Anticipating a Big Night

The sound of wood frogs calling is a most-welcome sign that Spring is near. I would say “here” except that this year they were heard in the first week of March, two to three weeks ahead of normal. Their breeding migration, known as a Big Night, typically occurs in early Spring on a rainy night with temperatures higher than 40F. Spring officially began a few minutes before midnight on March 19, which is also early. In fact, it is the earliest equinox date in 124 years!

Seasonal start and end points vary with the axis and orbit of the Earth. The biology of amphibians is also tied to Earth science, but more specifically to temperature and precipitation patterns that occur over Winter. New York biologists Gary Lovett, Al Breisch, and James Gibbs have published research that explains this relationship and that can be used to predict calling dates. They have also correlated a shift in frog biology that corresponds with regional climate change including warmer and wetter winters. Lovett’s research showed that frogs are calling an average of 11 days sooner than they were in 1949. Breisch and Gibbs (both great mentors to me) found a 13.6 day difference over the course of the 20th century.

Is this biological response to climate change a good thing? Scientists aren’t sure. Resilience to climate change is going to require that plants and animals adapt their life cycles to new conditions. However, adaptations take time to emerge and spread by the process of natural selection, and they must happen in concert with the evolution of other relationships in the ecosystem. For example, amphibians like the Wood frog cannot control their internal body temperatures except by altering their behavior. They are waking up from their winter slumber underground ahead of schedule because their metabolism rises with temperature. If their normal food sources (insects in the leaf litter) are not yet available, these frogs may suffer adverse consequences. Insect life cycles are often closely correlated to light intensity and to plant phenology, rather than to temperature, making food scarcity a real concern for early-rising amphibians. Birders share this concern over early migrants who show up here ahead of their food sources.

Local naturalist Paul Lewis has noted the early activity at area vernal pools this year. Vernal pools are the seasonal wetlands that several species of frogs and salamanders depend upon for breeding. Males typically arrive first and in the case of frogs, begin calling to attract mates. On so-called Big Nights, great numbers of animals migrate into the ponds and breed en masse. It’s over within a matter of days to weeks, depending on the species. Paul has been keeping track of first-calling dates for years and monitors a number of breeding pools in Westchester County parks and preserves. He has generously shared his knowledge with the community for about as long, including with me as a young biologist just out of college.

Make your own observations this spring. Listen for sounds, look for signs. Let us know your findings!

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