I just wanted to offer a word of reassurance about the possible reaction of 
these birds to traffic under their nests.  I live near the Y-Cr. Night- 
herons that nest on Sligo Creek either side of Piney Branch.  The birds 
here seem to positively crave company.  Nests have been either directly 
over the road, or next to a noisy playground, or right over the paved 
trail, next to the street.  I think they like to have something to watch 
while on the nest!  Sligo is a narrow park, but even along here, there are 
places they could get further away from constant people going by, if they 
wanted to.  Other pairs may react differently, of course.
Patricia Wood
Silver Spring