In the first long-term study of its kind, researchers have amassed data on how climate change affects the reproductive success of birds.

In the 12-year study, researchers focused on the greater ani (Crotophaga major), a long-lived species common to the tropical forests of Panama. They studied how drought conditions caused by the El Niño Southern Oscillation (ENSO) affects the breeding behavior of these birds. They discovered that the birds are not only less successful at breeding during hot, dry periods, they may also forego reproducing altogether. The data showed that 37% of adult birds attempted to breed in one El Niño year, as compared to 90% in a normal year.

Why? The researchers, led by Princeton’s Christina Riehl, put forward several hypotheses. Perhaps the insects the birds normally feed on are greatly reduced during ENSO years, leaving them with inadequate energy to reproduce. Alternatively, the seasonal climatic cues that initiate mating season may be weaker or absent in El Niño years, they suggested.

“If the effect of climate on breeding that we saw is true for other species, the resulting effect on biodiversity would be bad news,” said Riehl, an assistant professor of ecology and evolutionary biology who is associated faculty in Princeton’s High Meadows Environmental Institute.