An evolutionary case of fatal attraction.
By late March at Pomquet, the local black ducks are feeding and courting in cracks and openings that develop with each new tide in the harbour ice. Soon a male, or drake—easy to identify by the white cast of worn feathers on his back—leads a slightly smaller mate into the bushes and the forest. Together they inspect potential nest sites. A second male sometimes attaches himself to the pair. He is vigorously spurned at first, but gradually tolerated by the dominant male, who claims a small bay in front of our house as territory. Number Two is allowed to stay as an additional set of eyes. Two watchful males keep alert for predators, while the female, feeding head down, loads up on the nutrients her body needs for egg-laying.
I call the second drake the “Seymour” male, in deference to black duck expert and friend Dr. Norman Seymour of St. Francis Xavier University in Antigonish, NS, who studied these “threesomes” years ago. Presumably, if the resident barred owls that nest upstream in our woods manage to pluck number one male duck, the underling drake could step in as a known-to-the-hen suitor. The “Seymour” role may seem droll, but it’s probably more useful for a male black duck, in an evolutionary and population sense, than just hanging out with single drakes at the local mussel bar.
Besides plants and blue mussels, coastal black ducks eat periwinkles (snails) and limpets. A student of mine, concealed in a blind, once observed a black duck chase, catch, and consume a small flounder!
Evolution has taken an unusual twist with black ducks. Many moons ago, three or four “species” of ducks evolved from a single hereditary stock. Mottled ducks are one—a non-migratory species I’ve encountered in Florida and Texas. A small population of mottled ducks has been introduced to coastal South Carolina.
Mottled ducks are a “cousin” of the Mexican duck, which is found in secluded marshes and lakes in highland areas of Mexico, New Mexico, and southern Arizona. The mallard also arose from this same ancestor, as a western bird geographically removed from the eastern black duck. Mallards were a surface-feeding or dabbling duck of prairie potholes and reed-ringed sloughs. They are adaptable, nesting as well along rivers, lakes, and ponds. Black ducks, on the other hand, evolved a dark, chocolate brown plumage to blend in with the forest floor, the tannin-stained waters surrounded by evergreens, along the shadowy edges of forest bogs, spruce-rimmed lakes, and beaver ponds. Black ducks also adopted the hidden tide pools in windswept expanses of rich, productive salt marshes along the Atlantic coast as brood-rearing and wintering sites.
When the first European settlers arrived on this shore, black ducks were abundant in these fertile salt marshes from Newfoundland to North Carolina. Their range extended west, with flocks staging (gathering) on Great Lakes marshes each spring and fall while migrating to and from bottomlands along the Ohio, Tennessee, and Mississippi rivers. About 50 years ago, for some reason these birds went into a serious population decline. Breeding populations in forest environments are hard to census. Causes for their demise have been hotly debated in scientific circles. Permit me to offer some observations.
The prime Atlantic habitats of the black duck—tidal saltmarshes—were some of the most productive lands in the Maritimes. People regarded them as mosquito-infested wastelands that could be converted by dykes and drained for productive farmland. Taxpayer-financed Marshlands Reclamation Boards persist to this day.
In New Brunswick, Prince Edward Island and Nova Scotia, 65 per cent of saltmarshes (85 per cent in the Upper Bay of Fundy region) have been altered or destroyed. In addition to agriculture, present day tidal marshes are in-filled to create residential, business and industrial developments, or to build roads. They are also inundated with sewage, industrial wastes, excessive silt from poor farming practices, and chemical fertilizers. The custom of spring burning on saltmarshes destroys wildlife habitat, nesting sites, and depletes the nutrients in the marsh. Cattle, as well as off-road 4X4s and ATVs, are severely impacting many saltmarshes, trampling and killing vegetation that holds sand dunes in place, allowing the sand to blow away.
Female black ducks nest inland on the forest floor as far as 3.2 kilometres (2 miles) from permanent bodies of water. They lay six to 12 greenish eggs. After hatching, hens move their broods to marshes, bogs and beaver ponds where the youngsters feed upon mosquito larvae and other insects. They may rear their brood inland in relatively nutrient-poor freshwater wetlands, or move overland or downstream to food-rich coastal marshes. Alternately, some female black ducks nest in forests beside coastal marshes. Dr. Seymour’s studies have shown that female black ducks who remained in scattered inland wetland sites successfully reared about seven of their ducklings to the flying stage (called fledging). Hens who moved their young broods overland or downstream to the rich coastal marshes, however, had an average brood size at fledging of 3.5. Put another way, they were only half as successful. Why?
Attrition rates were noted in two ways. First, offspring died while they were being moved. How many of us have witnessed broods trying to cross a busy highway? How many more years will pass before provincial Departments of Transportation put modest-sized holes in the concrete barriers between opposing lanes, so that small mammals and waterfowl that make it halfway across are not trapped against these walls?
Brood mortality also happens after reaching the salt marsh. Why would concentrating black duck broods in nutrient-rich coastal marshes be counter-productive to black duck populations? In Atlantic Canada, farms and residential areas border many coastal marshes, reducing the forest to a fringe around the harbour. Pomquet is typical. Females nest on the ground under a slim line of trees remaining between the harbour and a ring of houses and farms. Incubation begins after all eggs have been laid and may take up to 28 days. While on the nest, the hen may be stalked by free-ranging cats and dogs from nearby barns and dwellings. She or her eggs frequently succumb to an overabundance of raccoons and skunks that scour the neighbourhood nightly. The abundant life of a salt marsh also attracts predators like northern harriers, red-tailed hawks and bald eagles, otter, mink, fox and coyote.
Beyond this degradation of coastal forest nesting and rearing habitat, the decline in black duck populations could be related to acid rain or hunting pressure. Both issues are hotly debated, and may have some responsibility, but neither has been proven by research studies.
The conversion of coastal and inland forests to rural landscapes interspersed with farm ponds is an attractive combination to pothole mallards that were deliberately released here or migrated from the West. Over the past 35 years, mallard populations in eastern North America have increased dramatically. But landscape is only part of the reason that blacks are declining as mallards increase.
The drake mallard, a dandy in his iridescent plumage, puts on a courtship display that diverts many black females from black drakes.You probably learned in biology class that one species does not breed successfully with another species, or that when it happens the resulting hybrid crosses do not produce “normal” offspring. Discard that fable. The black duck and mallard are so closely related that researchers have found more genetic differences within each species than between the two species! When mallards breed with black ducks, mallard genes are dominant. A key element here is that mallard and black courtship behaviour is very similar. The drake mallard, a dandy in his iridescent plumage, puts on a courtship display that diverts many black females from black drakes. Male mallards are generally more aggressive than male black ducks and will displace them from prime breeding habitats. Gradually, more and more female black ducks are turning their backs on males of their own kind.
As a result of all this inter-species love-making, hybrid crosses between black ducks and mallards are becoming the rule rather than the exception. Some hybrids are evident immediately. Others are not so easily identified. The most accepted criterion is the colouration of the wing feathers. If the bird has a bold white wing bar in front of its speculum (the middle or secondary feathers of the wing), many consider it to be a mallard. Other secondary characteristics may appear, such as green colouration on the head.
The black duck is gradually disappearing from eastern woods and waters. It may be too late to stop it.The only mechanism I see that may prevent the total assimilation of black ducks by mallards may be differences in wintering and breeding habitat. Whether or not there is a real habitat difference is an open question. The mallard is adaptive and charming. A pair of black ducks I once banded in Pomquet Harbour was still together in the same harbour eight years later. Mallards are now overwintering here as well. As mallard numbers increase, more female blacks will be persuaded to nest the mallard way, forsaking mates and remote inland forest nest sites near isolated wetlands. This tough, wary waterfowl of evergreen forests and coastal marshes is steadily being undone by human habitat changes that favour its flashy western cousin, and a fatal attraction. The black duck is gradually disappearing from eastern woods and waters. It may be too late to stop it.
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EAM
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... A wild card is the Canada Geese that were imported from Ontario into New Brunswick. Since then their numbers have increased to the point New Brunswick now has a early goose season that opens the day after Labour day and runs this year until Sept. 18. We4 have observed instances where these large birds were forcing out and taking over the nesting areas of mallards. Has anyone else noticed this? |
