New Forest Service study backs conservation at a landscape scale to protect a near threatened bird species

Understanding the factors that influence how species select their habitats is crucial to inform conservation strategies, especially for vulnerable species. A new study about how wintering individuals of the Kirtland’s Warbler (Setophaga kirtlandii) in the Bahamas use available space and food resources showed that the home range and core area of several populations within this island are tightly linked with the age of the vegetation and the way food resources are distributed in the environment.

Factors like food availability, predation risk, and competition between individuals of the same species influence how species use space. With birds that migrate from breeding grounds in northern regions to nonbreeding grounds in the tropics, these factors may result in multiple space use strategies, for example having fixed home ranges instead of moving in flocks.

The near threatened (IUCN 2023) Kirtland’s Warbler tends to exhibit flexible space use on its Bahamas wintering grounds. This Nearctic-Neotropical migrant overwinters in the Bahamas during the November to April dry season, when its food resources (fruit and arthropods) decline at some sites. These changes force many individuals to move from food-poor to food-rich sites, and in drought years, these shifts may result in significant variation in the density of individuals across space.

Knowing the extent and degree of overlap of the non-breeding home range of this species in their wintering grounds in the Bahamas, and which areas are most intensively used, i.e. their “core areas,” can provide unique insight into the factors that most affect space and resource use of this species. Previous work has addressed some of these issues but did not consider the full extent of space used by individual birds. Resighting observations were also restricted to accessible study sites which likely resulted in an underestimation of warbler’s space use.

A team of scientists from the USDA Forest Service (Joseph M. Wunderle, Eileen Helmer, Javier E. Mercado, Dave Currie), Antioch University of New England (Michael E. Akresh) and the American Bird Conservancy (David N. Ewert) used radio telemetry to quantify size and overlap of space use by sedentary Kirtland’s Warblers in Eleuthera, the Bahamas, to identify some of the variables influencing non-breeding space use. They also related home range and core area size to factors known to influence these variables, such as bird sex and age class, year, winter period, forest age since disturbance, vegetation structure, fruit biomass and fruit shrub foliage cover.

Researchers found that the warbler’s sedentary non-breeding home range was larger than most other wintering Nearctic-Neotropical migratory passerine birds studied to date. “While it is currently difficult to make comparisons with other migrant species, finding that the home range of the Kirtland’s Warbler in the Bahamas was so relatively large was consistent with our prior observations that availability of the warbler’s food supply was so variable. It seems that food availability in the areas they forage is highly variable which sort of forces them to be more flexible in their use of available space and habitats,” commented Wunderle, lead author of the study. Further, except for adult females, which were found to have smaller core area sizes than juveniles or males, age class and sex differences were found to hold no relationship with home range and core area sizes.

The warblers’ ability to exploit other food resources, such as arthropods, likely explain why neither home range nor core area size were correlated with fruit biomass or fruit shrub foliage abundance. Despite this, researchers suggest that less abundant fruit in older vegetation areas likely force warblers to expand their foraging activities to more distant places, thereby driving the observed increase in home range and core size area with vegetation age. Because arthropods are more abundant in mid-to-mature forests, they suspect that sampling artifacts may have obscured the role of fruit availability in this respect.

“It was also interesting to see that the abundance of fruit shrubs could explain so much of the home range overlap observed,” added study co-author Helmer. This was different to what they observed for core areas, where average overlap was more limited, especially between early vs. midwinter sites. “That is often indicative of neighboring warblers being more territorial or avoiding interacting with other individuals of its own species,” further noted Wunderle.

Findings in this study prompted authors to highlight that management at a landscape scale, as on the breeding grounds, will be required to sustain sufficient patches with food resources for this near threatened warbler. Further recommendations included focusing conservation efforts in areas where fruit production and arthropod abundance are least affected by late winter droughts and on younger vegetation areas with reoccurring disturbances, as those tend to favor fruit shrub establishment.

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