Wednesday, 8 January 2014

Australian Darter

Darters, the denizens of fresh water wetlands, are, to me, fascinating birds.
A male darter with view of back.

Another male darter - view from front.
Superficially darters appear to be closely related to cormorants however observation and some reading show they are very different from cormorants in many ways. Some taxonomists still include darters in the closely allied cormorant family, Phalacrocoracidae, however today darters are generally seen as a distinct group of birds comprising the family Anhingidae. The family is widespread in suitable habitats in both the New and Old Worlds.
Do darters share a relatively recent common ancestor with cormorants or have they arrived at the current point of similarity from a more distant ancestor through convergent evolution?

The main similarities are:
·      both occupy the same freshwater habitats
·      both have webbed feet and swim underwater to capture food
·      both can be found with outstretched wings drying out after an underwater feeding session, often sharing the same perches
However darters are significantly different in the following ways:
Darters have wing and tail feathers but the rest of the feathers on their bodies look more like fur, allowing them to remain underwater without the need to swim rapidly and aggressively to overcome buoyancy as cormorants do.
This feature underpins the different feeding methods of darters and cormorants. Cormorants actively chase and grab prey underwater by snatching with their hooked bills. Darters are lone hunters stalking prey slowly underwater – buoyancy would make it difficult to remain submerged for this hunting method.
They use their long sinuous neck and long sharply pointed bill to spear fish. The neck can be kinked and then suddenly straightened in a darting or thrusting manner, a little like egrets fishing, but instead of above the water, darters fish underwater. Unlike cormorants, darters shed all of their flight feathers at once, rendering them flightless during moult.
Darters often soar to great heights using air thermals - cormorants rarely do this.
Note the long sharp bill and fur like body feathers which help to reduce trapped air and reduce buoyancy.
Another close up head view - note the prominent white slash behind the eye and fine dry body feathers which look like fur.
A female Darter drying feathers - note flight feathers on wings and tail and the rest of the body is covered in fur like feathers.
Note the long neck which even when stretched out has a kink - this mechanism is used to thrust the sharp bill forward to spear fish.
When hunting, darter’s bodies ride low in the water or completely submerged with just the long snake like neck protruding, hence one of the bird’s common names – snake bird. Needle-beak Shag is another common name for this bird. The family and genus name Anhinga originated from indigenous people in what is now Brazil. Their name, which was modified by the Portuguese, meant demon or devil. It is hard to see how Anhingas earned the name devil bird?
The different common and scientific names given to the darter in Australian text books, field guides and bird lists, for example Australian Darter or sometimes Australasian Darter and Anhinga melanogaster or Anhinga novaehollandiae points to different views by taxonomists – are the Australian birds a separate species from other Old World birds?
The Anhingas in South America, New World birds (Anhinga anhinga), are a separate species - they look a little different but occupy similar habitats and behave the same – see photo below. 
The current view (refer HANZAB - see references below) seems to be that Old World Anhingas should be separated into three species - so there are four species of darter worldwide.
A New World Anhinga. This bird was photographed from a canoe while looking for Giant Otters on an oxbow lake in the Peruvian Amazon in 2011.
So next time you are in a freshwater wetland habitat keep an eye out for Darters. Even experienced birders can miss a single darter perched among a group of cormorants or a lone bird hunting with just a needle beaked head and snake like neck visible above water.
The following three photos show the female bird in above photos in a "taking flight" sequence.
A Darter's low buoyancy makes remaining underwater for fishing easy but when wet getting airborne is difficult. This bird is nearly dry - even so it still struggles to gain height.
Even after several wing beats this bird is still kicking the water for some assistance.
Further along and the bird is still close to the water - note the neck is kinked in flight.

After publishing the post earlier today I received some comment and photos by email from a friend and fellow bird photographer. The photos, taken in Kakadu National Park show a Darter with fish it has speared. They are so good and show perfectly how Darters spear fish that I thought they should be included. John Saxton has agreed to this so here they are. Thanks John.
The Darter is a female, she has speared this fish with her lower mandible.
This fish has been speared with both mandibles. Having skillfully captured the fish it is then flicked into the air and caught head first and then swallowed whole - another very skillful maneuver. All bird species swallow fish head first to avoid the spines being caught in their throats.
The following texts were used to prepare the notes for this post:
S M Marchant and P J Higgins, Handbook of Australian, New Zealand & Antarctic Birds (HANZAB), Volume 1, Ratites to Ducks
T R Lindsey, The National Photographic Index of Australian Birds, The Seabirds of Australia
G Pizzey and F Knight, The Field Guide of The Birds of Australia
M Morcombe Field Guide to Australian Birds
K Simpson and N Day, Field Guide to the Birds of Australia
I Fraser and J Gray, Australian Bird Names – A Complete Guide
Christidis & Boles 2008 List of Australian Birds
BirdLife Australia Working List of Australian Birds

1 comment:

  1. Another really interesting post. One day I hope to see how that manage to get the speared fish off their bill and into the air.