Nesting Birds: Pirates Thieves and Accomplices

Cunning strategies and nefarious practices that some birds use to secure nests or caregivers to their eggs. Are these ploys to ensure survival of their species or just bad mind? Faraaz Abdool looks at some birds that get others to do these jobs. (Photos by Faraaz Abdool)

 

It’s the start of spring in the temperate northern hemisphere and here in the tropics we’re also undergoing a shift in the seasons. Subtle changes in weather may be imperceptible to humans, but to the trees and animals, the signals are clear. Countless trees in the forest have burst into flower, some are just releasing their seeds, and others are already sprouting new leaves. To coincide with this boon in resources, many birds are also entering their breeding season.

Breeding season begins with courtship displays: A male Giant Cowbird engages his mate. Photo by Faraaz Abdool

 

While some species of resident birds in Trinidad and Tobago do breed all year, there is a marked increase in breeding activity at this time of year. From tanagers to mockingbirds to oropendolas, all are on the critical mission of sourcing a suitable nest location, obtaining the correct materials, construction, copulation, incubation, fending off predators, and finally the tiresome task of feeding and cleaning up after any number of fuzzy baby birds. Well, not exactly everyone. There are a few species of birds within our islands which do not subscribe to the conventional model. They are the birds waiting on the periphery, interested in the goings-on of the aforementioned tanagers, mockingbirds, wrens, oropendolas, and so on. Oftentimes they are chased off by devoted parents. But their success in bringing the next generation into the world depends on persistence and dogged determination to infiltrate the ranks.

Consider the Shiny Cowbird. Aptly named, for the males are indeed quite shiny and they do tend to associate with cows. Generally unassuming, they are tanager-sized birds with a stately posture. Sleek, contemplative birds with a sweet song, the males of the species may seem completely black but alternate between glistening green and purple depending on the incident light. Females are a dull, washed-out brown, and they are responsible for selecting a nest. Minor detail here is that this nest she selects belongs to another species.

Shiny Cowbirds are observant creatures and can often be seen perched, watching the frenetic nest-building activities of Palm Tanagers, Blue-grey Tanagers, Tropical Mockingbirds, or the endearing House Wren. At times these birds would actively chase the cowbirds away from the vicinity of their nest. But the cowbirds would always remain within view. Once a nest has been built and eggs laid, it’s time for the female Shiny Cowbird to make her move. 

 

The male Shiny Cowbird has a melodious song. When the young eventually leave the care of their foster parents, these vocalizations lead them to their kin. Photo by Faraaz Abdool

 

The same drama unfolds on a larger scale with the Shiny Cowbirds close relative, the Giant Cowbird. Towering over its smaller cousin, the Giant Cowbird shares many characteristics including its breeding behavior – officially termed “brood parasitism.” Within the sphere of brood parasitism, the species earmarked by the brood parasite as the foster parent is referred to as the host species. Being large birds, the Giant Cowbird must seek similarly-sized hosts to rear its massive offspring. In Trinidad, only two species are targeted as hosts by the Giant Cowbird, the Crested Oropendola and Yellow-rumped Cacique. Only the former exists on Tobago.

Both species of cowbirds are social, most often being observed in small groups. Both sexes of Giant Cowbird are glossy black with striking red eyes, however males are considerably larger than females. Males typically keep their distance from the host colonies, approaching only to lure attendant parents away from the nest so that his mate can fly in and deposit her eggs. Most often, however, the cowbirds wait until the flock heads off to feed during the day.

Brood parasitism is a complicated, ever-evolving arms race between the parasite and host. It occurs across the world with some species evolving truly remarkable levels of mimicry. In southern Africa some host species have evolved complex throat patterns that are only visible when the baby bird opens its mouth to be fed – a sort of failsafe mechanism for the parent to verify the identity of its chick as it feeds it. This should work, except that the resident brood parasites have almost perfectly copied this throat pattern for their young! Here in TT, the Striped Cuckoo found on Trinidad decides what colour its egg would be depending on which host nest it is depositing its egg into! Such levels of deception become necessary for a species that kills its nestmates upon hatching!

Piratic Flycatchers tend to prefer perches that are at the very tops of trees, from which they can comfortably survey their surroundings. Photo by Faraaz Abdool

 

Another atypical breeding strategy unfolding in our forests during this time is that of the Piratic Flycatcher. This is another species that deserves its name, for it truly is a pirate. Piratic Flycatchers have a clear preference for pendulous, globular nests – one species fits this description, the Crested Oropendola. While some other species may build similar nests, the Piratic Flycatchers seem to prefer oropendola nests and can often be observed perched high in the canopy, overlooking an active colony of Crested Oropendolas.

The piracy procedure involves incessant harassment to the point where the original owner of the nest abandons the structure. This harassment may involve constant invasion, egg-tossing, and even the removal of hatchlings. Everyone has a threshold, and eventually the nest is abandoned and the Piratic Flycatchers can move in. They do not expect another species to raise their young – they simply need to capture a nest.

The strategy of nest piracy is not unique among birds, but it is much less common than brood parasitism. Some methods may appear to be less virtuous than others should we seek to apply our moral compass to the hearts of birds, but we are only now beginning to put the pieces together. Studies on Giant Cowbirds’ interactions with Chestnut-headed Oropendolas in Central America revealed startling insights on the relationships between parasite and host, where in some instances the oropendolas didn’t mind the intervention of the cowbirds in the slightest, as the young cowbirds fed on botflies in the nest, protecting the juvenile oropendolas from infestation in the process. The more we observe birds, the more we discover.

 

Ref: The Hidden Powers of Animals: Uncovering the Secrets of Nature (Karl Shuker, 2001)