have to consider events throughout their full annual cycle. The full annual cycle describes a bird’s ecology across the year. A migratory bird’s annual cycle can be divided into four primary phases: breeding, migration away from the breeding grounds, an overwintering period, and migration back to the breeding grounds. Events within one period interact with events in subsequent periods to drive the biology of migratory species.
Threats to migratory birds are often well-studied on their breeding grounds, but much less is known about what threats exist to them during migration (Cohen et al. 2017), during the winter (Rockwell et al. 2012, Marra et al. 2015, Rushing et al. 2016, Akresh et al. 2019) or how these stages and events interact (Marra et al. 2015). Over the past 30 years, our researchers have headed to Jamaica’s coastal black mangrove forests and adjacent dry forests every winter to study the birds that spend their non-breeding season there alongside tropical resident birds. Our studies in Jamaica have produced key insights into how winter ecology — interactions with habitat, food, weather, climate, and other factors — impacts the birds, not just at that time, but also during the following migration and breeding season and subsequent years.
Some of the most notable results of this research have come from the American Redstart (Setophaga ruticilla), a common warbler species in North America. By monitoring their wintering populations in Jamaica over the long term (30 years), we have found that winter habitat significantly impacts the condition, probability of survival during migration, and reproductive success of American redstarts later that year on the breeding grounds. In one study, we showed that redstarts occupying lower-quality scrub habitat in winter lost up to 8 percent of their body weight — a significant amount for a small bird — and left, on average, six days later for breeding areas. A disadvantage like that carries over to the breeding season and puts them at a significant disadvantage compared to their healthier counterparts who occupied more productive mangroves during the winter.
Barren forest in a dry year in Jamaica (left) vs. lush forest in a wet year in Jamaica (right).
In addition to the effects of winter habitat, winter climate can have similar impacts on the health of these warblers. More rain during the non-breeding season produces vibrant new foliage and hence, more insects during the non-breeding season, which nourish redstarts and other wintering migrants. The amount of rain and subsequent food availability influence these birds’ health, and the health of a bird when it departs for its breeding grounds influences their probability of surviving migration and the number of young it will produce that year. Given climate-induced drought conditions across the Caribbean, it becomes even more important to unravel more of the mysteries of how climate and habitat quality during the winter ultimately shape the lives and populations of migratory birds.
We continue to evaluate the impacts of climate on migratory birds in the different stages of their annual cycle, by examining a range of likely projections of key climate variables – temperature, precipitation, and drought stress – that could be used to assess future effects of climate change on species’ vulnerability.
It’s become extremely evident through our studies that winter climate and habitat quality impact reproductive success, survival in a later season, overall population abundance, breeding season range limits, and natal dispersal. Today we know that breeding productivity of some migratory species, including songbirds, shorebirds, and monarch butterflies, can be influenced in a large part by events that occurred during the previous season, thousands of miles away. And although most avian mortality likely occurs during migration, it is probably a consequence of prior events that occurred during the breeding or over-wintering period. Without a better understanding of how events interact throughout the annual cycle, we will not be able to understand how human activities or natural selection ultimately drives the ecology and evolution of these remarkable animals. Our research continues to explore not just how events throughout the annual cycle, but also the life cycle, of these species influence multiple aspects of the biology of birds.