Eastern Birds Shrinking in Response to Climate Change

Scarlet Tanager / Photo by Vincent Lucas (a.k.a. leppyone)

Occasionally I see questions about why bird banding is useful (or even assertions that banding is outdated). Banding can serve a variety of purposes, from identifying the birds present at a location to tracking individual birds to studying avian diets or environmental toxins. Here is one example of how even the most basic measurements can be useful, if they are done consistently over a long period. A station in western Pennsylvania has been banding birds since 1961 and taking measurements of the birds' weight and wing chord. ("Wing chord" is the distance from a bird's wrist to the tip of the primaries.) Over the period from 1961 to 2007, the birds' measurements steadily decreased.

They examined the records of 486,000 individual birds that had been caught and measured at the ringing station from 1961 to 2007.

These birds belonged to 102 species, arriving over different seasons. Each was weighed. It also had the length of its wings measured, recorded as wing chord length, or the distance between the bird's wrist to the tip of the longest primary feather.

Their sample included local resident bird species, overwintering species, and even long distance migrants arriving from the Neotropics.

What they found was striking.

Of 83 species caught during spring migration, 60 have become smaller over the 46 year study period, weighing less and having shorter wings.

Of the 75 species migrating in autumn, 66 have become smaller.

In summer, 51 of 65 breeding species have similarly reduced in size, as have 20 out of 26 wintering species.

The differences in size are not big.

"On average, the decline in mass of spring migrants over the 46 year study was just 1.3%," says Dr Buskirk.

"For a 10g warbler that's a loss of just 130mg."

But some species are losing more weight.

For example, the rose-breasted grosbeak has declined in mass by about 4%, while the Kentucky warbler has dropped 3.3% in weight and the scarlet tanager 2.3%.

According to a biological principle known as Bergman's Rule, individuals that live further north will be slightly larger than individuals within the same species that live further south. The reasons for the link between climate and size are unclear, but the link is well attested. Thus if the climate is warming, one would expect birds to become gradually smaller. That appears to be what is happening in this case. The link to warming is strengthened by the fact that species that spend part of the year in the tropics have shrunk faster than resident species.

Whether this will affect the birds species' long-term viability remains to be seen. So far, there does not seem to be a link between smaller physical size and population declines. Many of the species that have decreased in physical size are doing quite well otherwise. Those populations threatened by climate change appear to be struggling with other climate-related problems, such as droughts or the timing of migration. Still, the trend is worth monitoring, for what it tells us about the changing climate and the bird populations.