Reproduction
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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). |
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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.