According to a report that I missed back in June, many species of birds that migrate along the east coast of the United States are arriving earlier than they have in the past. The interesting part: it’s short-distance migrants only. Birds like the Red Knot or the Great-crested Flycatcher, which migrate from South America, are maintaining their original migration schedules, while those that winter in the U.S., like, say, the Song Sparrow, are cheating north earlier and earlier.

Scientists at Boston University and Manomet Center for Conservation Sciences used 33 years of bird capture data (1970–2002) from Manomet, Massachusetts to examine variation in spring migration times for 32 species of North American passerines. They found that, although first arrival dates actually were slightly later each decade, first arrival dates alone indicate a very small migratory response to climate change. The real measure of whether birds are migrating earlier or later, is the mean arrival dates. And those, according to the authors, have occurred significantly earlier over time.

They speculate that birds that winter in the U.S. are able to notice a warming trend and respond to it, because warm weather in the south generally means warm weather farther north. Consistently earlier warming trends, attributable to global warming/climate change*, influence plant growth and, presumably, food availability for both insectivores and herbivores. Local weather conditions can, it seems, influence migration dates, contrary to long-standing wisdom about migrating birds.

Long-distance migrants, though, because they risk much more when they set out, cannot rely on this possibility, and hence remain where they are until they “know” it’s time to go.

*This study appears in the June issue of the journal Global Change Biology, which “exists to promote understanding of the interface between all aspects of current environmental change and biological systems, including rising tropospheric O3 and CO2 concentrations, climate change, loss of biodiversity, and eutrophication.”