Lou -
Since nobody else has answered your question about Purple Martins, I spent
several days hunting around the house for my Purple Martin records.
Although Purple Martin roosting behaviour has been well described in the
literature for many years (see:

Salisbury , MD had a very large Purple Martin roost, which I first
discovered in 1979. It was there until 1986, after which  the ca. 2 acre
abandoned orchard where they roosted was cut down. I don't have any idea
how long the roost had been at the orchard location, but when I discovered
it in 1979 it was LARGE. I don't have notes for 1979 or 1980, but I
estimated ca. 50-100,000 birds in 1981. In subsequent years the number was
closer to 20-30,000 birds. These estimates were always done in the evening.
Roosts in the East of 20,000 - 100,000 birds are not uncommon.

Nobody visited the roost in the morning so we never developed any
experience about how the birds left the roost. In 1985 I did get up at 4:30
AM one morning and went outdoors. Purple Martins were circling overhead;
and a friend that lived about one mile away (away from the roost)
experienced the same thing. The roost was only 4 blocks from my house, so
the birds were presumable from the roost.

My wife and I banded in the roost from 1982 through 1985 with the help of
Carol and Donald Broderick, Jim Stasz, who also lived locally at that time,
and a few other people. Kathy Kliemkiewicz was the master bander in 1981,
while Gail and I were in subsequent years.

Some results:
  In 1984 some birds (125 +-) were at the roost by June 11. Of these about
50% were second year males or older.
Subsequent roost size estimates were made as follows:
June 25, 350;
July 8, 1200-1500;
July 31, 5000;
August 4, 10000;
August 18; 20000+;
August 20-24, 20-30,000+;
August 28, 20,000; and,
Sept. 11, 2-3000.

In 1984 we banded a total of 1,770 Purple Martins. Adults banded exceeded
juveniles until August 5. By August 10, juveniles banded were 4-5 times the
number of adults. By August 27, adults were virtually non-existent (at
least in our mist nets.) There are clearly lots of questions that can be
asked about the chance of catching adults versus first year birds, but we
were not able to address these issues. Although we did occasionally re-trap
a first year bird we never re-caught an adult bird. But we can't really
draw any conclusions from this because we only caught a tiny fraction of
the roosting birds and the number of adults caught was only 351/1770 (in 1985).

It is interesting to note that we did catch birds banded at colonies
elsewhere. The most distant birds were from Benfield, MD (approx. 9 miles
NNW of Annapolis in 1985) and Williamsburg, VA (in 1982). Each birds had
been banded as a young bird at a colony 5-6 weeks earlier. Kathy
Kliemkiewicz color banded birds in 1981 at Saxis, VA and in 1981, at least,
we were able to easily see many of these birds along the power lines where
they gathered before entering the roost.

The bird we banded that was not found dead in a subsequent year at an
Eastern Shore nest box was a bird we banded August 17, 1983 that was
recovered November 2, 1983 in Bonita Springs, FL.

You might be able to find a local roost if you go to the location you saw
the birds. Go sometime around sunset and look for birds in the sky. If
there is a roost nearby (less than a mile), birds should be drifting toward
it. I should mention that there was one day when we stood at the roost and
wouldn't have known there was a roost there until about 10-15 minutes
before DARK, at which time the birds appeared and immediately dove into the
roost. The birds were visible for a period of only 5-10 minutes. We would
not have know the roost was there if we hadn't been there during that five
- ten minute period!

I hope this helps.

Charlie

>Today, 8/17/84, from 7:30 to 9:00 a.m. I was at Mariners Point Park in
>southern Harford county where I witnessed the largest aggregation of
>Purple Marins
>I've ever seen.  Whether unusual or not I've no idea.  There were at least
>3,500 and possibly as many as 4,500 Purple Martins on the power lines at the
>entrance to the park.  At least 90% of the birds were either female or
>juvenile
>birds.  I saw very few identifiable (adult) males.  Do the males take of
>for the
>south earlier than the females and juvies?  Interesting to note that a few
>starlings were scattered in among the roosting martins -- cheek to jowl so to
>speak.
>
>Lou Nielsen
>Reisterstown, MD
>

Charles Vaughn
1306 Frederick Avenue
Salisbury, MD 21801

410-742-7221