[Bristol-Birds] communal roost of Yellow-rumps and curious feeding behavior

From: "Wallace Coffey" <jwcoffey@xxxxxxxxxx>
To: "Bristol-birds" <bristol-birds@xxxxxxxxxxxxx>
Date: Mon, 15 Feb 2010 21:19:29 -0500
South Fork Holston River
Sullivan Co., TN
10 February 2010

An apparent winter communal roost of Yellow-rumped Warblers
was feeding along the South Fork of the Holston River in the
vicinity of the Central Holston Community, last week.

My best estimated was about 200 warblers.

I visited the group three days and found them actively feeding at 
the waters edge.  I first saw them 10 Feb 2009.  They were very 
active and difficulty to count due to darting among the vegetation 
over a measured stretch of 600 yards along the river.  They seemed
to be like a flock of starlings alighting in a field.

In addition, they were feeding in a flycatcher or swallow like method -- 
sweeping along the water surface and snatching prey.

Their behavior was 
remarkable.  Frequently,
four or five would sit in 
the lowest branches 
of small trees and other 
plants, waiting to spot 
tiny insects emerging at 
the water surface.  The 
warblers were perched 
from just a few inches 
to two or three feet 
above the surface.  They
faced the river.
They would sally over 
the water and sweep 
the surface for maybe 
three to 10 feet, 
getting bugs in the air and regularly snatching them in their bills from 
the water surface.

The most remarkable observation was seeing up to four
birds actually flop into the river and pop right back into the
air, not unlike a fishing Osprey.  They were quick to get out
of the river and did not stay to look about as an Osprey
does when it has plunged for a fish.  I was surprised 
they did not seemed to be soaked and would fly to a branch
and continue to hunt.  As far as I could tell,  I never saw the 
same warbler go into the water twice.

It seemed obvious that if this were
a standard method of hunting, many 
birds hitting the surface might soon 
attract, predator fish.  

Those going into the water did not 
seem particularly phased by the 
experience.  They quickly went back
 to hunting.

(in photo above, Yellow-rumped starts near vertical dive)

While this appears to be far from a normal method of capturing
prey, as far as the Yellow-rumped is concerned, one can
imagine that it may happen more often.  Many birds had the
same opportunity but either didn't need to use that approach
or were unwilling.  It is possible that going into the water is
a more daring skill level for some individuals and not an 
intentional option for others.  

Going into the river, with a full generating current flowing past,
could simply be an error in judgment or flight approach problem
for a particular sweep towards tiny prey.  Perhaps the individuals
I observed were less experienced birds that did not have their
skills developed.  Some could be somewhat deprived of food and
were stressed into over attacking the bugs they were gathering.
Otherwise, when an insect was stirring at the surface, a warbler
better get there quickly before another got there first -- this could
be a preferred food source, considering the long cold spell.  
Competition might be greater than we might notice with more
casual observation.

The flock was not rolling past one another as they were feeding.
They mostly moved short distances and took advantages of
hunting perches and available prey.

Feeding along the shorelines of streams and rivers in bottomlands
is a characteristic of Yellow-rumps.  So the two most significant
observations were (1) large and apparent winter communal group
(with possible roosting as such) and (2) individuals going into the
flowing river to capture prey.

Let's go birding . . . .

Wallace Coffey
Bristol, TN
[Bristol-Birds] communal roost of Yellow-rumps and curious feeding behavior

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