Several calls that sounded like the plastic whistles of a hoard of overeager umpires announced the arrival of the black bellied whistling ducks, dendrocygna autumnalis, beforethey could be seensoaring around the bend of the pond(A great video of the black-bellied whistling duck and its shrill call at Brazos Bend). As they flew directly towards us, the six birds weaved haphazardly around each other, assuming no particular formation or shape.
These waterfowl immediately caught my eye and intrigue. They looked nothing like the white and brown ducks that my brothers and I fed in local ponds fifteen years ago. Although their shape resembles the “quackers, ” as my brother Ian liked to call them, that gobbled up our bread pieces and saltines, their pumpkin orange beak and brown, black and white feathers make for a unique, beautiful bird. As I glanced from the group flying in to the other ducks in Brazos Bend, I noticed that none of them have identical coloring or patterns. This passing thought led me to further investigations of bird feather development and a bit of reading on exactly what makes the black-bellied whistling duck so striking.
Searching through the literature for explanations led me to a fascinating paper that chronicled the black-bellied whistling duck feather development from conception through adulthood. In Growth and Plumage Development of the Black-Bellied Tree Duck, Brian W. Cain describes the plumage development of 21 birds in laboratory conditions. After collecting eggs in southwest Texas, he observed his own version of The Ugly Duckling. And yes, duck embryos are quite ugly (see the great photographs he took below throughout the process). As an individual pursuing a career in reproductive health, the opportunity to learn about the prenatal maturity of another species enthralled me!
The first heartbeat of the duck embryo can be seen after only 4 days. At this point the duck is a mere crescent-shaped blob of sorts with a gigantic black eye spot. The characteristic orange bill soon follows at 10 days and the first pecking noises of the chicks trying to escape their prenatal home come around day 28. Twenty-nine to thirty days in an egg sounds a lot easier than nine months in another mammal if you ask me, and seems to suit black-bellied whistling ducks just fine. I was surprised to discover they are born without their characteristic colors but are instead in a fuzzy layer of black and yellow fluff (see the great photo taken by Bolen et al during their 1964 study below). Their adult feathers don’t start to emerge until between 3 and 9 weeks.
The feather development occurs later for birds that have a lot of brothers and sisters as opposed to birds from smaller broods. The paper actually included a lot of information on the weight and development of birds depending on brood size (see graphs below J). In the graph, it can be seen that the birds from smaller broods grew to outweigh the birds of larger broods by nearly 200 grams on average. Although the weights of the large and small broods followed a similar trajectory in the first 49 days of life, towards the end of development the weight ducklings in broods of 15 or more was significantly less than that of smaller broods. This decreased their chance of survival and delayed their development. Larger birds gain the capacity to fly around day 56 where as the smaller birds usually wait until day 63. Overall the birds show a growth pattern that seems pretty similar to humans first growth spurt; they’re born, they shoot up in the first several weeks and then level off at their adult weight (see graph of growth below). The figure shows growth trends over time, a logarithmic growth leveling off around day 30 at roughly 800 grams. They progress to their full body weight in a much shorter period of time than humans, but the idea is the same.
The birds flying towards us in Brazos Bend had survived the postnatal trials and appeared to be fully grown. The six adults ducks settled in shallow water near the edge of the pond. I imagined they were a family, as black-bellied whistling ducks often migrate with their young even past development. They certainly looked similar enough to be a family- though I admit I am not an expert at distinguishing between different ducks of the same species beyond gender differences. Surrounded by grassy water plants they were hardly shy. In fact they practically surrounded the white ibis poking around for food there. The ducks were feeding as well, occasionally bobbing their orange bills into the water in search of seeds and small edible goodies, their typical diet. The two bird species ignored each other, each going about its own business. After foraging for a few minutes, the pack of six took off once more. They flew out of sight, likely to another shallow area of the pond; the black-bellied whistling duck almost never swims to depths greater than their height.
In fact, sometimes it is hard to believe that they’re ducks at all – They don’t like deep water, they don’t behave like ducks (more like swans or geese), and they often nest in trees. Their behavior is quite bizarre for a duck.
Let’s start with their mating ritual. In Natural History of the Black-Bellied Tree Duck in Southern Texas, Bolen, McDaniel and Cottam supply enough fun facts about the bird to win any duck trivia game and describe their swan-like mating dance in detail. Displaying moves any lady would be flattered to receive, the male woos his life partner with swooping figure-eight neck gestures, head dipping and the occasional dive. Instead of a post-copulation cuddle, the two birds triumphantly dance together, splashing, sticking out their chests, and swinging their necks about merrily.
As I watched the six birds fly out of sight, I continued to wonder if they too were a family unit. They had similar coloration and I could hardly tell between them. This distinction is further complicated in black-bellied whistling ducks by the fact that males and females have similar coloring and size. The nearly identical (or at least with my bad vision) pairs raise their young together and often families stick together even after the young learn to fly. When the birds are young, they are known to exhibit the goose-like behavior of swimming between their parents or the swan-like behavior of climbing on their parent’s back. This family structure helps protect the young from predation and ensures they make it to their first flight.
And those that do make it to their first year of flight are very lucky. Before the obstacles of predation, nutrition and basic survival, the eggs must survive a common practice in the black-bellied whistling duck world called “dump-nesting. ” Described well in Ground Nesting by Black-Bellied Whistling Ducks on Islands in Mexico by Markum and Baldassarre, dump-nesting occurs when more than one female lays eggs in a nest and often results in a large number of unhatched eggs. In their investigation of ground nests in Laguna la Nacha, Tamaulipas, Mexico 41.8% of the 496 ground nests they studied were dump-nests. In a different study by Bolen et al, they documented a 70% rate of dump-nesting in south Texas. Dump-nesting causes an unusual clutch size a low survival rate of eggs. In a study by Delnicki, Bohn and Cottam, they observed nests with up to 101 eggs. In one nest only 38 of the 99 eggs hatched. Talk about terrible odds!
With all of these studies being mentioned, you might be saying to yourself “whistling duck, tree duck, now which is it? What’s the deal?” This name switching reveals one of the most interesting behavioral characteristics of the black-bellied whistling duck. Along with the sharp call, they are equally known for their strange nesting habits. Although they sometimes nest on the ground, as referenced in the above studies, they mostly nest in tree cavities and often perch in trees. Now that is a strange creature – looks like a duck, nests like an owl, mates like a swan – Oy vei! The 6 black-bellied whistling ducks I saw fed by the water and flew off, but in my next exploration of Herman Park and on future field trips I hope to spot one squatting on a branch.
Until next time, have a fowl time and keep reading this fowl blog!