Commensalism is a relationship between two species where one species benefits and the other species neither benefits nor is harmed. Such relationships exist in the natural world between various species. One such example is the relationship between two bird species, a Short-toed Snake Eagle (Circaetus gallicus) a raptor and Indian Silverbill (Lonchura malabarica) a passerine. This article is based on our continuous observation of the nesting of the same pair of Short-Toed Snake Eagles, for a period of four years between 1998-2002.

Short-toed Snake Eagle incubating its egg

As the monsoon recedes and winter sets in, the eagles start preparing for the nesting season. The courtship starts with them circling in the sky and chasing one another mid-air. This ritual continues for a few weeks. The male often brings a snake to attract the female and perches on a rock or large branch with the snake popping out of its beak. The courtship and bonding are further solidified through mid-air spins (cart wheel display – a courting ritual usually displayed by raptors) while their talons remain entangled. Some of these spins last quite long and the birds drop too close to the ground; at times it feels like the eagles may crash!

A single egg is laid by the Short-toed Snake Eagles each year, resulting in high mortality. The eagles’ nest is so packed that it is impossible to get a photograph where the silverbills’ nest is visible too.

After few weeks of courtship, the nesting process begins with the eagles starting to scout for a suitable tree and branches to build their nest. Once the selection process is done, one by one twigs are laid in a way that they inter connect, to form a strong bond. The nest building process for Short-toed Snake Eagles is long and could take about 4-6 weeks. While these eagles are busy placing one twig over the other to form a stable nest, they are being watched by a pair of Indian Silverbills. Just like the eagles, these passerines are also preparing for their nesting season. But unlike the eagles, these passerines need to protect their nests from potential predators, especially snakes. Nesting within the vicinity of the nest of the eagles provides them with the best kind of protection. 

Representative image of Indian Silverbills, courtesy Tapas Biswas via Wikimedia Commons, under the Creative Commons Attribution-Share Alike 4.0 International license.

These Indian Silverbills build their small ball-like nest within the gaps of the Short-toed Snake Eagles’ nest. Usually these nests are built either just before the eagles lay their eggs or just after. The silverbills lay about 4-6 eggs and raise their chicks while the eagles are busy incubating their sole egg. The incubation period for the eagles is about 5-6 weeks and it takes another 6-8 weeks for the fledgling to leave the nest. Within these 12-15 weeks of the nesting period of the eagles, the silverbills build their nest, incubate and raise their chicks. The clutch of the Indian Silverbills is usually about 4-8 eggs which are incubated by both parents. The incubation period is about 10-15 days and for the fledglings to fly out of nest it takes another 2 weeks.

The eagles do not prey on passerines and restrict their diet to snakes and lizards. Hence, the silverbills are not only safe but also get protection from predators like snakes and lizards. Since the silverbills do not destroy the nest or the egg, they cause no harm to the eagles. Such a relationship is referred to as Commensalism.

We will be back with more examples of other symbiotic relationships in nature like Mutualism and Parasitism. Stay tuned!