Chestnut-headed Oropendola (Psarocolius wagleri)
Illustration: Source Unknown
I've said it once and I'll say it again: never underestimate blackbirds. Blackbirds are perhaps the most underrated songbirds; they are too often discounted, brushed off, or simply ignored. When they do attract attention its usually for all the wrong reasons. They're maligned as agricultural pests, though many species are more beneficial to farmers for eating insects, than they are destructive in consuming crops. They're derided as ugly, even though a great many species are beautifully coloured - orioles are blackbirds, let us not forget. They're considered noisy, clamorous and obnoxious, when in truth many species have remarkably intriguing, if not utterly beautiful, songs; consider a meadowlark singing on a warm spring day. But such nearsightedness is a fool's understanding. The truth is we must never take the blackbirds for granted, we must never underestimate them, especially the tropical ones!
Among the most conspicuous, and certainly the most charismatic tropical blackbirds are the oropendolas. They're big, badass, flashy (as far as blackbirds go), and their vocalizations are stranger than Norwegian prog rock. Their nests, which take weeks to construct, are incredible feats of engineering: pendulous, intricately woven, and arranged in conspicuous colonies hanging above the forest canopy from the limbs of a enormous emergent, or in an isolated farmland tree. Oropendolas are full of surprises. For example, they've been seen catching hummingbirds. But the most fascinating aspect of oropendola natural history may be their reported relationships with wasps, bot flies and cowbirds.
In Panama, some Chestnut-headed Oropendola (Psarocolius wagleri) colonies are built in trees that are also home to colonies of highly aggressive predatory wasps. The wasps unwittingly assist the oropendolas in two ways. First, they keep most would-be predators at bay; after all who wants to mess with a bunch of ornery flying hypodermic needles? Second, they keep the colony more or less free of parasitic Philornis bot flies. Wasps are hunters after all, using their venomous stings to defend their colonies and subdue their prey. Philornis bot flies are common parasites of neotropical birds. In oropendola colonies Philornis lay their eggs on nestlings, when the eggs hatch the larvae burrow beneath the bird's skin and begin syphoning off the precious resources that the baby birds themselves need to grow. When those neighbourly vespids are present, nestling oropendolas seem to suffer fewer instances of parasitism, but in colonies depauperate of wasps, nestlings can suffer from relentless bot fly parasitism. Infestations can actually be so intense that they can cause widespread nestling mortality.
Enter the Giant Cowbird (Molothrus oryzivorus). Giant Cowbirds, like our familiar North American Brown-headed Cowbirds (Molothrus ater) are brood parasites. They don't build their own nests and raise their own kids, rather they seek out the nests of other species and foist all parenting responsibilities upon them. Brood parasitism has evolved several times among different families of birds. Cowbirds and cuckoos are perhaps the most famous example, but numerous other birds do it too: some finches, honeyguides, and even ducks. Some brood parasites are fairly choosy, laying their eggs in the nests of a specific host species. Giant Cowbirds, for example, mostly parasitize oropendolas and the closely related caciques, though orioles can make suitable surrogates in a pinch.
Being parasitized doesn't come without consequences. Usually, a host will experience reduced success in raising their own offspring if they also have to raise a parasite's chick. This can be for a number of reasons, like host egg ejection by female cowbirds, or accelerated growth rates of cowbird chicks compared to host chicks. In some cases, hosts will simply abandon their nest and begin the breeding process again. Yellow Warblers (Setophaga petechia) will construct a brand new nest directly on top of their old one, smothering not only the cowbird's eggs but their own as well. Because of the risk that parasitism will result in reduced nesting success, oropendolas normally aggressively defended their nests from Giant Cowbirds. When a cowbird approaches a nest, the colony may irrupt in excitement and the nest owners will do their best to see the interloper off. But at some Panamanian oropendola colonies, astoundingly, Giant Cowbirds are apparently allowed to parasitize the nests! Why?
Colonies where parasitism is reportedly allowed are those which don't have aggressive wasp neighbours to help keep the vicinity clear of bot flies. Remember, that bot fly infestations are most severe when wasps are absent. But Giant Cowbird nestlings in their constant hunger, appear to actually remove bot fly larvae from their oropendola nest mates. They pick the larvae right off their fellow nestlings. In colonies with high a incidence of bot fly infection cowbirds may actually help improve the chances of oropendola survival. In colonies with few bot flies (because of more wasps) cowbirds are only a detriment, so they are vanquished.
This curious piece of natural history, the association between oropendolas, wasps, bot flies and cowbirds has been cited time and again in the scientific and popular literature. It shows up in print frequently enough to give the impression that this complex set of avian-insect interactions is common and widespread; in actuality, it has only been documented in one study dating back to the 1960's. It seems to have never been observed again in Panama, where it was originally described, or anywhere else in Latin America. It's not as though no work has been done on oropendolas since the 1960's. In fact, some studies have even looked at other aspects of Giant Cowbird interactions with other oropendola and cacique species, but have not reported the same types of interactions. Certain authorities on blackbirds have even called into question the validity of the work itself. Certainly, more study is needed. Hopefully, further work in the neotropics will rediscover the oropendola-cowbird mutualism, but if not, there are doubtless even more complex and intricate ecological interactions to be found, and I eagerly await their discoveries.
