Density

Reproductive success (number of young produced per pair per year) in relation to the density of Black-throated Blue Warblers (individuals per 64 ha) on the main study plot at Hubbard Brook from 1986 to 1999 (data from Holmes, Rodenhouse, and Sillett).

One of the most exciting results to come out of the long-term research on Black-throated Blue Warblers at Hubbard Brook was the strong relationship between density and reproductive success. As the number of birds in the study area increased, the average number of young raised decreased from a high of approximately four to a low of about two, so the average reproductive success was lower at higher densities.

Researchers discovered that high densities of warblers had two effects on the population. First, warblers were packed more tightly into the best habitat, so that territories became smaller and the success of all the birds was lower. Second, when the population was large, birds also set up territories and bred in habitat that they didn’t breed in when the population was smaller. This habitat isn’t quite as productive, since birds breeding there had even worse success than those packed into the better habitat. Through these two pathways, the reproductive success of the whole population was lower when the population is large.