Rare swift nesting success surprises researchers

All 11 nest boxes for the critically endangered black swift in Eurajoki have produced successful broods this year, a rare piece of good news for a species whose Finnish nesting numbers have fallen by 60 percent since the 1980s, Yle reports.

The black swift was classified as critically endangered in Finland in 2019. Ari Rantamäki, bird officer at the Pori Ornithological Society, calls the decline dramatic and cites three main causes: a global drop in aerial plankton that the birds rely on during their long migrations, the loss of nesting sites as buildings become more energy-efficient, and a shortage of nest boxes.

Rantamäki, who has ringed 117,500 birds over his career, describes the ringing process as a kind of “bird clinic.” Chicks around two weeks old are fitted with uniquely numbered rings, weighed, and have their wing length measured to determine their age within days. The data allows precise tracking of individual birds, as no other Finnish species uses the same numbering system.

Black swifts are monogamous, often living up to 20 years with the same mate. They migrate to Africa for the winter but return each spring to the same nest, which is legally protected. Destroying a nest now requires a permit and the provision of replacement sites.

Local bird enthusiast Pentti Pere built the nest boxes on his Eurajoki property, originally intended for starlings. The horizontal design unexpectedly attracted black swifts, and the rich feeding grounds have kept the boxes occupied every year. Pere maintains the boxes through the winter, ensuring they are ready for the swifts’ return from Africa each spring.