On April 17, 2016, a one-year-old female wood duck was taking her first steps toward motherhood by exploring nest sites.
The tawny bird with white eye rings and iridescent blue wing patches was investigating several artificial nest boxes that our team had erected along a wooded stream near Davis, California. There was nothing unusual here, except that many of these nest boxes were already occupied by other females.
That year this female made 195 visits to 34 different nest boxes that were already in use, including 30 visits to her favorite box. It turns out that this duck, whom we call by her tag code E9BA0 (or E9 for short), wasn’t just exploring her neighborhood. Our genetic studies show she was laying her eggs in other wood duck hens’ nests. More specifically, she laid a total of 12 eggs in four nest boxes, all in a row and just a few boxes down from where she had hatched the year before. E9 never incubated any of those eggs, relying instead on the nesting females in those boxes to do so.
E9’s behavior wasn’t entirely surprising—wood ducks (Aix sponsa) are known to be brood parasites, meaning they lay eggs in other birds’ nests (conspecific brood parasites such as the wood duck do this to birds of their same species). The word parasite doesn’t normally invoke an image of a bird laying an egg in a nest. But sneaking an egg into another nest is not so different from a worm in your gut: It potentially takes away resources from the mother who ends up caring for the egg (that’s one hypothesis, anyway).
When we began this work in 2014, we thought that most wood duck hens would incubate their own eggs and set up home in one site, whereas a few others might travel around a bit, perhaps parasitically laying some eggs. We completely underestimated the wily wood duck. According to our new data, some females visit an incredible number of nest sites, repeatedly visit many nests over an entire breeding season, and frequently lay eggs in other females’ nests.
Such was the case with E9, and her sneakiness paid off—host mothers hatched 10 of her 12 eggs. When E9 returned the following year, we wondered whether she might settle down, but she did not. Instead, she visited even more boxes (42), never incubated a nest, and parasitically laid 10 eggs, of which 8 were hatched by the foster mothers. Quite clearly, E9 was not destined to be a stay-at-home mom.
E9 may seem extraordinary, but it turns out that other females behave in similar ways—roaming widely and visiting many nest sites. Wood ducks typically prospect for a nest site at the beginning of each breeding season, so it is not unexpected that females explore potential nest sites early in the year. But we didn't expect the remarkable wide-ranging behavior and nest visit patterns that we observed. Box visitations are sometimes exploratory and don’t always result in an egg laid. In addition to monitoring the birds’ movements over the past eight years, we used genetics techniques to determine ducklings’ parentage. These analyses revealed which moms are true nest parasites on their neighbors.
As birds go, duck parents have it fairly easy. As soon as the offspring hatch, they are pretty much ready to go and can feed themselves. Female ducks provide a modicum of protection, warmth, and guidance—and that’s about it. This relatively light workload makes it possible for female waterfowl to raise a lot of babies, and they do. What has long been a mystery is why a female would risk the investment in her eggs to the care of another female.
As their name aptly implies, wood ducks such as E9 are cavity nesters that require a tree hole in which to make a nest. The woodie, also known as the Carolina duck, is unquestionably one of the most colorful birds unique to North America and Cuba. Sporting a green- and purple-hued crest flung over the back of his head like a rockabilly pompadour, the male also has bright red and white markings on his bill, a white chin patch, and a rusty speckled breast separated from his golden flanks by a scarf of black-and-white feathers. The female, more muted in color but no less impressive, has white teardrop spectacles ringing her eyes, blue iridescent feathers on her wings, and delicate tawny and white checkering on her breast. During spring in the hardwood forests throughout their range, one might be lucky enough to happen upon a flutter of wood ducks, males and females, whistling and whirling frantically among the trees, echoing their plaintive “oo-eek oo-eek” refrain.
Woodies’ peculiar habit of nesting in a tree was part of the reason for their decline in the 1800s (they were also extensively hunted for their dazzling feathers and delectable meat). Timber harvesting and agricultural expansion resulted in the loss of mature trees, and the availability of natural nest sites severely declined. Even in a healthy mature forest, cavities large enough to host a 600-gram female and an almost equal mass of eggs are likely at a premium. One challenge that hens may avoid through brood parasitism is the inability to find an available nesting spot. Faced with the task of finding a vacant nest site, wood ducks may circumvent this limitation by using someone else’s.
Wood ducks’ notoriety for laying eggs in one another's nests made them an ideal study animal for exploring questions about this behavior and its evolution. But we couldn’t get details such as those we collected about E9 until recently. Questions that long seemed unanswerable about where the birds go and which eggs end up where can now be explored. After more than 30 years studying brood parasitism in ducks, we’ve finally been able to get some answers, thanks to advances in radio-tracking tags, videography, and genetics. We’ve found a complex social network of wood ducks that we are still unraveling, one that defies single explanations for nest parasitism but instead shows that this behavior offers females flexibility to adjust their reproductive investments in the face of changing conditions.
Why Ducks Parasitize Other Nests
Laying eggs in another female’s nest once seemed so unusual and inexplicable that early authors dismissed it as an aberration and referred to it disparagingly as “egg dumping,” as though females were simply ridding themselves of excess or unwanted eggs. But an influential paper in 1980 by Yoram Yom-Tov of Tel Aviv University changed our thinking by considering the behavior from an evolutionary perspective. Yom-Tov and subsequent authors proposed several hypotheses to explain why females might lay eggs in the nest of a neighbor and how such behavior might benefit the practicing parasite.
Many females do establish a nest of their own, lay their eggs in that nest, and care for their offspring—this behavior is what we might think of as a typical nesting strategy for a bird (illustrated as Option A at right). But other females are like E9 and lay some or all of their eggs as brood parasites. We wanted to know why this latter behavior might be advantageous enough to evolve in some ducks.
One possibility is that females are constrained from nesting on their own (Option B). Perhaps these females cannot compete successfully for a nest, or nest sites are in short supply, as is likely the case for a cavity-nesting wood duck. Alternatively, it might not “pay” (in reproductive terms) to nest on one’s own, because of physiological stress, lack of experience, or poor body condition. Nonetheless, some reproduction is possible by laying a few eggs parasitically, without having to go all in.
Another possibility (Option C) is that females get the best of both worlds. If there is a limit to how many eggs a female can tend in her own nest, laying additional eggs in another hen's nest can increase total reproductive output without having to provide the care for those extra eggs and offspring. (The “gambler” variant of this hypothesis is that females put their eggs into many baskets to hedge their bets against losing all their eggs in a single nest to a predator—a catchy idea that does not hold up under formal mathematical analysis.)
Finally, there is the possibility that some females are pure parasites—they never raise their own young and only lay eggs in the nests of other females (Option D). In doing so, these “professional” parasites are freed from any parental care duties and instead invest the time and energy saved into making extra eggs or living longer.
There is one other twist to this story. What seems to be parasitic behavior might actually be a form of cooperation. Cooperative behaviors that are costly on an individual level can evolve if they increase survival and reproduction of relatives that might not have reproduced otherwise (See “Why Some Animals Forgo Reproduction in Complex Societies,” July–August 2014.) In many duck species, daughters return to their natal (birth) area to breed, where they can closely interact with their relatives (in most other species of birds, the male returns to his natal area). Malte Andersson of the University of Gothenburg in Sweden offered this novel idea to explain why conspecific brood parasitism is disproportionately common in waterfowl: Perhaps it is an interaction among kin, with female family members sharing the costs of parental care and enhancing their own evolutionary success through the shared genes they help propagate. This perspective turned the concept of parasitism on its head. Rather than a competitive dynamic, perhaps brood parasitism among females within the same species is instead a form of cooperation between relatives—less a case of parasitism and more one of shared duckling daycare. But if ducks don’t just parasitize the nests of their kin, there may be other reasons at play: some cost to nesting that can be avoided by a female that adopts brood parasitism, either partially or fully.
The challenge we faced was how to test these hypotheses. To study this behavior, we first needed to know which females are the parasites. To do that, we needed to identify the nests females were visiting, and determine how many of those visits resulted in an egg laid.
Studying Wood Ducks
A typical day in the field begins early, with the morning sun reflecting off the water of a slowly moving stream or slough where our nearby nest boxes are attached to the trunks of large old oak trees. Carting a 10-foot extension ladder on our shoulders, we tromp dutifully from nest box to nest box, pushing through patches of star thistle or swishing through tall grasses abounding with ticks. The springtime sun filtering through the leaves of the valley oaks lining the creek and the symphony of bird song accompanying us as we travel more than compensate for the thistles and ticks. Upon reaching a nest box, we carefully plug the entrance hole with a custom cover, securely lean the ladder up the tree, and climb quietly to the box to discover what lies within.
We wanted to observe these birds in the wild, with a large number of birds for our study. Natural cavities are scarce, and should one be lucky enough to find one, accessing the cavity may come at risk of life and limb. Cavities are often located in old snags, rotted in the middle, and with an unsettling tendency to topple, bringing both tree and biologist to the ground with a resounding crash.
Studying birds in nest boxes avoids these problems. Each nest box is about 0.6 meters tall and about 25 centimeters wide and deep, with an entrance hole measuring 7.5 x 10 centimeters cut in the front. Boxes can be easily nailed to a tree or mounted on a post or pole, making nest checks easy and reliable. Moreover, given the scarcity of natural cavities, nest boxes have become an important management tool to enhance the availability of nest sites for wood ducks, and so our work would also contribute to these conservation efforts.
Checking every box is like an Easter egg hunt, and there is always a small rush of anticipation. If eggs are inside, we carefully number and measure them. Wood duck eggs are about the size of a chicken egg, although more oval in shape and thicker shelled, with a buffy off-white or beige color. In a nest, they are tucked tightly together, covered in soft down that the hen plucks from her breast to keep them warm once incubation begins.
If instead a female peers up at us with her fashionable white eye spectacles, a whole new game is afoot. Now we move into banding mode. We gently remove the hen, and quickly place a uniquely numbered aluminum federal bird band on one leg. We then measure her weight, wing length, and bill length and we collect a few drops of blood from her leg (using a small needle) on a piece of filter paper for genetic analyses. Then, back in the nest she goes, where we let her readjust for a few minutes, and off we go.
Our field research has taught us much about wood duck breeding biology, but we had to overcome several impediments to delve deeper into the mystery of parasitic egg laying. We generally have no idea who the moms of the eggs are. The eggs, being from the same species, look similar and cannot be reliably matched to their moms. Likewise, we have no way of telling whether the same female laid eggs in one box or many.
That’s where technological innovations opened the door, revealing an underworld we could not have imagined. Our first major advance was the use of radio frequency identification device (RFID) technology, which is the same technology as the chips injected into pets to allow an owner to find and identify a wayward cat or dog that has lost its collar. The trick is not just attaching the chips to the birds but having enough chip readers deployed to capture these electronic signals as birds move about in their natural environment. The technology is not new or revolutionary, but we had hundreds of nest boxes to monitor and a limited amount of funding. The innovation that permitted such a large-scale tracking study was all about making things cheap.
We devised a low-cost RFID reader that we could make for about $30. At that price we could scale up to track entire populations, and since 2014 we have implanted the chips (formally called passive integrated transponders or PIT tags) in all females and all ducklings in several populations of wood ducks nesting near Davis, California. Each tag is the size of a rice grain and is easily injected just under the skin in the back of a bird using a specialized syringe. Birds appear almost oblivious to this quick procedure. A reader fastened to the nest box energizes the tags, which then send a burst of data that includes a unique identification number for the animal. With this setup we can identify and log every PIT-tagged bird that enters a nest. Thanks to the efforts of doctoral candidate Tez Stair and master's degree student Ami Olson, we installed readers on more than 200 nest boxes and implanted tags in more than 500 females and 5,000 ducklings. This massive undertaking has now yielded more than 2 million RFID reads.
With this approach, we were able to discover that females, including E9, explore an incredible number of nest sites and repeatedly visit many nests over an entire breeding season (early spring to midsummer).
Multiple Flavors of Parasitism
The RFID network fundamentally altered our understanding of how wood duck females evaluate and use the most essential resource for reproduction—a nest site—but it is only part of the story. As illustrated by E9BA0's movements, at least some of the roving females not only visit multiple nest sites but also lay eggs in those nests as well.
Not all females exhibit such wanderlust, however. Some females appear to be homebodies and restrict their attention to one or a very few sites (for example, see female 7F64B in the first part of the video below, as contrasted with the movements of E9BA0 starting at 0:11 and in the video above).
As we sat in windowless labs compiling data from several hundred nest boxes equipped with RFID readers, it soon became apparent that there was a range of female types, from “rovers” who visit far more boxes than expected by chance, to “stayers” who visit far fewer. This variation can be seen clearly when we look at an entire population of females at a single site. In our study at Roosevelt Ranch near Zamora, California, in 2020 (see graph below), many females visited 15 or more sites in a season (some as many as 40), while others were recorded on only 2 to 4 boxes. But how does this nest-visiting behavior relate to parasitic egg laying? Are the rovers the parasites, while the stay-at-home moms are the incubators?
To fully answer those questions, we needed another tool to determine whether any eggs in the nest belong to the visiting interlopers. Here again advances in technology—in this case DNA genotyping—have revolutionized how we tackle such questions.
We can now extract DNA from a small drop of blood and determine the individually unique genetic signature (genotype) of every individual. Moreover, with a sample from both mom and the offspring, we can also determine with high certainty whether the baby is actually hers. (The technique is based on highly variable regions of DNA of every animal that serve as a genetic fingerprint. Offspring get one copy of their genes from mom—with all her unique variations—and one from dad, permitting analysis of parentage and kinship.) We have taken blood samples from every female we banded and almost every duckling that hatched, and thanks to doctoral students Cara Thow and Melissa Jones and postdoctoral fellow Caitlin Wells, the next layer of the wood duck underworld is now being uncovered.
Our genetic studies revealed that there are multiple kinds of egg laying among females in our populations. In any given year, some females breed only as “traditional” parental females that lay their own eggs in a nest and incubate the clutch, as is typical of most bird species. But parasitic egg laying is rampant, and up to 80 percent of nests contain at least one parasitic offspring and more than a third of the eggs laid are parasitic. Clearly, maternal skullduggery is a well-established practice among the wood ducks of California.
What was more surprising is that there is more than one type of parasitism: Some females lay eggs only as a brood parasite in a given year, whereas other females establish a nest of their own and incubate the clutch, but then also lay some parasitic eggs in the nests of other females during the same breeding season—a little on the side, so to speak (Options B and C). Accordingly, in any given year, a female might pursue one of three different tactics: parasitize only, nest only, or combine nesting with parasitism. These pathways are about equally common, although the frequencies vary among years and among study sites. Our work, with the application of the new RFID and DNA technology, is finally allowing us to gain a more complete understanding of the context in which parasitism occurs.
We have found that females switch tactics from year to year, and it appears that there are few if any lifelong “pure parasites” (Option D). Younger females are more likely to adopt the parasite-only tactic, perhaps as a less risky entry into the world of adulthood and reproduction. We have also discovered that the females that adopted the nest-and-parasitize tactic laid more eggs and hatched more ducklings in a given year than females that only nested, and those females, in turn, did better than birds that only laid eggs as parasites.
Our new findings suggest that no single hypothesis alone explains parasitic behavior. Rather, the emerging picture is that parasitism in wood ducks, and possibly other species, is part of a flexible life history that allows females to adjust to variable and unpredictable social and ecological conditions. When conditions are poor or females are young or inexperienced, the parasite-only tactic might be a low-cost, low-benefit option that provides at least some reproductive success. When conditions are excellent or females are of exceptionally high quality, sneaking a few additional eggs into a neighbor’s nest (nest-and-parasitize) may yield extra fitness dividends without the cost of caring for those extra young. This integrated framework not only embraces the multiple explanations proposed by Yom-Tov and others described earlier (meaning that the hypotheses are complementary rather than competing), but it also provides an explanation for the rich variation in the frequency and context in which conspecific brood parasitism occurs in other animals, including birds, fish, and insects. (A more detailed description of this conceptual framework is presented in the animated figure below.)
We also now can begin to address the question of whether female kin help each other with daycare. Our results so far suggest that there are strong patterns of female philopatry (returning to their natal site) and kinship networks among females, supporting the possibility of kin-affiliated interactions in wood ducks. At a population level, however, parasites are not more closely related to hosts than expected by chance, so something other than or in addition to simple kin-based sharing of duckling daycare is at work. We do find some pairs of hosts and parasites that are closely related, so perhaps the interactions are more nuanced. With graduate student Katharine Cook, we are now examining whether the interactions among females in these kin-related pairings differ from pairs of unrelated birds.
To do this, we mounted motion-activated trail cameras with night-vision capability, and the results were astounding. We observed an astonishing array of aggressive and tolerant behaviors when females encounter each other on a nest (see video above). Some females fight as though for their lives, pecking and pulling violently at each other’s feathers and squealing stridently. Others nestle down quietly, side by side with nary a peep of protest. One video (above, starting at 1:08) shows a female laying an egg literally on the back of a particularly passive partner. We don’t yet know why these interactions are so variable—are the affable interactions among kin or possibly just among duck friends? Are strangers more likely to engage in fights? Are there dominant hens who rule the roost? These new observations are causing us to look much more closely at the wood duck social network.
The Social “Nestwork”
The nest visitation patterns revealed by the RFID data suggested yet a deeper dimension of the social underworld of the wood duck. We were surprised to find that some nest boxes were never used, while a nearby nest box within a few hundred meters can attract as many as 15 or more different females (see figure below). The record in 2017 was 47 hens visiting a single box.
It’s unclear why there is such variation in nest box attractiveness. We examined a myriad of physical features that might influence a nest box’s appeal (nest height, orientation, microclimate, distance to water, and so on); nest visibility influenced whether a newly erected box was used initially, but nothing emerged as a reliable indicator as to whether a site would be used again or how frequently. Only a single factor predicted whether a box would be used in the future: whether it had been used in the past.
The observation that large numbers of females visit the same box, often at the same time, suggested that woodie females were incorporating social information into their nest site decisions, perhaps following strong cultural conventions—much like favorite pubs in a college town where the beer price is the same, the music is just as loud, but a crowd throngs to one, while the pub next door remains empty. In essence, it's a nest box popularity contest.
It’s unclear what sets these nest box fads in motion. We hypothesize that it is the social connections that females establish, possibly as ducklings, that may persist through life. Perhaps, just like a high school clique, these social ties set off a follow-the-leader chain reaction: If one hen lays eggs in a box, others follow and do the same. Such social cueing might also explain the phenomenon of “dump-nesting,” where large numbers of eggs accumulate rapidly in a nest box, often without being incubated (hence the term). This behavior would seem to be seriously maladaptive, but it may be that following the leader would be highly effective in conditions where nest sites were dispersed through the woods and harder to find. In heavily managed areas where habitat is limited and high densities of visible nest boxes are erected, such a behavior may be counterproductive if too many females are attracted to the same nest box. Understanding the dynamics of these behaviors will be important for nest box conservation efforts.
We are now using social network analysis tools developed by human demographers for platforms such as Facebook and Google with our RFID data to explore these ideas. With these methods, we can determine whether kin groups of females are more likely to associate, nest, and lay eggs together, or whether groups persist over multiple years in a social neighborhood.
We are just scratching the surface of this new layer of the underworld, but initial analyses by doctoral student Tez Stair indicate that some females are indeed integrated tightly into close social groups, at least in terms of patterns of shared nest use, or “social nestworks.” Combining the power of the RFID data with population-wide genotyping and our long-term data on individual females will enable us to explore these social networks in much greater detail.
Driven by a resurgence of interest in Charles Darwin’s ideas of sexual selection and the importance of male–male competition, males have occupied center stage among studies of reproductive behavior for the past half century or more. Conspecific brood parasitism reminds us that females have just as much at stake with regard to successful reproduction and that social competition over reproduction occurs in females as well. Moreover, we know that wood ducks are not the only birds that practice conspecific brood parasitism. In addition to several other duck species, this reproductive strategy occurs in geese, coots, moorhens, swallows, starlings, and many more. We are only beginning to understand the complexity and importance of conspecific brood parasitism and other alternative reproductive tactics practiced by females within the context of social competition.
Brood parasitism, within and among species, is now recognized as a classic example of how females pursue multiple avenues toward passing on their genes. It is one of many female strategies that diverge from the widespread avian pattern of monogamy—in some species parental care has been lost altogether, while in others, females collaborate to lay in a shared nest and cooperate to raise young together. Clearly, there is no single best way to be a mom, and each new discovery, like the nest hopping behavior of wood duck female E9, opens new avenues of investigation into fundamental questions relating to the evolution of clutch size, parental care, life history variation, and behavioral plasticity.
It will be interesting now, with our wealth of data at hand, to discover what wood duck females do over the course of their entire life span. Ultimately, fitness is determined not only by the number of eggs laid or ducklings hatched in any given year, but also by survival and the number of chances to play life’s lottery. By following females that have been PIT tagged and genotyped as ducklings throughout their lives (typically 2–4 years but possibly up to 10), we will learn how these alternative tactics are packaged through life, how female interactions and partnerships vary over time, and what the reproductive payoffs might be.
Fascinating studies in other species are finding that mothers may have extraordinary effects on the phenotype and development of their offspring through their investment in nutrients, hormones, and other constituents into eggs, and even through the ways they incubate the eggs. Social interactions with siblings or brood mates can also have a profound influence, something researchers refer to as social niche construction. With doctoral student Mitch Hinton, we are now turning our attention to the critical first weeks in a young wood duck’s life to better understand how such early maternal and social influences might shape a duckling’s development and set a youngster on a particular pathway for life—rover or stayer, parasite or nester.
Until the advent of new technologies such as high-throughput genotyping and low-cost remote-sensing tracking, we had no idea just how rich and complex these duck societies might be. In addition to what we learn about conspecific brood parasitism, these analyses may tell us something even more fundamental about the deeper social structure and connectivity of females in a wild bird population; we may be surprised to find that these duck societies are not so unlike our own.
AUTHORS
John M. Eadie, Bruce E. Lyon, Eli S. Bridge
PHOTOGRAPHY
Bruce E. Lyon
PRODUCERS / DIGITAL CREATORS
Katie L. Burke, Digital Features Editor
Robert Frederick, Digital Managing Editor
VISUALS
Art Director and Illustrations: Barbara Aulicino
First Graph: Data courtesy of the authors, visualization by Katie L. Burke and Barbara Aulicino
Second Graph: Data courtesy of the authors, visualization by Robert Frederick
Videos of Mapped Duck Movements: John M. Eadie, Bruce E. Lyon, and Eli S. Bridge
edited by Robert Frederick
VIDEOS
Opening Video: Mark O'Connell/Pond5
Second Video: Bruce E. Lyon
Synthesis Figure Animation: Robert Frederick, adapted from Lyon and Eadie, 2008.
https://doi.org/10.1146/annurev.ecolsys.39.110707.173354 © 2008 by Annual Reviews. All rights reserved.
Video of Hen Interactions: Katharine Cook, edited by Robert Frederick
Final Video: John M. Eadie
OTHER CREDITS
Coding: Scott Thomas, Systems Solutions Development · Robert Frederick, Digital Managing Editor
Bird Call Audio: Michael Andersen/Macaulay Library/AllAboutBirds.org