Redwing blackbirds pose a question about provisioning

I am on my way to Chicago’s Lincoln Park’s winter market. On my way to the lovely Notebaert Nature Center, where the stalls sprawl over two floors, I wander through the park. So here I am, at North Pond, early on a misty, rainy Saturday morning, notebook in one hand, George Orians's thrilling but unfortunately titled book, Some Adaptations of Marsh-Nesting Blackbirds (Princeton: Princeton University Press, 1980) in the other. I’ve brought my dog-eared copy of his book along because I had learned a few days before that an alert city birder had spotted the first redwing blackbirds of the season. Orians tells me that, in the nesting season blackbird parents are under great pressure to obtain enough food for their nestlings. In this situation, as I remember from Darwin's study of finches, natural selection operates powerfully on the birds' behavior; for the number of healthy young that can be raised depends directly on the amount and quality of food that the parents can capture and bring back to the nest.

What’s all this have to do with provenance? As Lizzie’s father urges her in a famous scene in Pride and Prejudice, “Read on!”

At any rate, as most of you will know, redwing blackbirds nest in vegetation growing in shallow water. They are strongly territorial. In the spring, the males arrive first and establish territories that include an area of marsh and adjacent dry land. About two weeks later, the females arrive, and, after visiting several male territories, each female settles on one, mates with the territorial male, and builds her nest within his territory. The females do all the nest building, incubation of eggs, and almost all the feeding of the nestlings, although after the young have left the nest the males also feed them. Most of the diet consists of aquatic insects that have just emerged from the water, supplemented by insects that inhabit the dry areas.

OK, I’m getting to the topic of provenance. My readers -- if I have any! -- have surely observed that these transitional zones between water and marsh -- ecotones is the lovely ecological term for these in-between spaces -- vary greatly in the abundance of insects on which blackbirds can feed, hence the density of blackbirds is roughly correlated with insect abundance. But Orians observes that other factors also play a role. For example, blackbirds avoid nesting in areas with a continuous stand of trees that rise more than 30 degrees above the horizon, even when insect food is abundant, because hawks typically select such trees for their nests.

I promise, I know where I’m going with this. Now comes the real challenge to understanding. Male blackbirds make extremely important choices about nesting territories well before their young hatch and require an abundant source of insect food. Their choices, I think, are analogous to the ones that humans make in finding residences near sources of produce. Unlike humans, who can rely on all sorts of aids in locating provisions, from the Yellow Pages to iPhones -- for instance, I had checked on the hours of the Winter Market on the web shortly before leaving home--, male blackbirds, at the time that they make these choices, have almost no guidance. On this cold, damp, March morning at North Pond, very few aquatic insects have emerged, so the choice of territory must be guided by something other that the sheer abundance of insects. Somehow the males make choices, selecting out of extensive areas of marsh those localities that later produce the richest harvest of aquatic and terrestrial insects. I might suppose that these choices are guided by memory and tradition, the blackbirds simply remembering where they nested last year or where their parents raised them. But marshes change rapidly, year to year, as a result of ecological changes -- varying water levels, invasion by carp -- that drastically reduce the populations of aquatic insects.

Hence the absorbing question: How do redwing blackbirds gain a representation of a future situation that will produce an abundant harvest of aquatic and terrestrial insects? Or, to put the question in less high-falutin’ language: How do redwing blackbirds consider their situation and their future?

That's the topic for my next entry.