When a cattle egret (Bubulcus ibis) flew up to sat on a tree at dawn, I had to take out my camera to take some photos. After having spent some time in my initial years of watching birds, I’ve given up looking at these white egrets. They are very common, and since the four species are not terribly easy to tell apart, I don’t usually bother to either record them or to look hard. Cattle egrets are the easiest to tell apart from the others; they always have some yellow on them. In the breeding season it covers their head, neck, and back. In winter, the yellow recedes to a small patch on its forehead, as you see in the photo above.
To identify the other white egrets you have to look at the feet and beaks. Since the birds are usually seen in extremely muddy places, this is not easy. The little egret (Egretta garzetta) has a dark beak, dark legs, and yellow feet. In breeding season it has two long plumes hanging behind its nape. The large egret (Casmerodius alba, formerly Ardea alba) has dark legs and feet. Its beak is dark in the breeding season, and yellow otherwise. In both seasons it has a dark line extending from the beak under its eyes and beyond. This is called the gape line. If you see it flex its somewhat longer neck, you might see a kink in the neck. In the photo above (taken last February near Jamnagar), the gape line clearly extends beyond the eye, and there is a definite S-shaped kink in the neck, both telling us that this is a large egret. The intermediate egret (Mesophoyx intermedia, formerly Ardea intermedia) is the most confusing. Its legs, feet, and beak are very similar to that of the large egret, but the gape line stops below the eye. Also, its neck is a little shorter, and does not kink into an S-shape. Distinguishing them is always a puzzle, and I’m never sure that it worth taking the time to solve.
I’ve never paid much attention to the cattle egret (Bubulcus ibis). After all, they are common as dirt. If you go past villages, you see them trailing after cattle. If you go driving through jungles you find them following rhinoceros, elephants and herds of deer. In cities I’ve seen them following lawn mowers. In the monsoon, they put on their mating finery. The pure white feathers turn a bright ocher on the head and neck, and their beaks and legs turn into the same colour. Once I was photographing a bunch of them inside IIT when a girl stopped by and asked “Are they very rare?” I told her, “Not very,” and she left a little disappointed.
But this very common bird turns out to have an uncommon story. Until the middle of the 19th century CE, the bird was restricted to the tropics and sub-tropics of Asia and Europe. Then it began to take over the world. It was seen in the Cape province of South Africa in the early 20th century. At about the same time it was sighted in South America; then in 1933 it was reported in Old Providence Island in the Caribbeans, and spread to North America soon after. In the 1940s it touched down in Australia. There is no record of these birds being transported across the seas; to the contrary, there are scattered observations of small flocks spotted crossing oceans. In the 21st century it has spread from its original habitat in Spain and Portugal to northern parts of Europe.
If these birds are able to cross oceans, then they must have done so in the remote past as well. So the new thing is not its dispersal across the world, but its recent establishment in almost all continents. It seems likely that this has something to do with ecological changes brought about by humans. The scientific speculation that it has spread following cattle brought by humans, (and the conversion of forests to pastures) has gained currency since it was first applied to observations made in the Caribbeans. But England and Ireland, where the egrets established themselves only in 2008, have had cattle through recorded history. So cattle raising cannot be the only trigger for the century of expansion of the cattle egret’s range. Could it be that climate change, in the form of hot summers and mild winters, also has something to do with the spread of the egret?