Black-naped Tern: Defense vomiting
Posted by BESG on 23 April 08, Wednesday
Contributed by Roger Deng
Roger Deng was also around the rocky islets off the northern shore of mainland Singapore where breeding Black-naped Terns (Sterna sumatrana) were confronting an intruding Grey Heron (Ardea cinerea) that April 2008 morning, posted earlier by Lee Tiah Khee.
It is well known that nesting birds, especially with young, will attack intruders. Smaller birds have been known to attack larger birds, including raptors. And terns are no exception. They mobbed the heron as soon as it flew near the colony. And despite the large size of the intruder, the terns were relentless in their mobbing.
There were other photographers as well, a number of whom posted impressive images of the confrontation on NaturePixel.org forum .
However, Roger was apparently the only one who managed to document something the others missed. He photographed the terns employing their secret weapon in their attempt at driving the heron away.
Look carefully at the image above. There are four terns mobbing the heron. Three of the terns have discharged a liquid-like substance, appearing in the image as three series of droplets, one of which landed on the heron’s head. Apparently, some birds like herons, gulls and vultures vomit unpleasant substances in self-defense (Podulka, 2004). But there is no mention of terns, so can this be the first record? This vomit, an oily mix of flesh and fluid that they regurgitate to feed their young, is reported to be foul-smelling. It is used to repel potential predators that approach their nests. The vomit may even be acidic, that can have an effect on the feathers and possible the eyes of the victim.
Can the liquid raining down on the herons be excrement? The action was too fast and the distance a little too far for Roger to confirm whether the liquid came from the anterior or the posterior end of the bird. But is there and instances of birds using excrement for defence?
The relentless attack finally forced the intruding heron to retreat (above).
This posting is a cooperative effort between NaturePixels.org and BESG to bring the study of bird behaviour through photography to a wider audience.
Reference:
Podulka, S. (2004). Defense bahaviour. Pp. 6.52-6.56 in: Podulka, S., Rohrbaugh, R.W. Jr & Bonney, R. eds. Handbook of bird biology. Ithaca, NY: The Cornell Lab of Ornithology.
Categories: Interspecific
Comment by Hai~Ren
Made Wednesday, 23 of April , 2008 at 6:10 pm
Vomiting of stomach contents might not be an efficient strategy, especially if the species of bird feeds its chicks through regurgitation. A bird that repels predators by vomiting would then have to expend energy obtaining more food.
Where it comes to excrement as a weapon, the fieldfare, an European species of thrush (Turdus pilaris), repels nest predators by dive-bombing and showering them with excrement.
Comment by YC
Made Wednesday, 23 of April , 2008 at 7:43 pm
Thanks Hai-Ren, most interesting. Based on your reference, I am now wondering whether the terns are using vomit or excrement? Any idea anyone?
Comment by Haniman
Made Wednesday, 23 of April , 2008 at 9:45 pm
I think its definitely excrement. Food is better saved for the hungry chicks then to be wasted on the heron.The excrement theory is more sound to me.
Comment by shundeng
Made Friday, 25 of April , 2008 at 6:43 pm
Hai-ren and Haniman: Fulmars (Fulmarus glacialis)feed their chicks by regurgitation and use vomit-defense. We cannot say that if it is less efficient then it wouldn’t be possible. Evolution works in funny ways! 
