Overwintering birds finally get their time in the spotlight

Avian wintertime visitors to America seem to be completely snubbed by wildlife conservation efforts. A ton of effort is directed, quite reasonably, at breeding bird populations and their habitats during the summer months, while the need to preserve wintering habitats has been targeted mainly at shorebirds and waterfowl, plus Neotropical migrants who breed in North America and winter in South America. But for some reason, the wintertime needs of birds that breed in the boreal forests and winter in the more temperate forests of southern Canada and the US have been all but ignored.

These “Neotemperate migrants” number in the billions and make up a hefty proportion of winter bird communities in the US, particularly in the South and West. And although their breeding habitat is among the largest and most intact habitats in the world, many of these boreal breeding species are in decline. According to one study, 49% of migratory species that breed in the boreal zone declined between 1966 and 2012. It’s more than likely that at least some of the blame for those declines can be placed on the quality of their more southerly winter habitats. As the boreal region becomes modified due to climate change and concomitant human development, future declines are all but inevitable.

In order to begin assessing the needs of Neotemperate migrants, University of California, Davis wildlife biologists Kristen E. Dybala, Melanie L. Truan, and Andrew Engilis Jr. decided to focus on a single ecosystem and monitor its bird communities for an entire year. They chose two watersheds in California’s Central Valley. By the end of the study, they had conducted 315 surveys, with 41 during the summer and 66 during the winter in the lower Cosumnes River watershed near Sacramento, and 93 summer and 115 winter surveys in the lower Putah Creek watershed near Davis. In all they detected 138 species.

Species richness during the summer and winter were statistically equivalent, and perhaps more surprisingly, avian phylogenetic diversity was actually higher in the winter. Of 34 winter visitors, 33 of them are known to breed in the boreal forests to the north, and a third of them are in significant decline across North America, which further underscores the importance of temperate habitats for North America’s birds.

Because the quality of winter habitats may contribute to those declines, the researchers argue that their study “provide[s] evidence that maintaining and restoring high-quality riparian habitat for Neotemperate migrants and year-round residents in California is an important conservation responsibility.”

However, it’s not sufficient to assume that the sorts of habitat considerations that breeding birds need are identical to those that wintering birds need, even in the same riparian corridors. Because of seasonal alterations in predation risk, food availability, microclimate, or habitat structure, birds’ requirements may change seasonally as well. Some may change foraging strategies, while others change habitat types or dietary guilds. Indeed, the birds observed in this study were less evenly distributed in the winter, with more heterogeneity in species from site to site, which suggests that the resources that wintering birds require are more patchily distributed than those needed by breeding birds, even for year-round residents.

Given the seasonal differences in habitat use, the researchers argue that local winter biodiversity hotspots could be targeted for protection, and that whatever features are missing from “coldspots” could be targeted for restoration. – Jason G. Goldman | 25 September 2015

Source: Kristen E. Dybala, Melanie L. Truan, & Andrew Engilis Jr. (2015). Summer vs. winter: Examining the temporal distribution of avian biodiversity to inform conservation. The Condor: Ornithological Applications 117, 560–576. DOI: 10.1650/CONDOR-15-41.1.

Header image: A Green-winged teal (Anas crecca), a winter visitor. via Shutterstock.com

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