Every spring, the streets of Réunion Island in the Indian Ocean are littered with petrel fledglings that have gone astray. The problem is that the birds are nocturnal and are attracted to light, which is a deadly combination on Réunion because artificial lights have skyrocketed with the island’s development.
“Fledglings are attracted by lights during their first flight to sea and many fall to the ground with fatal injuries, are killed by predators or die of starvation,” say Matthieu Le Corre of the Universit? de la Réunion in Saint Denis, France, and three coauthors in the May issue of Biological Conservation. The birds presumably fly toward lights because they feed on bioluminescent squid.
Réunion Island has two endemic petrels that are critically endangered: Barau’s petrel (Pterodroma baraui), which has about 5,000 breeding pairs, and the Mascarene petrel (Pseudobulweria aterrima), which has fewer than 250 breeding pairs.
From 1996 to 1999, a media campaign encouraged local people to rescue fallen petrels and fill out a survey indicating the nearest source of artificial light. Le Corre and his colleagues focused their analysis on Barau’s petrels because they comprised most of the fallen birds. The researchers used fallen birds as a measure of deaths because unrescued fallen petrels seek hiding places, where they starve or are eaten.
During the four-year study, rescuers found an annual average of 400 fallen Barau’s petrel fledglings. Nearly 80 percent of the falls were caused by street lights and flood-lit sports facilities, which are the most widespread light sources on Réunion Island. More than 90 percent of falls were in April, when most Barau’s petrel fledglings leave their colonies of nesting burrows.
Le Corre and his colleagues estimate that artificial lights kill up to 40 percent of Barau’s petrel fledglings each year. While the Barau’s petrel population has not yet declined, this is not surprising. Artificial lights have probably posed a major threat to petrels for less than a decade, and the birds live so long (up to 25 years) that there are still plenty of breeding adults to mask the fact that fewer fledglings are surviving to adulthood.
While figuring out why so many petrel fledglings are dying was relatively easy, figuring out how to help them is anything but. Ideally, artificial lights would be shielded and some would even be turned off during April, the peak fledging season. Acknowledging that this approach may not be practical, the researchers suggest a rescue effort. This might work because 90 percent of the fledglings found were still alive. However, it’s too soon to say. “We have no idea of what happens to the hundreds of fledglings rescued each year after their release: do they die as a consequence of the stress or internal injuries induced by their failed first departure, or do they behave as other fledglings?” say Le Corre and his colleagues.
Le Corre, M. et al. 2002. Light-induced mortality of petrels: A 4-year study from Réunion Island (Indian Ocean). Biological Conservation 105:93-102.