Readers, please send in your good wildlife photos, as my tank is running low.
Today, after a hiatus, evolutionary ornithologist and ecologist Bruce Lyon has returned with one of his science-plus-photo posts. His captions are indented and you can enlarge Bruce’s photos by clicking on them. His topic is one that’s been on my mind this year: adoption and brood desertion in ducks.
Given the recent duck conflict and infanticide on Botany Pond by Jerry’s office, I thought readers would be interested in some information about adoption in ducks. In 1986 I did an experimental study on adoption in Barrow’s goldeneyes (Bucephala islandica) with John Eadie. Eadie, now a professor at UC Davis, is a leading expert on waterfowl ecology, behavior and management. He wrote the foundational synthesis paper on brood parasitism and adoption in waterfowl just as he was starting his PhD at the University of British Columbia, and he studied these ideas with field work on goldeneyes at a gorgeous study site in the Cariboo region of central British Columbia. We joined forces during his PhD work to do an experiment with his study population. I will start with some natural history photos of the ducks and then get to our study.
Below: John Eadie on recent trip to check his original goldeneye study population in BC.
Below: Typical Barrow’s goldeneye habitat: a mix of wooded and grassland areas. This was early spring (May) and the aspen trees have not yet leafed out and the marsh vegetation is brown with last year’s dead growth. Goldeneye nest in tree cavities, and the trees have to be somewhat near water. Nests up to 2 km from water have been reported. Within a day or two of hatching, the ducklings jump from the nest and follow their mother on an overland march to the wetland where they will then grow up.
Below: More goldeneye habitat, a small lake in the forest. It’s early spring and the aspens are just leafing out so their leaves are a lovely pea green color.
Below: A pair of Barrow’s goldeneyes on a small pond.
The same pair:
Below: The same male as above but now by himself—his female was likely off visiting her nest, perhaps to lay an egg. Males associate with the females prior to and during egg-laying but then abandon the wetlands once incubation begins. The females alone raise the kids. Note the male’s spectacular iridescent purple head complete with the diagnostic white crescent near the beak.
The Barrow’s goldeneye may soon be ‘extinct’, but in name only. The American Ornithological Society seems poised to change the names of all North American species with eponymous names (Jerry posted about that here). I have been trying to think of an appropriate new name for Barrow’s goldeneye and it is hard coming up with good names (fun naming contest—suggest your names in the comments). The females are similar in appearance to female common goldeneye (Bucephala clangula), so a name based on the female seems tough. A geographic name is also a challenge since Barrow’s goldeneyes overlap with common goldeneyes in Western North America, and Barrow’s goldeneyes also have a peculiar disjunct distribution—Northwestern North America, Eastern Canada and Iceland (the bird was first known to modern science from Iceland, hence the latin species epithet islandica). Perhaps something like purple-headed goldeneye will do? Or maybe history will prevail and it will be the Icelandic goldeneye.
Below. A very clean and dapper looking male. I love the two tone feet. Barrow’s goldeneyes are one of my favorite birds; they are charismatic and live in hauntingly beautiful habitat. In flight, both goldeneye species make a characteristic loud whistling noise with their wings, giving rise to their nickname ‘whistlers’. It is an evocative sound that I will always associate with the wild north woods.
Below: A goldeneye clutch of eggs inside a nest box. The eggs are greener than those of most other ducks.
Below: A female goldeneye with her brood.
In goldeneyes, there are two ways that females end up raising the ducklings of other females—brood parasitism, where a female lays eggs in the nest of another female, and adoption after hatching. Both are common, and an important contribution from Eadie’s synthesis paper was the idea that adoption might in some cases be a form of post-hatching brood parasitism. The paper also highlighted the importance of considering the costs and benefits to both the female adopting the ducklings (the recipient) as well as the donor, who is getting her ducklings adopted by the recipient female. It is not always clear whether the donor female or the recipient female is causing the adoption to happen so it is important to consider the consequences to both females and their kids.
At least a couple of different explanations might account for adoption in waterfowl. First, adoption increases the number of ducklings in a brood and there may be safety in numbers—if predation occurs, the risk per individual duckling goes down. In this case the adopting female should be happy to adopt ducklings and increase her brood size, and the donors would also gain this benefit. This cooperative view of adoption dominated the field for a long time, reflected in the fact that amalgamated broods in waterfowl are often called crèches, from the French word for a nursery where babies are cooperatively cared for. Second, adoption might reflect a form of post-hatching brood parasitism where the donating female benefits by fobbing her chicks off on someone else. Her kids get the benefit of having a parent around but the donor is off the hook for caring for kids and invests less time and effort in breeding that she would otherwise. The cooperation and parasitism hypotheses are not mutually exclusive because both donor and recipient might benefit from adoption. Finally, some have suggested that adoption is not adaptive but a mistake—female goldeneyes are territorial and often fight with each other, and broods get mixed up during fights.
Below: An unusual Canada goose (Branta canadensis) nest on the roof of an old cabin at our study area. One study of Canada geese claimed that parents actually kidnap young from other parents—apparently having additional goslings in the brood is so beneficial that the adults steal kids from each other.
Below: Goldeneye ducks are great for studying adoption for a couple of reasons. Large lakes can have more than one female with ducklings and adoption is common. The ducklings have white cheek patches that are ideal for marking with colored permanent markers to keep track of which broods the ducklings come from. We have lots of colors to use and we only color one cheek (left blue chick is a different brood than right blue cheek). This provides enough combinations so that each adult female on a lake gets her own unique brood color for her ducklings. Then, when a duckling is adopted its cheek color gives it away as an adoptee (it differs in marking from the foster mom’s kids) but we can also know which donor brood (and mom) it came from.
Below: John Eadie giving a duckling its unique brood cheek color (blue left in this case). The colors last for a few weeks and are easily seen with a telescope during brood surveys.
Below. An example of a brood with adopted ducklings. The two photos show the right and left cheeks, respectively. The hen’s own kids are yellow right and she adopted kids with yellow left and green left. We kept track of the adult females with plastic ‘nasal saddles’ attached to the beak; each hen gets a unique color combination. Colored leg bands would be useless for identifying swimming females.
John and I did a brood size manipulation experiment to study both the recipient and donor perspectives of adoption. In other species of waterfowl, researchers noted that adoption often occurs during a period of intensive predation on ducklings, leading to the conclusion that adoption likely functions to reduce the risk of predation per individual duckling, as described above. However, John and I realized that there could be another explanation for the link between predation and adoption. Intensive predation can quickly reduce a female’s brood size well below the number of ducklings she leaves the nest with, changing the benefit she can expect to gain from caring for the brood relative to the costs of staying and caring for them. If the expected costs exceed the benefits, the adaptive response would be for the mother to desert the brood and save her investment for future reproduction. Studies with several birds, and some mammals like bears, provide evidence that adaptive desertion of offspring sometimes occurs.
John and I applied these ideas to adoption and proposed that adoption might be result of adaptive brood desertion. We dubbed this donor-driven explanation for adoption as the ‘ditched duckling hypothesis’ and we predicted that these deserted broods would besmaller broods. Desertion could then lead to adoption in two ways. First, females might desert their brood and the ducklings then find a foster mom on their own. Alternatively, females could invade the territory of another hen, which would invariably result in a fight between the two hens, and during the fight her kids might mix in with the other kids. The sneaky donor is then ‘chased off’ by the territory owner, but her kids now have a foster mom. We tested the adaptive desertion part of hypothesis by experimentally reducing brood sizes at hatch—we took ducklings at hatch from some nests and gave them to other females, either by adding them to another nest box at the same stage or by getting females with young broods to adopt the kids. We left many broods unmanipulated to serve as ‘control’ broods.
Below: The brood size manipulations strongly affected whether a female ditched her ducklings: smaller broods were far more likely to be abandoned than control broods. There also seemed to be a clear threshold size: 100% of the broods smaller than four ducklings were deserted while only 20% of the broods with four or more ducklings were deserted. This graph from our paper shows this key result and compares the sizes of broods where females stayed and where they left. Importantly, we also found a link between desertion and adoption: a few of these deserted broods ended up being adopted. Whether ditched ducklings got adopted depended in part on how many potential foster moms were available on a given lake.
Below: A female with tiny brood of two ducklings. This bird was not part of our experiment but tiny broods like this in our experimental study were invariably deserted by their moms.
Data from John Eadie’s first couple of years of thesis work suggested that adoption might benefit the adopting females—his observational data showed that an individual duckling’s chances of survival to the end of the season was higher in larger broods, as predicted by the safety in numbers idea. However, observational (non-experimental) studies can be misleading because some other hidden factor might actually be causing the pattern. For example, ducklings might survive better in larger broods because they have better mothers: in birds generally, older more experienced mothers often have bigger broods and are better able to care for their kids. Experimentally changing brood size is essential to scramble any correlation between brood size and mother quality, and thus directly assess the effect of brood size itself. When we did this, we found no relation between experimental brood size and duckling survival. In other words, we detected neither a cost nor a benefit to the adopting hen in terms of the survival of the kids.
An additional experimental suggests that there may sometimes be costs to the mothers that adopt ducklings. We did some duckling addition experiments to see if moms would happily adopt kids—they were similar to Jerry’s attempts to get the stray mallard ducklings he encountered adopted by one of the females on Botany Pond. Putting a duckling in a lake near a female with a brood causes the released duckling to begin calling which gets the attention of the mother with the ducklings. She then swims over to check out the duckling. Having the ducklings be the same age as the adopting moms kids turned out to be key. When we released newly hatched ducklings near female who also had newly hatched ducklings they were invariably adopted without fuss—she treated them as she would her own. However, when we tried to introduce smaller ducklings into broods of older ducklings the moms would invariably attack the ducklings and try to kill them (we were able to rescue virtually all these experimental ducklings and get them safely into broods with ducklings of the same age). The discovery that females sometimes aggressively reject foster ducklings suggests that there may be costs to adoption in some contexts but we do not yet understand what these costs are.
Below. A female aggressively chases an introduced duckling away from her brood and the duckling wisely flees. Before our study John assumed that female goldeneyes would readily adopt ducklings so this aggressive behavior came as a real shock. We named the spit of land where we observed this particular chase Epiphany Point—this single observation of female aggression changed the way John thought about adoption in these ducks.
Below. Not wanting to end on a depressing note (parents beating up kids), let’s end with a bluebird of happiness—specifically, a mountain bluebird (Sialia currucoides). These bluebirds are very common in study area. This is a male bluebird at his nest in an aspen tree. Aspens often have lots of holes, which serve as nest sites for bluebirds, goldeneyes and lots of other cavity-nesting species as well. Given the abundance of aspen trees, cavity-nesting birds are particularly abundant at our study area.