Two species

an encounter with a wary ermine… and some thoughts about tree swallow nests

On a murky day toward the end of February, I went with a friend on the Boy Scout camp trail. Rain and warm temperatures had turned the snow to unpleasant deep slush and puddles in some places. As usual, we were just looking to see what we could see—and it wasn’t much. There were some crows picking through the wrack on the beach, a tiny group of bufflehead moving farther offshore, and a few geese on the far side of the big meadow. Not even any curious seals popping up to inspect us, no sea lions cruising by. A bit disappointing!

We cut through some of the groves on the big berm behind the beach, where the mosses were happily showing off their many shades of green. One spreading tree sheltered several duck decoys. Then, as I was stepping over a few roots, a movement near the toe of my boot made me stop. A small white head with bright black eyes was peering up out of squirrel-size hole in the ground. I signaled to my friend (who walks faster than I do) to come back. Meanwhile the white head disappeared, but briefly, only to re-emerge once more for a quick look-see. The owner of the head did not like two monsters looking at it, so even though we backed well away and waited, it did not reappear. With its wintry white coat, the ermine (a.k.a. short-tailed weasel; called a stoat in the U.K.) would have been very conspicuous on the snowless ground under the trees. We don’t see ermine very often, and this was the highlight of the walk that day.

The same day, in the afternoon, three female mallards arrived on my icy home pond. One of them had scouted the place two days earlier, and now brought along a couple of friends. They were out of luck, though; no open water and no seeds on the ice. The ducks weren’t the only critters that were anticipating spring, however. The previous week, a bear had crossed the ice into my yard, no doubt allured by the aroma of the peanut butter feeders, and left dirty footprints on my downstairs windows. That was not the only bear report for the Valley—ADFG tells me that there have been other early risers (or poor sleepers) this winter.

Some recent reading included a book called White Feathers, by famous naturalist Bernd Heinrich. It’s about tree swallows, those beautiful aerial acrobats that also sing sweetly—some birds seem to have it all! They are cavity nesters, using natural tree holes and readily using nest boxes.

Among many other observations, Heinrich noted that the tree swallows using his nest boxes had a strong interest in white or light-colored feathers, sometimes collecting them from some distance away. Male swallows were especially interested, although females sometimes showed interest too. Small feathers might make a cozy nest, but they had a special use for long, whitish feathers, chiefly during the later stages of egg-laying and the incubation period.

Of course, I wanted to know if our local tree swallows collected white and light-colored feathers too. And they do: inspection of nest boxes here and in Gustavus found white and whitish feathers around the clutches of eggs.

Those long, white feathers are arranged around the edge of the cup that holds the eggs, placed with the quills poked into the bottom of the nest around the eggs, so that the plumes stand up and arch over the eggs. The feathers clearly are not a cuddly cushion for the eggs, and not a snuggly blanket around them; smaller feathers might do that. They might conceal the eggs, but feathers of any color could do that. So why white ones? Are tree swallows the only species that adorns its nests in this particular way?

Photo by Jessica Millsap. This image was taken as part of the Audubon Tree Swallow Project, under permits from the Alaska Department of Fish and Game and the US Fish and Wildlife Service.

Tree swallows are fiercely territorial, aggressively defending an area and sky-space near the nest, sometimes engaging in knock-down-drag-out fights that end in injuries. They defend a chosen nest cavity against other tree swallows and other cavity-nesting species, including wrens, woodpeckers, bluebirds, starlings, chickadees, and others. The supply of suitable cavities is generally limited and competition for them can be ferocious. In some cases, tree swallows even oust chickadees that have already laid eggs and appropriate the cavity.

The long, whitish feathers, arranged to arch over the eggs, would show up well in dark cavities, easily visible from the nest opening. Heinrich suggests that they might possibly be a visible signal that tree swallows occupy that cavity. When the adult swallows are out foraging, such a signal could be useful in turning away other cavity-seekers and thus avoiding injurious battles. More observation and research needed!


Mate choice (1 of 2)

…it’s complicated!

When a female wood frog, ready to mate, arrives at a small pond full of singing males, she is jumped by the nearest male, who grabs her around the neck and locks his thumbs together, so he cannot be dislodged. She apparently has little or no choice in the matter—it is first come, first served.

But in many vertebrates, females can and do choose their mates. Sometimes choice is based, at least partly, on the male’s property—his territory, defended against other males and providing nest sites or food sources or protection from weather. For example, male yellow-headed blackbirds carve out territories in a marsh. When females arrive, they cruise around, checking out each male and his property. One of the factors determining their choice of mate is the availability of suitable nest sites at the edges of clumps of cattails or bulrushes—the more edges he owns, the more females he gets.

Males of a wide variety of insects present females with nuptial gifts of food, and females select males on the basis of the size of the gift. The female gains energy for egg production, at least, and if the ability of males to find good gifts is hereditary, she also may get good genes for her offspring.

Sometimes females base their choices on the qualities of the male himself. It might be his song, or the vigor of his courtship dance, or his colors. For example, in the Lower Forty-eight, the size and brilliance of male plumage pattern is the basis for choice by female house finches. Male house finches have red feathers on the head and chest. Females prefer males with intense red coloration and large red chest patches. The red pigment is carotenoid-based, and carotenoids come from the bird’s diet, so the red depends on what the male has eaten (and his ability to convert components of food to red pigment). Thus, house finch females may be choosing males that are the best foragers or have the most efficient metabolism.

A small warbler called a yellowthroat nests in marshes and shrubby swamps. Males have black masks and yellow chests. Careful research has shown that yellowthroat females have marked preferences, but that these preferences differ from region to region: In Wisconsin, females like males with bigger black masks, but in New York, they like ‘em with bigger yellow chest patches. What the females get from making these choices is not clear.

Female preferences also affect the ability of males to obtain extra-pair copulations (many socially monogamous birds engage in very active mating activity outside the pair bond). Mountain bluebirds, for instance, vary in the intensity of blue plumage, and males with brighter blues are more successful in attracting extra-curricular females. Similarly, intensely colored male tree swallows and yellow warblers are preferred by females that copulate with males outside their pair bond.

The bowerbirds of New Guinea and northern Australia have gone a step farther, by transferring the signals to females from themselves to elaborate structures (bowers) that a built solely for the purpose of attracting and courting females. Different species construct bowers in differing shapes—avenues, towers, huts—and decorate them with colorful objects. Each species uses different kinds and colors of objects; some like blue, some like white or yellow, and so on. Within each species, female bowerbirds cruise around and visit the bowers of the males and judge each male on the construction and decoration of his bower. After she makes her choice and mating takes place, she goes off to build a nest and rear chicks by herself.

What’s the payoff to females for making these choices? In some species, there is a direct benefit in terms of resources such as nest sites or food. In other cases, males with brighter colors turn out to be better providers for the offspring. When males do no parental care, females might at least get good genes for their offspring. And in other cases, choosing a ‘sexy’ male might mean that the sons of the choosing female will also be sexy and successful in attracting females. The so-called sexy-son hypothesis obviously requires that the attractive characteristics are genetically based and inheritable.

Of course, male animals make choices too. Males of some species choose on the basis of color or pattern, just as females do. For instance, for whatever reason, male barn owls prefer to mate with females that have lots of lovely black spots on their white breasts. However, in many vertebrates, males seem to be somewhat less choosy than females.

When female creatures choose males, we do not suppose that the choices are conscious in the human sense. (Nor can we say that all human female choices are necessarily conscious!). All that is required is that there is variation among males and that females can discern the differences and act accordingly.

The result of mate choice (and of the competition to be chosen) is, in the big picture, an incredible diversity of color and form in the animal kingdom—diversity that is not directly related to making a living or simple survival but, instead, is related to mate selectivity and being attractive to the opposite gender.