Hi Maeve and others -

This is not an unusual event at all.  I have studied cormorants on Lake 
Champlain for the last 7 years and have observed this often.  The 
cormorants are most likely from the Four Brothers Islands, just over the 
Vermont boarder in New York.  They forage in groups and follow each 
other from place to place.  This social foraging is common in colonial 
waterbirds, such as cormorants and other seabirds.  So what you observed 
sounds like the group either stopped foraging or was looking for a big 
group of fish.  Since they were fairly close to shore, they would most 
likely encounter schools of yellow perch. 

Cormorant social foraging: a quick synopsis.  When the cormorants seek a 
place to find fish, there are usually very few of them that actually 
search for food.  Most of the cormorants follow others to foraging 
locations.  If you are into game theory, the play what is called a 
producer-scrounger game.  Those that search are the producers and the 
rest are scroungers.  Often the cormorants will raft up next to their 
nesting islands in the morning and wait.  Suddenly, they all take off in 
a focused direction - probably following one of the producers.  Now the 
producers may have either independently discovered some school of fish 
or is heading off to a location that it foraged at the day before.  Once 
they get to that location, they might raft up again if they have not 
found fish.  Once they do find fish, they will eat in a group.  The 
cormorants at the head of the group will dive and catch fish, the next 
few will follow suit and the whole flock will start to move forward.  As 
the first divers come up for air, they will inevitable fall to the back 
of the group.  These birds will then fly forward, producing a pattern 
that looks like cormorants playing leap frog.  Once the school of fish 
becomes unavailable (probably because they find hiding places) the 
cormorants stop foraging.  They will raft up again.  It is at times like 
this that I have also seen them change directions in unison.  I suspect 
they all are now playing the scrounger, and are waiting to see if 
another cormorant flies by from another foraging area.  Sometimes they 
will swim in the raft for an hour or two.  Then suddenly, they all fly 
off somewhere else to forage.  I find this behavior interesting, as you 
can tell by my long post.

On another note, I would encourage caution when trying to age cormorants 
by plumage.  I know the field guides show that all black individuals are 
adults and that those birds with light plumage in the breast and neck 
are subadults; however, this is not well described in the scientific 
literature and only holds for certain times of the year.  Early in the 
breeding season, I think this pattern holds well.  But later in the 
summer (like now) and through the winter, this pattern really falls 
apart.  Juveniles hatch mid to late June and fledge about 6 weeks 
later.  The juveniles will have a dark brown plumage that I think is 
indistinguishable from adult plumage unless you have the bird in hand.  
Soon after this, the adults and maybe the juveniles appear to go through 
a molt and replace body feathers with lighter colored plumage.  I can 
say this because of series of observations from individually marked 
birds.  I have seen photos that were taken during the winter of 
cormorants that we color banded as adult breeders (that were all 
black).  During the winter, these birds looked just like the subadults 
that field guides describe. 

Enough already - I know.  Thanks for the post on the cormorants.  I 
don't hear positive or curious comments about them often.

Adam

-- 

Adam Duerr
Vermont Cooperative Fish and Wildlife Research Unit
University of Vermont
212 Aiken Center
Burlington, VT 05405
802.324.0854 (cell)



> Date:    Thu, 9 Aug 2007 13:32:41 EDT
> From:    -  Maeve Kim <[log in to unmask]>
> Subject: raft of cormorants
>
> This is a late listing because my beloved iMac was out of commission for a 
> few days. 
>
> On Sunday, I saw something that was completely new to me: a raft of 
> approximately 350 double-crested cormorants in the lake at Shelburne Farms. The birds 
> weren't going anywhere, so this is an actual count, not an estimate. Many were 
> juveniles. They all were floating serenely, every single one facing in the 
> same direction and then all but one rebel reversing direction together. Is this 
> common? Do they nest or roost on one of the nearby islands?
>
> Maeve Kim
> Jericho Center
>
>
> **************************************
>  Get a sneak peek of the 
> all-new AOL at http://discover.aol.com/memed/aolcom30tour
>
> ------------------------------
>
> End of VTBIRD Digest - 8 Aug 2007 to 9 Aug 2007 (#2007-221)
> ***********************************************************
>