New breeding population of Coscoroba Swan discovered

A new, isolated breeding population of Coscoroba Swan has been confirmed in Bolivia, some 600 km from the nearest known nesting grounds.

In July 2022, three nesting pairs were discovered in the department of Santa Cruz. Previously, Coscoroba Swan was deemed a rare vagrant to Bolivia, with only three records, all of which come from Santa Cruz.

The species nests in marshes and freshwater bodies in southeastern Chile and Argentina, wintering in southern Brazil, Paraguay and Uruguay. According to Birdlife International, Coscoroba Swan is not endangered, although the global population is small (between 6,700 and 17,000 adults).

A Coscoroba Swan with young in Argentina in 2010 (Peter Gasson).

The Bolivian breeding records involve a pair that raised five young at Palmar de Las Islas, in the municipality of Charagua, in early July 2022. This was followed by a second pair with seven chicks was observed on the Kaukaya lagoon, in the municipality of Gutiérrez, on 17 July 2022, when and then a third pair were found at the same site with an active nest.

Amazingly, 18 adults were counted at Kaukaya lagoon – the largest gathering ever reported in Bolivia. By September, only six adults remained, probably due to falling water levels caused by drought and withdrawals for irrigation, and perhaps also due to hunting.

The closest known nesting sites are in Salta, Argentina, some 624 km from Kaukaya Lagoon and 769 km from Palmar de las Islas.

The Bolivian breeding was documented in a report in Ecología en Bolivia.