Shrinkadees

This morning there was a chickadee in my backyard singing, “Hey Sweetie”. It had fluffed up its feathers, so it looked just slightly more intimidating than it would otherwise. Puffed or not, this was still a small bird, likely weighing about as much as a AAA battery. More than just being small relative to this naturalist, this early morning chickadee was about as small of a chickadee as you’ll find during the year. While chickadee body weight is relatively stable across seasons (source), during the winter chickadees lose about 10% of their body weight every night (source). If we looked at the chickadee in a historic context, it would also be small. To understand this, we first need a quick weather report.

ABOVE: The daily range of reported temperatures (gray bars) and 24-hour highs (red ticks) and lows (blue ticks), placed over the daily average high (faint red line) and low (faint blue line) temperature, with 25th to 75th and 10th to 90th percentile bands.

While mid-January to early February is typically the coldest period of the year in Vermont, this year has been particularly warm, with daily highs and lows consistently well above the historic average. This is not anomalous. Most of the warmest days and months recorded have been in the last 50 years and Vermont is about 2°F warmer than it was in 1900 (source); globally January was almost 3°F warmer than pre-industrial averages (source). 

These early season thaws and milder, shorter winters have changed the timing of many seasonal events. Take amphibian migration. When I moved to Vermont in 2008 I was told not to expect amphibian migration earlier than the first or second week of April, but it now regularly occurs in March and it is not impossible to imagine that amphibians will be moving tomorrow night in some of the areas where snow cover has already mostly melted away!!  

The shift in climate has also pushed populations of species mostly north and upslope into areas well outside of their historic ranges (source). And for some species, like our chickadee friend, a warming climate has spurred a measurable evolutionary impact.

Researchers in Connecticut combed through museum specimens of chickadees (and a few other songbirds) dating back to 1874, and compared their body weight to modern birds. They found that body weight decreased over the 150 year interval, which has a corresponding 1°F. This conforms with Bergmann’s rule: body size increases the farther north you go. This trend follows in most mammals and non-migratory birds. The larger an animal is, the lower the ratio between surface area and volume is. And the lower the ratio, better an animal can retain heat. This also connects to Allen’s rule (A for appendages) recognizes that appendages get smaller the farther north you go. Think of a jack rabbit’s enormous ears compared to the small stubby ears of an arctic fox.

Bergmann’s rule was initially used to describe what happens within a species as you measure populations along latitudinal gradients. So what is really fascinating about this study is that it shows Bergmann’s rule playing out in the fourth dimension, acting through time. More recent research (source) supports this, finding that smaller birds are actually shrinking faster. So as Vermont continues to warm, we should see animals continue to get smaller.