Most birds build some sort of nest where the eggs are incubated. In many species, the female does that job, and in many others both females and males participate. But in a few species, it is the male the does most or all of the nest-building. Perhaps the most famous cases are the brush turkeys and mallee fowl of Australia. Males of both species build mounds of decaying vegetation or vegetation and sand, in which females deposit their eggs. Heat generated by the decaying vegetation incubates the eggs, while the females go off to do other things.
Closer to home, male nest-building activity is well-known in several species of wren. There are nearly 90 species of wren (family Troglodytidae), categorized into about 19 or 20 genera (depending on the taxonomist). They are spread out all over the Americas, with only one species occurring in Eurasia. Males are often involved in nest-building, in some cases working with a female to construct the nest in which she will incubate the eggs.
Pacific wren males build nests used as focal points for courtship displays. (Photo by Helen Unruh)
In about 15 wren species, multiple nests are built within a male’s territory, occasionally by both male and female, but often built mainly by the male (no data available for six of these species). The function of those “extra” nests (not used for eggs) has been a puzzle to ornithologists for a long time. They are sometimes used as roosts for males, while the females are incubating, or for fledglings, and so they are often called “dormitory nests.” Left unexplained is why those particular species might need protected roosting places while so many other species manage without them.
A commonly suggested function of the extra nests is that perhaps they help distract the attention of predators — they are decoys, decreasing the risks for the main nest. That might work for species nesting in relatively open habitats (marshes, etc.), but probably not so well for species nesting in thick understory or other concealing habitats — at least if the predators hunt visually. If they hunt by olfaction, the smell of the real nest is very likely to be different than the smell of other nests, even if those are used as roosts, so the idea of decoy seems less applicable then. In any case, there appear to be no data about the possible effectiveness of the supposed decoys.
Another idea about those extra nests originated back in the 1960s from detailed studies of marsh wrens. Male marsh wrens claim territories in cattail and bulrush marshes, and build multiple nests, weaving a sort of hollow ball attached to the vegetation. When a female visits the territory, the territory owner accompanies her as she inspects the empty nests. As he does so, he engages in visual and song displays near each such nest. If she likes what she sees (and hears), she may choose one of those nests, put in some finishing touches on the lining and the entryway, and use it for her eggs. Sometimes more than one female settles on a male’s territory, in a polygynous mating. The area around the nests is essentially a courting center, not much used for foraging.
Males of winter wrens and Pacific wrens (formerly put in one species) also make multiple nests at which the males display to potential mates. Sedge wrens are closely related to the marsh wren and these males also display near the nests they build.
While it is possible that the use of “extra” nests as part of courtship pertains only to those four species (and if so, then why?), it seems strange that the idea has not been explored or reported for other wrens that build multiple nests. Information about eventual reproductive success of the males is scarce and conflicting, but it would be essential to understanding how the habit may have evolved. There are lots of questions still to be answered…
Addendum: in a previous essay, I reported the unusual behavior of a female wood duck, who figured out how to get seeds to drop from a hanging seed feeder onto the pond surface, where they became lunch for ducks. A few days after those observations, I watched a female (the same one?) that had refined the technique. Now instead of striking a glancing blow, she perched on top of the feeder, flapped her wings vigorously, bouncing up and down. This was very effective in dumping seeds out and quickly required filling the feeder again. Smart duck!
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.
