Herons on ice.

The temperature actually reached 6 degrees today, so here's another shot from this weekend:

Great Blue Heron (Ardea herodias), Starved Rock Lock and Dam,
LaSalle Co, IL 1/24/2014

It seems odd to see a Great Blue Heron hanging out on ice -- it's hard to fish through, unless you have a drill. But it's become an expected sight in northern Illinois in the winter, if not a common one. They're what's sometimes termed a "half-hardy" species, meaning one that will only migrate as far as is necessary to survive. Some birds, inevitably, will misjudge that distance, and perish from cold and starvation.

That seems like the kind of selective pressure that should have pushed the species towards a more stereotypical migration pattern, where the birds keep going to a place that has little chance of freezing over, and therefore there's a much better chance of surviving. And yet, there he is, on that ice floe, and he wasn't alone. So what's going on?

The answer comes down to variation, in two places. First, the birds: each bird uses its own criteria to decide upon a wintering location, and those criteria vary based upon both the bird's physical condition and its genes. The second variation is, of course, the weather. In a cold winter, those birds at the "winter north" edge of the variation will likely die. But in a warm winter, those same birds save themselves much of the time and energy their "play it safe" southern neighbors are investing in migration. Which means that in those years, the risk-taking northern birds have more time and energy to invest in reproduction, and are likely to end up producing more offspring.

This sort of variation is important for a population to quickly adapt to shifts in its environment. Evolution is often presented as "when a beneficial mutation shows up, natural selection will cause it to replace the older version of the gene". But, as the Great Blue Heron above shows, most of those mutations actually happened many generations ago, and the necessary variation is already in the population, just waiting to go. Fixing one allele in a population through natural selection actually requires a fairly strong, consistent selective pressure over quite some time, which is unusual when we're talking adaptations to climate.

However, as our winter weather has warmed significantly over the last century, we've seen these half-hardy migrants wintering farther north than ever. As an example, Great Blue Herons were unrecorded on the Evanston Christmas Bird Count until 1969, and only averaged 0.6 per year until 1989. Since then, they've averaged 8 per year, with a high of 21 in 2012. (1) If we assume that this change is purely genetic (actually pretty unlikely -- most of these half-hardy birds probably incorporate a lot of behavioral plasticity in their genes), then the rate at which the population as a whole shifts will depend upon the basic reproductive rate of the population as well as how much better the risk-taking, northern birds actually do. But it's going to be a lot faster than if the population had to wait for the right mutations.

We have a tendency to think of a species as a thing, as an abstract that individuals rarely quite meet. But, as this heron shows, the abstract is our own mental construct. A species is actually made up of a bunch of individuals, each of them embodying a small part of the pool of variation that is a much more productive way of thinking about populations.

(1) National Audubon Society (2010). The Christmas Bird Count Historical Results [Online]. Available http://www.christmasbirdcount.org [1/27/2014]