Reproductive success (number of young produced per pair per year) in relation to the availability of caterpillars (mg per 100 leaves) on the main study plot at Hubbard Brook from 1986 to 1998 (food data were not available for 1999; data from Holmes, Rodenhouse, and Sillett).

As researchers predicted, in years when caterpillars were more numerous, the birds were able to raise more young. How did this work? It turns out that it’s not because nestlings starve in years with fewer caterpillars, but instead birds adjust the number of nests they build to the food availability. In really good years, birds even start over once their nestlings have left the nest! So it’s possible to raise two groups of nestlings in a single breeding season, but the number of birds that try to do this is greater when there is more food.


Learning module developed by K. Langin, H. Sofaer and S. Sillett for the Hubbard Brook Research Foundation (2009).