I find much to like about house wrens, including their scientific name: Troglodytes aedon. It’s very appropriate for this vigorous singer and cavity nester. Troglodyte is from the Greek for “hole or cave dweller,” while aedon, also Greek in origin means “nightingale.” But, to be honest, it’s a challenge to love this contradictory animal.
Naturalist John James Audubon, clearly captivated by it, described the house wren as a “sweet little bird” that was “sprightly, active, vigilant, and courageous.” In contrast, essayist Margaret Renkl pronounced it “a tiny, feathered terrorist,” wreaking havoc on the nests, eggs, and nestlings of other song birds in her Tennessee yard. In fact, it’s both: perky in general and vicious in its relationships with other birds. (See the reference section at the end for links to cited sources.)
The illustration of the house wren that Audubon included in The Birds of America is quite revealing:
He showed the birds nesting in a hat. There’s no cavity too outlandish for the house wren when it comes time to build nests and, furthermore, the nest-building impulse leads the little bird to construct multiple nests during the breeding season, many of which go unused. The purpose of these non-breeding nests, often called “dummy nests,” has eluded researchers for a long time and, as I explore below, we’ve yet to fashion an accepted explanation for the behavior.
The issue of dummy nests is quite real for me. I have been monitoring a group of seven bird boxes built according to specifications appropriate for bluebirds, but used by a variety of song birds. My particular array of boxes lies in a wooded section of my community which means that bluebirds are unlikely to be residents: this immediate setting just isn’t their kind of territory. Instead, so far this season, the boxes have been occupied variously by Carolina chickadees, tufted titmice, and house wrens. The first two species have used boxes at one end of the wooded stretch I patrol while the last, the wrens, have been active mostly at the other end of this trail.
The pictures below show house wren eggs recently laid in one of the boxes and a parent perched on top of that box.
House wrens build a distinctive nest of a wide variety of materials, typically starting with a base of twigs. In addition to the one box where eggs have been laid and brooding is underway, I have found two other boxes with solely the telltale first layer of twigs of a wren nest: dummy nests, I’ve concluded. One is shown below - this is a view with the side of the bird box swung open. This nest has never been used.
And therein lies a key question: why? Why would male house wrens (it is apparently the males that do this) expend the energy to build nests that won’t be used for propagating the species?
My first introduction to the explanations advanced for this behavior was in the rather dated but still useful A Field Guide to the Birds’ Nests (1975). Of wren nest building in its breeding territory, it states: “Male arrives first, establishes territory, builds dummy nests of twigs in all or most of available nest sites. Female may or may not accept prechosen site; may or may not accept male’s incomplete twig nest.”
As I read some of the literature on the subject, I was quite pleased to come across a recent review of the research by Elise Isabella Macqueen and Graeme Douglas Ruxton. The authors consider all of the peer-reviewed literature on this avian behavior (for all species, not just wrens) and posit that it proffers four “non-exclusive mechanisms” to explain this phenomenon. They review the evidence for these explanations and discuss the logical “plausibility” of each.
First, though, it’s useful to briefly discuss three explanations that, according to Macqueen and Ruxton, prior authors had suggested for this behavior: (1) the birds were practicing their nest-building skills, (2) they were expending “excess sexual energy prior to pairing,” or (3) they were establishing territory (the one promoted by the field guide quoted above).
They summarily dispose of all three of these explanations, characterizing them as “now widely accepted to be implausible.” Of the first (getting in practice), the authors note that the dummy nests usually are poorly fashioned compared to the nests actually used and that the activity is not confined to young males. With regard to letting off steam, they said that males in many species continue the activity even after breeding. Finally, when it comes to marking territory, Macqueen and Ruxton assert that this would be a singularly inefficient way to do that compared to singing and displays.
According to the authors, the four explanations that are currently advanced to explain this nest building activity posit that the dummy nests:
(1) mislead predators or parasite species ("predation avoidance hypothesis")
(2) sexually signal that the builder is a high quality male ("sexual selection - signalling [sic] hypothesis")
(3) offer shelter ("fledgling shelter hypothesis"), and/or
(4) defend against other species taking over an occupied nest ("nest usurpation defence [sic] hypothesis").
The authors are often quite critical of the evidence advanced in support of any of these hypotheses, outlining how the methods used to test them might have been improved. The paucity of conclusive evidence for any one, despite a fairly substantial amount of research effort devoted to this cause, is striking, though perhaps to be expected given the observational and experimental challenges of working in the field with birds. Macqueen and Ruxton deconstruct the underlying logic of each of the current hypotheses, devoting more attention to the first two, while skating quickly over the last two.
Here then are brief summaries of their analysis of each hypothesis, with a focus on what they say about the inherent logic behind them.
Predation Avoidance The underlying assumption for this hypothesis is that predators (other birds or other animals) will give up a search in a particular area for an occupied nest if they encounter several empty nests. Macqueen and Ruxton suggest this hypothesis could be tested experimentally but find little evidence in the research of such testing.
That said, the authors write, "We are doubtful of its logical plausibility." It is, they assert, "an unlikely functional explanation for building multiple nests." They question the central assumption that a predator would abandon a search which, past experience might suggest to it, could yield a rich reward. Though for some ground dwelling predators the costs of continuing a heretofore fruitless search might be relatively high, for birds looking for prey they might not be. More telling, the authors assert that the search conducted by predators wouldn't be random among all local nests, but would be guided by key signals like smell and noises.
Sexual Selection/Signaling According to Macqueen and Ruxton, at its heart, this hypothesis assumes that the building of multiple nests signals to potential mates that here is an individual with superior genes who has the physiological resources to build several nests, or that only an individual with a highly desirable territory, offering much available nesting material, could do this.
The difficulties for testing this hypothesis are substantial. Close scrutiny of the behavior of individual birds is required. Interestingly, the authors find that all the studies advancing this hypothesis, based on the relationship between the breeding success of males and the number of dummy nests they built, involved species that were polygynous, that is the males having more than one mate. (House wrens can be polygynous.) Such animals would, as a matter of course, build more than one nest. Perhaps, they speculate, through natural selection this behavior has become "exaggerated."
The authors conclude that the signaling hypothesis is "logically plausible." But they suggest that alternative signals - singing or courtship feeding - could have potentially greater potential payoff requiring less investment. Singing is a signal that reaches a broader audience and area than does an array of dummy nests. More than multiple nests, courtship feeding is an immediate and direct indication of an attribute critical to successful breeding.
They write:
This suggests that building multiple nests would be an inefficient mode of signalling. However, if non-breeding nests do act as signals, this inefficiency could explain why it is seen in so few species.
I suggest that some other kind of signaling might be going on, a possibility looking back to the territorial explanation that the authors say has been dismissed. The wren might be making effective use of its thuggish reputation to warn other birds, perhaps other wrens included, that they better not encroach on its territory. I came across this idea in the discussion of dummy wren nests in Sialis, the nice website devoted to bluebirds that Bet Zimmerman Smith maintains. She "guesses" that non-wren species might recognize the dummy nest as the work of the aggressive house wren and decide it would be better to nest elsewhere.
Fledgling Shelter This third of the current hypotheses is given short shrift by the authors. Though non-breeding nests might at some point serve as shelter, they are built early in the breeding season, well before there are fledglings needing shelter. In fact, given how poorly many of them are made, they are likely to be in serious disarray by the time fledglings might want to make use of them. (This doesn't apply to bird boxes that might provide shelter regardless of the state of any nest inside.)
Nest Usurpation The authors spend little time with this hypothesis which is advanced by only a single study. This hypothesis contends that dummy nests might be part of defending against other bird species taking over a nest or might serve as quick replacements when a nest is overtaken. One logical limitation to this hypothesis is that backup nests are useful only if the usurpation occurs during the female's egg-laying period. Otherwise, the couple, whose nest is taken over, will have no immediate need for a backup nest. The female will need time to become biologically ready to conceive and lay eggs again, time that would allow for building a new nest.
Two takeaways from the Macqueen and Ruxton literature review: we don’t have conclusive evidence explaining why birds build dummy nest, and all current hypotheses, to one degree or another, have logical limitations.
The authors do characterize the hypotheses as “non-exclusive” which means that aspects of more than one might be at work. As a result, it may not be a matter of accepting one and rejecting the others, but understanding that the behavior is complex, not easily explained, and the result of selection working through multiple avenues. That would, I think, compound the difficulty of fashioning an appropriate research methodology to prove any explanation that is advanced.
Yes, this is complicated.
References
John James Audubon, The Birds of America: From Drawings Made in the United States and Their Territories, 1841.
Hal H. Harrison, A Field Guide to the Birds’ Nests: United States East of the Mississippi River, Petersen Field Guides, 1975.
Elise Isabella Macqueen and Graeme Douglas Ruxton, The Adaptive Function of Construction of Multiple Non-Breeding Nests in Birds, IBIS, Volume 165, 2023.
Margaret Renkl, The Questions Started With the Wren, The New York Times, July 7, 2025.
Bet Zimmerman Smith, Dummy and Abandoned Nests - Why Wasn't It Used?, Sialis.
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