Wrybills of New Zealand
A recent newspaper article described how research into the feeding habits of the largest animal on Earth, the blue whale, has shown that when pursuing krill, the small crustaceans on which they feed, most roll to the left in shallow water but to the right in deeper water, with a few consistently behaving in the opposite way. The researchers saw this as “sidedness”, another example of which is the tendency of humans to be either right or left-handed, due to slight differences in brain development.
The article took me back to the only birds I have seen to show a preference for one side over the other, the wrybill, a small wading bird native to New Zealand and the only bird in the world with a beak that is bent sideways.
Wrybills breed on shingle riverbeds in the South Island of New Zealand where they use their thin sideways bills to extract invertebrates and fish eggs from under stones, many too heavy to overturn.
On a visit to New Zealand last autumn we saw our first wrybills on several muddy estuaries in their North Island wintering grounds, feeding like ordinary waders by probing the mud. It was surprisingly difficult to get a head-on shot of a bird (pictured) to show the bent bill.
I was intrigued by the uniquely adapted bill for I had assumed that half would have bills bent to the left and half to the right. In fact, the beak is always bent to the right and when feeding they cock their heads to the left (In crossbills, which have a bill specially adapted to prise the seeds from fir cones, the tips of upper and lower mandibles cross because they are bent in opposite directions, sometimes left over right, sometimes the other way.).
In evolutionary terms, the wrybill’s nearest relatives are two other New Zealand plovers which both have straight bills implying that the common ancestor of all three had a straight bill also. It seems that, to take advantage of its special way of feeding to exploit food sources not available to other wading birds, the wrybill’s brain has developed differently leading to its skull and bill becoming one-sided rather than the bill bending at random.
There are other examples of this one-sided approach in the animal kingdom, so flatfish which live on the sea bottom have both eyes placed on one side of the head, on the left side in the case of the turbot and on the right in the sole. To take another example from the great whales, sperm whales always have the blow-hole on the left side of the skull rather in the middle.