Drying its feathers by the water’s edge the Australasian Darter can cut a decidedly prehistoric figure. With its large webbed feet and dagger-like bill, the darter is an accomplished diver and spearfisher. After their diving sessions they can often be found like this, perched near the water drying its wings. This has led to some misunderstandings over the years.
When I was younger I was often told that darters, and the related diving birds the cormorants, lacked oil in their feathers that give the famed resistance to water of other waterfowl like ducks. More recently, we have come to understand that this is untrue. The darters and cormorants still preen their feathers with oil from the uropygial gland on their back, just above the tail. This oil preserves the feathers’ insulating properties and maintains their condition for flying. The reduced buoyancy in these diving birds is actually a result of the physical structure of the outer feathers. They are subtly different, at a microscopic scale, to the feathers on a duck.
The duck is completely enveloped in an impervious cocoon of feathers, allowing water to literally roll off its back. The darter’s feathers let a bit of water penetrate just below the surface to reduce its buoyancy and allow it to dive with less effort. When we see the darter drying its feathers it is really just letting that water drain from the outer layer to get it back to a more manageable flight weight – the skin beneath its feathers will still be bone dry.
The Field Nats had their quarterly wader count on Sunday and picked up a huge variety of water birds at the sewage ponds including a single Black-tailed Godwit. Later in the week, a single pelican flew in for a brief stay at the ponds.