A male Black-throated Blue Warbler caught in a mist-net after responding to a tape recording of another male's song (photo by M. Cline). Shortly thereafter, the bird was safely removed from the net, given leg bands, weighed and measured, and released unharmed.

The final piece of data required for the purposes of this module is warbler density. In order to determine the number of Black-throated Blue Warblers within the study area, the researchers have to mark each individual so that the same one isn't counted twice. A standard protocol in bird studies is to capture birds in mist-nets (see photo) and to place a numbered aluminum and several colored plastic bands on their legs in a unique combination. The bands are of negligible weight and do not impede the birds' movements. They are essential for the purposes of this study because individuals can then be identified by looking at them through a pair of binoculars and noting what band combination they have - for instance, red-aluminum-white-blue or black-aluminum-blue-black. Usually, males are caught early in the season when they are defending their territories, luring them into a net by playing a recording of another male's song. Females don't respond to playback as readily, so they are usually caught later in the season while they are getting on or off their nest. In the end, the total number of individually-marked birds within the study area is used as a measure of warbler density.

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