I was at Centennial Lake from 6:30-8:00 this morning (Friday March 25).

4  Great Blue Herons
8  Ring-necked Ducks
6  Red-breasted Mergansers
1  Hermit Thrush
1  Fish Crow
2  NORTHERN ROUGH-WINGED SWALLOWS (probably)

Details on the Swallows:

I saw them twice, at 7:00 and 8:00 am.  Both times they flew in from the 
south, made one or two passes low over the water, found nothing to eat, 
and left.  This was at the dam end of the lake, a little bit east of the 
boat ramp.

The birds had brownish backs (couldn't tell back color very well due to 
cloudy day with crummy lighting conditions).  Despite the lighting, it 
was clear they had light brown bellies, especially near the head.  They 
definitely did not have the blazing white bellies of Tree Swallows.  I 
thought they might be juvenile Barn Swallows, but when I looked in the 
Sibley and National Geographic guides, they showed that even a juvenile 
Barn Swallow would have some red or orange on it.  These definitely 
didn't.   They didn't look large enough to me to be Purple Martins.

The early date in the Yellow Book for Rough-winged Swallow is March 16, 
with a dotted line extending from about March 20 to April 3.  The early 
dates for Bank and Cliff Swallows are March 29 and March 27, 
respectively, and those birds are a lot less common so I don't belive 
these birds were either of those species.

-- 
June Tveekrem
Columbia, Howard Co, MD
tweekiebird AT toadmail DOT com