“What’s that?” exclaims a lady loud enough for us to hear. She has noticed a large black bird perched on a branch of a barren, submerged tree with wings and tail spread out like a cloth hanging on a line for drying. What makes me really turn back and look at the family is the reply from the lady’s teenage daughter, “Mom this is Darter. It goes under water to catch fish. When it comes out, it has to dry wings because its feather does not have oil – water sticks to them.” My first reaction is ‘smart girl’. I remember some issue of waterproofing with features of Darter.
Incidentally, I also find Darter conspicuous in Ghana (Bharatpur Bird Sanctuary) and among the most noticeable. The description of the bird is quite complicated and I am simply quoting this from online source for those who may be interested – The Oriental darter or Indian Darter is a large bird, measuring 85 to 100 cm in length and wingspan is 115 to 130 cm. The bill is long and measures 7 to 8 cm and it has long slender neck. The crown and neck of the bird are brown, darkening downwards to become black in the body plumage. The wing coverts and tertials have silvery streaks along the shaft. The iris is white with a yellow ring around it.[1] For those who are not interested in detail, the photographs can suffice.
It’s not a shy bird – Goes about ‘business as usual’ – tourists or no tourist, disturbance or disturbance, I am not sure, but, may be, fish or no fish! It happens to be among my first photos of the shoot during the trip. At times, it has made a forced entry into frame while I am shooting something. It comes around and be in foreground of the frame!
On one occasion, I see it drying itself and suddenly it dashes (almost crashes) into water and disappears. I wait and wait with camera ready. I am excited with the thought that it will emerge with a fish in beak and I will have a good opportunity for photography. Alas! It has just disappeared with not a sign. I am sure, it has tricked me!
And on another occasion, we are just relaxing sitting on a bench and watching the water hyacinth filled wetland in front. There is a pair of Bronze-Winged Jacanas busy feeding, on what, we do not know. And, here flies in a darter and settles in a Babul tree partially submerged in the wetland. But where has it settled? It manages to merge so much with the tree, that it becomes a puzzle to find– no, it has not spread wings or tail and just set like a vertical line with slender neck straight up along with the beak pointing to sky – is it praying?
What amazes me further is the flexibility of the neck – it turns and twists it so much, so far, and so tediously that I feel, Baba Ram Dev can learn a trick or two for yoga from Darter for stage shows.
I recollect that during our three days-two nights, we do see Darter where there are wetlands, but yes, they are not in swarms, few here and some there – thus attraction remains.
As I am writing this, several questions regarding feathers – waterproof or not and oiled or not -crop up in my mind. I feel I should satisfy myself. And I also realise, it will be a good idea to pass on scientific explanation to readers regarding lack (or whatever) of waterproofing in Darter’s feather.
I have been sure that just a quick internet search and viewing few pages will be enough to gather the stuff. But, it turns into complicated basic science questions, which Wikipedia alone cannot answer.
But, let me start with Wikipedia page on Oriental Darter /India Darter (Anhinga melanogaster). It simplifies things – ‘it has wettable feathers and it is often found perched on a rock or branch with its wings held open to dry.’[2]
Wettable – what is that? We have simple questions – Are the darters feathers waterproof or not? Are they coated with oil or not? Wikipedia does not answer these.
In this context, we refer to another facet of darter’s ecology. It’s known to spend most of the time roosting, resting or swimming. It is known that darter hunts for fishes inside the water. Another article mentions the issue differently ‘Since their feathers are not completely waterproof, they absorb water and are less buoyant, allowing for faster swimming and diving. Darter swim with their wings extended and paddle with their webbed feet. After swimming, darters sit on branches in the sun and spread wings to preen and allow them to dry. [3]
This article explains the usefulness of darter’s feather for being ‘not completely waterproof’ as an asset. Our questions are still not answered even by this detailed article. Rather another question is added here. It says ‘spread wings to preen…’
This brings us to a paradoxical situation as one of the roles of preening is spreading oil on feathers! Yes, I explore some literature about preening. First of all, several species of birds, particularly waterfowl spends hours in preening. Why it is that important? One good article explains –
Such is the act of preening, where each individual feather is meticulously maintained. As ducks check and re-check their bodies, they are actually cleaning each feather, removing any insects or lice, re-aligning the feather to its maximum aerodynamic position and then conditioning it.
This conditioning is the subject of much of the aforementioned scientific debate. Ducks, as well as a number of other bird species, actually coat their feathers with an oily substance that attributes to their apparent waterproof bodies. This is done through the uropygial gland, better known as the preen gland (and, also known as oil gland).
The preen gland is located near the tail of a duck, has a small, nipple-like opening, and is covered in short tufts of dense feathers.
A review of literature shows that this gland has been the subject of much scientific study due to its apparent mystical powers.
However, what we know for certain is that, by working the gland with their bill, ducks retrieve the substance on their head and face, and then spread it over their body. Such gives their feathers a shiny, oily appearance, and the unbelievable ability to shed water.’[4]
Another article regarding Darter particularly suggests, ‘They also squeeze their feathers through their bill to remove excess water and repel water with oil from their enlarged preen gland at the base of the tail.’[5]
The preening angle adds a contradiction. It is clearly suggested in several articles that ‘Birds pick up oil on their beaks, by rubbing against the uropygial or preen gland near the tail, and then rub it over their feathers. This coating insulates the interlocking barbules in the feather. As water cannot penetrate through the oil coating, the feather is waterproof.’[6]
I decide to look at an academic, detailed feature on feathers. This explains, ‘The interlocking Velcro-like structure on many bird feathers creates the smooth, flexible, and resilient surface that supports flight and sheds water. Arranged in an overlapping pattern on a bird’s body to expose the waterproof tips, contour feathers allow water to roll right off a bird’s back. Birds constantly maintain their waterproof coat through extensive grooming, or preening to ensure that every feather is in good shape.’[7]
What brought us closure to answer to our questions is the note on preening in a book – Not all birds have a preen gland. Ostriches have lost their preen gland over time as their ‘earthbound’ feathers do not need to be groomed for flight. African Darters also lack preen gland and the absence of fatty oil allows their feathers to get wet – an adaptation to hunting underwater. [8]
I speak to several experts and most of them assume Indian Darter does not have preening gland because it has to dry features but it is not confirmed. To add to the confusion, I have got a good photo of Darter from this Ghana visit with clearly visible preen gland at the base of the tail. Not only that, the bird has taken its beak right up to it! Is it a pseudo gland? Or the gland does not have oil as found in many species?
Sorry, friends, I have gone too far and must conclude. I feel, nature has deliberately made Darter like this – feathers are ‘wet-able’ or ‘not completely waterproof’ or ‘not oiled’ in the tourist’s word, so as to allow it to be ‘less buoyant’ and be able to ‘make fast movement underwater and hunt easily’, and Darter do preening probably to maintain feathers and not to oil them.
I must say, Darter has proved a tough nut but I have liked cracking it this far. May be some original wildlife biologist takes this over from here and make this very simple information available, simply.
Pushp
[1] http://indianbirds.thedynamicnature.com/2015/04/oriental-darter-anhinga-melanogaster.html
[2] https://en.wikipedia.org/wiki/Oriental_darter
[3] http://animaldiversity.org/accounts/Anhinga_melanogaster/
[4] https://www.realtree.com/the-duck-blog/are-ducks-really-waterproof
[5] http://www.besgroup.org/2009/02/25/darter-or-snake-bird-sunning/
[6] http://www.sciencefocus.com/qa/how-are-birds-feathers-waterproofed
[7] https://academy.allaboutbirds.org/feathers-article/
[8] https://books.google.co.in/books?id=fsuroRrJ5zAC&pg=PA35&lpg=PA35&dq=Preening+gland+in+Darter&source=bl&ots=ffQ8UqvRcY&sig=qthdeg3ENU7J08gTOYAIb8-kyek&hl=en&sa=X&ved=0ahUKEwiLpuL3kpbXAhVGOo8KHXIWD5oQ6AEIXTAN#v=onepage&q=Preening%20gland%20in%20Darter&f=false
Pushp Jain 