One of the many grid flags that are used to map territory boundaries and nest locations within the bird plot at Hubbard Brook (photo by S. Sillett). This particular one is "J-1.5", meaning it is along the J line in between the 1 and 2 lines (lettered lines run perpendicular to numbered ones). Flags occur every 25 m along these lines.

Each year, a team of field workers arrive at Hubbard Brook in early May and await the spring arrival of migratory birds. They survey the study area daily, listening carefully for the characteristic song of the Black-throated Blue Warbler (listen yourself: ). Males arrive first, and when they do their territory locations are mapped (see photo). They are then monitored on a daily basis to detect females (males follow around their females to guard them, making it easy to figure out whether a male is paired with a female or not).

The arrival of females is always an exciting time, because at that point the warblers are monitored even more intently in order to find nests. Females can be sneaky during the nest-building phase, by being quiet or moving through dense vegetation where it is hard to track them. But, over the years researchers at Hubbard Brook have figured out the best nest finding strategies. First, they find the female, either by watching the male, looking for movement in the understory, or by listening for "chip" notes that are produced by the female. Then, they watch to see if she picks up spider webs, birch bark, dead grass or any other material that could be used to build a nest. If she does, then the researcher will try to follow the female back to her nest, all the while being as quiet as possible so as not to disturb her. Of course, sometimes it takes a few tries before the nest is found.

Four nestlings packed into a Black-throated Blue Warbler nest (photo by N. Desnoyers).



Once a nest is located, a flag is placed on a nearby tree and the nest is visited every few days to determine when the female starts to lay eggs, how many eggs she lays, and if the nest gets depredated or survives. When the nestlings are 6 days old, they are briefly removed from the nest to place bands on their legs, to measure and weigh them, and to collect a small blood sample. Each band is a small aluminum ring with a unique number that can be used to identify the bird if it gets caught again. A few days after banding, the nests are visited again to determine if the nestlings were successful in leaving the nest - a process called "fledging." This information forms the basis of figuring out the birds' reproductive success, which we define here as the average number of young produced (i.e. that "fledge") per warbler pair in a given year.


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