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The Kolkata Metropolitan area at present plays host in winter to
about 85 (120 historical) migrant species. Of these some are extremely
rare and may not be observed every year. About 50 of these are
regularly recorded by one or more observers.
Over the last 7 years, members of
Bengalbird googlegroup have been tracking 1st arrivals of these 50 odd
species in the Kolkata and West Bengal area. Migration data plays a
critical role is assessing how birds adapt to change in environmental
conditions. By and large, bird species have a regular migration cycle
and finding a baseline enables us to observe and record variances.
Such statistics are extremely scant in
eastern India and this is our 1st effort in establishing such a
baseline through community participation in which more than 70-80
birders participate actively over the years. From our data we have
chosen 25 species for this presentation based on visibility, ease of
identification, regularity and sequence of arrival.
The earliest species to arrive in our area are the Barn Swallows
and Brown Shrikes, who may start arriving as early as in July. The
waterfowl pick up the rear and start being observed around end October
and early November. Members observing migrants report 1st arrivals
which are posted to the group forum. Two different sightings are
usually needed to confirm the 1st date of arrival (unless the 1st
sighting was photographed).
The following charts highlight the range of arrival periods. They
are based on 1st date of sighting of the species in our area over the
period 2005 to 2010. The coloured areas indicate the possible arrival
time.
Arrival trends are
mirrored in the chart below. It is based on
1st arrival dates of
different species over successive fortnights. The chart
is
not species specific, but
arrival specific.
For example, if a White Wagtail arrives in the 1st fortnight of
September 2005 and September 2010, both records carry equal weight
in the final output.
In 2010, 12/25 species arrived in September which
is an useful test
for the trends shown.
Sumit Sen ©
Kolkata, India
Oct 2010.
