Up to scratch
BIRDS which are ground feeders, especially seed-eaters, often have to search hard to find sufficient food. One of the methods they use is to scratch to discover food hidden under grass, leaves or soil.
British birders will be familiar with the rustling sound made by a Blackbird among dry leaf litter; when watched, the bird can be seen to sweep with its beak and flick its foot backwards.
With many species just one foot is used, but a few use both feet simultaneously and this is known as double or bilateral scratching. In North America
some sparrows, as well as juncos and towhees, use bilateral scratching.
When feeding in leaf litter or loose soil, a bird will hop forward and then back while at the same time kicking both feet backwards and into the surface beneath to disturb it and reveal any food. It has also been noted in a few other North American species.
This behaviour is so distinct that it has been used as evidence for how members of the New World sparrows and buntings have evolved. It is not found among buntings in Eurasia and Africa, nor among the circumpolar buntings (longspurs and snow buntings) which occur in both the Palearctic and Nearctic.
Original theories were that the Emberizidae speciated in the New World before spreading into Asia and through to Europe, but if this were true then it is likely that the scratching behaviour would also be found in Eurasia. More probably, the Old World species diverged from the New World birds before this behaviour evolved.
Recent DNA analysis on the
Emberizidae suggests that the amount of genetic divergence in Old World buntings is roughly equivalent to that in New
World sparrows which means it occurred at about the same time after ancestral stock spread to both regions.
Interestingly Northern Cardinals and some icterids also feed like this, and these are closely related to Nearctic sparrows. In Africa, the whydahs (Vidua spp) also use bilateral scratching, while in Eurasia the only species known to use this method is Bearded Tit.
So the next time there is a chance to watch a North American sparrow species in Britain, if you’re lucky enough to see it, pay special attention to what the bird is doing. White-crowned, White-throated, Savannah, Song and Fox Sparrows, as well as Dark-eyed Junco and Eastern Towhee, all use this method of feeding. So does Lincoln’s Sparrow – long overdue for addition to the British and Irish lists.
Chris Harbard