I enrolled in my university’s Field Bird Biology Lab this semester, a lab I’ve been looking forward to since my freshmen year. We had our first field trip this Sunday to Brazos Bend State Park. Even though it was raining on and off the entire time, we saw a lot of cool birds, such as the anhinga and the vermilion flycatcher! But the bird I found the most interesting was definitely the black-bellied whistling duck.
My first encounter with the black-bellied whistling duck, or Dendrocygna autumnalis, was in the form of one of a high-pitched chattering whistle. You can listen to it here on the All About Birds website: http://www.allaboutbirds.org/guide/Black-bellied_Whistling-Duck/sounds. (It’s a good thing Douglas Adams, a British comic writer, was not a taxonomist, since the black-bellied whistling duck certainly does not “quack like a duck”!) Looking around to identify the source, I saw groups of about four to six of the ducks flying in a v-formation overhead. I couldn’t believe at first that a duck was making the odd whistling sound, but I suppose they fit their namesake! I saw them again at the edge of one of the ponds at Brazos Bend. There were two parent ducks and their four juvenile offspring. It was easy to tell the difference between the juveniles and the adults because they were smaller and lacked the red bill and feet of their parents. Unusually for ducks, there was no sexual dimorphism—that’s a fancy word that means the males and females look different. The family called noisily as they flew off together, the big white patches on their wings flashing brightly against the stormy sky. They all returned a short while later, still calling loudly. I thought it was very cute that the family stayed together, especially since the juveniles seemed almost to adulthood.
Just a quick heads-up: I’ll be referring to the black-bellied whistling duck by its banding code BBWD, since my fingers were starting to get tired! For more information on banding (or alpha) code, you can go here: http://www.birdpop.org/alphacodes.htm. It makes it a lot easier to jot down the birds you see while you’re in the field.
After my class trip, I decided to look up more information about the BBWD, and learned that it’s different from most ducks in other ways besides its odd call. I found a bunch of papers on Web of Science (http://apps.isiknowledge.com/WOS_GeneralSearch_input.do?product=WOS&search_mode=GeneralSearch&SID=1DC@GdGFO2Al5LLoN6@&preferencesSaved=), so hopefully you’re part of a university network so you can read them too! There’s links to all the papers I read at the bottom of my post.
Black-bellied whistling ducks are part of the “tree duck” group, and they were originally called “black-bellied tree ducks” since they usually nest in trees. There are eight types of tree ducks, two of which are found in the U.S.—the BBWD and the fulvous whistling duck, so already you can tell that the BBWD is special.
One way that the BBWD is different is that most ducks exhibit extra pair copulation, while the BBWD is much more faithful. Eric G. Bolen, who extensively researched the BBWD, discovered in his 1971 study that they tend to have lifelong pair bonds—in other words, they mate for life. Bolen banded mates to track whether or not they stayed together, although there was some difficulty since some of the birds died between mating seasons. Bolen found that seven pairs stayed together from one mating season to the next, and one pair stayed together for at least three years! Even more amazing, pairs stayed together even if their first clutch of eggs was destroyed, and “renested,” or laid a new clutch. Bolen also observed that there was approximately a one to one ratio of adult males to females, further indicating that BBWD mate for life. He noticed too that the adult ducks tend to return to their original nesting spot; out of the ducks that returned, 42 percent were male and 58 percent were female, which also points to lifelong mating. Perhaps that’s part of the reason why men who were had unfaithful wives were called cuckolds (derived from cuckoo), not ducks!
Richard McCamant and Eric Bolen did a follow-up study in 1977 to see what would happen if one of the mates died. For the BBWD, the male and the female take turn incubating the eggs. One duck hangs out in the pond while the other sits on the nest, until the one incubating flies to the pond, signaling that it’s time for the one lazing about to do nest duty. McCamant and Bolen simulated a death by capturing the incubating duck and holding it captive for a few days. In all cases, the nest was abandoned since the duck in the pond never received a cue from its partner. In a way, it’s a little romantic—the remaining partner is lost without its mate!
The black-bellied whistling duck is the only whistling duck that nests in natural cavities. Bolen, with the help of McCamant, did a 12-year study on whether the BBWD could nest in nest boxes. They collected data on nest success—whether an egg hatched—for natural cavities, unprotected nest boxes, and protected nest boxes. The protected nest boxes had metal barriers to prevent terrestrial predators from eating the eggs. Unsurprisingly, the protected nest boxes were the most successful, with a success rate of 77 percent, versus 46 and 44 percent for unprotected boxes and natural cavities, respectively. They also collected data about clutch sizes. They discovered that a single female duck usually has about 9 to 18 eggs; contradictorily, only 15 percent of the nests had this many eggs, and there was an average of 20 hatched eggs per nest. How did this happen?
One of the more interesting facts that came out of this study was that the BBWD can have rather large clutch sizes due to compound or dump nests. This means that females of the same species lay eggs in another female’s nest. You’re probably expecting maybe 30 eggs in one nest. What if I told you there could be 100 eggs in one nest? It’s unbelievable, but true! In their 1976 study, Don Delnicki, Clarence Cottam, and—once again—Eric Bolen observed a nest box that had 17 eggs on one day, and a whopping 50 eggs two days later. Since waterfowl usually lay one egg per day, this means 17 other females laid eggs in this nest! By the time the eggs began to hatch, there were 99 eggs in the nest. They observed that only 38 of the eggs hatched, probably due to the lower chance of older eggs hatching and to the inability of the ducks to incubate all 99 eggs.
I assumed the BBWD had dump nests because there weren’t enough suitable nesting sites. Strangely, there would be dump nests right next to nest boxes that were available! Bolen and McCamant suggested that the dump nesting phenomenon is due to the gregariousness of BBWD. Whether or not this benefits the amount of young is up in the air; although the survival rate of ducklings from dump nests was the same as that of non-dump nest ducklings, it is hard to account for the loss of potential ducklings in dump nests.
If you’d like to learn more about the black-bellied whistling duck, you can check out its Birds of North America Online page here: http://bna.birds.cornell.edu/bna/species/578/articles/introduction, or you can look at all the articles I read while researching this most unusual duck (links below.)
Bolen, E.G. 1971. Pair-Bond Tenure in the Black-Bellied Tree Duck. The Journal of Wildlife Management. 35:385-388. doi:10.2307/3799620
This paper discusses the tendency of black-bellied whistling ducks to remain faithful to their mates for at least two years. (Link: http://www.jstor.org/stable/3799620)
Bolen, E.G. and McCamant, R.E. 1977. Response of Incubating Black-Bellied Whistling Ducks to Loss of Mates. The Wilson Bulletin. 89:621.
This source talks about what happens when a mate is loss during nesting time. (Link: http://www.jstor.org/stable/4160994)
Bolen, E.G. 1967. Nesting Boxes for Black-Bellied Tree Ducks. The Journal of Wildlife Management. 31:794-797. doi:10.2307/3797986
This is Bolen’s pilot experiment for his 12-year study, in which he investigated what was the most suitable nesting site. (Link: http://www.jstor.org/stable/3797986)
Bolen, E.G., McCamant, R.E. 1979. A 12-Year Study of Nest-Box Utilization By Black-Bellied Whistling Ducks. The Journal of Wildlife Management. 43:936-943. doi:10.2307/3808277
This paper lays out the results of a 12-year study. It discusses trends in utilization of nest boxes, the distance from the nest boxes to water, nest success, egg success, clutch size, and other topics. (Link: http://www.jstor.org/stable/3808277)
Bolen, E.G., Cottam, C., Delnicki, D. 1976. An Unusual Clutch Size of the Black-Bellied Whistling Duck. The Wilson Bulletin. 88:347-348.
This article looks into the phenomenon of dump nesting in black-bellied whistling ducks. (Link: http://www.jstor.org/stable/4160748)
An adult black-bellied whistling duck in flight. Its red bill and big white wing patches make it easy to identify while flying (if its call doesn’t clue you in first!)
Here you can see its signature black belly.
This is part of Brazos Bend State Park, where I saw the black-bellied whistling duck.
A distribution map for the black-bellied whistling duck. If you’re from the U.S. and have seen the BBWD, consider yourself lucky! As you can see, this duck is present in only the southern part of Texas.
This table is from Bolen 1967. It shows the success rate of different nesting sites, and has implications for controlling the population of black-bellied whistling ducks.
This figure is from the Bolen and McCamant 1979 paper about their 12-year study of nest boxes and the BBWD. It shows the frequency of different clutch sizes, and how many of the nests had at least one egg hatch. Around 70 percent of the nests are dump nests.