On a recent game drive in northern Tanzania, in eastern Africa, our driver pulled to a stop next to a group of acacias decked out like Christmas trees. Hundreds of five-inch grass spheres decorated the branches.
Along with students from a Hope College biology class, I gazed in wonder at the nests of the Red-billed Quelea, a sparrow-sized weaver that breeds in colonies. As many as 400-500 of its nests can hang in a single tree. Colonies can contain millions of nests and cover several hundred hectares — almost unbelievable! (A hectare is equivalent to 2.5 acres.)
The male is quite handsome. During the breeding season, he wears red, sometimes yellowish, feathers on his crown, neck, and breast and sports a black mask around a thick red bill. Non-breeding males and females have a cryptic tan plumage. The species is widely distributed in sub-Saharan Africa but absent from west-central rainforests.
The onset of prolonged rains at the beginning of the wet season, usually in November, stimulates breeding. Because colonies are so densely packed, the birds begin nesting at the same time, and all stages are virtually synchronous.
Each male starts building his hanging nest but doesn’t complete it. Instead, when construction is about half-finished, he sings and displays to attract a female. If one comes and accepts him, they pair, and she helps complete the nest.
The colony we were observing was in the late stages of feeding nestlings and at the beginning of fledging. The scene was chaotic: Adults flew in with food while nestlings gaped and older nestlings stood on nests. Some young queleas even made their first short flights.
Besides stimulating breeding, the rains also cause important food sources to become available at the right time. Caterpillars, grasshoppers, and other insects — main food items for young queleas during the first few days after hatching — continue their development and emerge from the soil. Then the adults bring milky seeds from developing grass plants that germinated with the early rains. After fledging, the youngsters leave their parents early and become independent, feeding on grass seeds.
Red-billed Quelea is an opportunistic species that relies on rainfall for its breeding season, rather than temperature or photoperiod. And when the rains continue, so do the nesting queleas, sometimes bringing off four broods. The rains also affect the juveniles. While they typically become reproductively active at the age of one year, if the rains continue long enough, the youngsters may breed as early as nine months. All of this adds to the great reproductive potential of the species.
Queleas form dense, highly synchronized flocks that, from a distance, look like clouds of smoke. As a flock flies closer, the birds’ wing-beats sound like high wind. It is common for people who observe a large flock rise from the ground to insist that a grass fire is developing. Flocks have been reported in the tens of millions. It is believed that quelea populations grew 10- to 100-fold beginning in the 1970s, when farmers started devoting more and more acres to millet and other cereal crops.
Queleas have many natural enemies, including herons, storks, hawks, owls, hornbills, rollers, kingfishers, shrikes, and corvids as well as snakes, lizards, and several small mammals, but predation hasn’t put a dent in the population.
Red-billed Quelea has been an agricultural pest for centuries. In fact, paintings in the tombs of pharaohs show farmers cracking whips to scare off queleas.
“Its main characteristic is that it occurs in extremely big numbers,” says retired biologist Clive Elliott, who worked with the species during most of his 31 years with the United Nations Food and Agriculture Organization. “Nomadic super colonies can grow to millions of birds, making queleas not only the most abundant bird in the world but the most destructive.”
Two million birds can eat 20 tons of grain a day, Elliott says. Since the adult population is estimated to be 1.5 billion (2009), FAO estimates annual agricul- tural losses of $50 billion.
Planters of small farms are especially vulnerable because a large quelea flock can wipe out their fields quickly. Growers commonly respond by becoming human scarecrows. They walk through their fields banging pots and hollering. It’s effective, but they have to do this all day for a month.
Government agencies spend millions trying to control the queleas. A common method is to spray night roosts with fenthion, a controversial organophosphate insecticide that it is highly toxic to birds. (It was removed from the market in the United States in 2004.) A second method is to detonate a mixture of dynamite and diesel fuel under a roost. The huge blast rises up, killing most birds instantly. Both techniques dispatch non-target species along with millions of queleas each year but do not appear to reduce the overall population significantly.
Red-billed Quelea’s preference for grass seeds, including domestic cereal crops, makes it a nemesis to farmers throughout sub-Saharan Africa. But its sheer numbers — whether at water holes, in trees, in the air, or in nesting colonies — will overwhelm and amaze you. — Eldon Greij
Eldon Greij’s column “Amazing Birds” appears in every issue of BirdWatching magazine. Subscribe. This article appeared in the October 2014 issue. Eldon is professor emeritus of biology at Hope College, located in Holland, Michigan, and the founding editor of Birder’s World magazine.
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