I went on a short bike ride on the C& O canal from my house (near single lane Bridge along MacArthur) up to the American Legion bridge and back.  It was the first local birding I've done in a month so plenty of first-of-season birds for me.
 
Highlights:
 
Common Merganser - two males swimming in Cabin John creek, just where it ends at the canal.  Have never seen them there before and will keep an eye out for females and young.
Double-crested Cormorant - back on the colony opposite mile marker 8.  I could not count the nests due to high water on the Potomac.
Peregrine falcon - no sign - probably in the box, but high water restricted vantage points.
White-eyed Vireo - one singing near single lane bridge on MacArthur.  They don't breed here, so presumably just passing through.
Warbling vireo - about 5 heard in total
Northern Parula - one every 1/3 mile or so on the canal
Prothonotary Warbler - two singing males, one by the cormorant colony and one about 1/4 mile further down
Yellow-throated Warbler - only one, at the bridge over the canal down from the path from the single lane bridge
Yellow Warbler - singing male at about mile 7.5
Yellow-rumped Warbler - about 20 at various spots
American Redstart - one near Y/T Warbler spot
Louisian Waterthrush - one singing near Lockhouse 8, another working the edge of the canal just up from there
Both orioles, down by the river as well as at the end of our street
 
Good birding,
 
Clive Harris
Cabin John, MD