Having read Gail Mackiernan's post on the 15th of a Prothonotary and a Tree  
Swallow dueling for one of my nest boxes, while ignoring the other, I went 
over  to Rileys and witnessed the Warbler entering with nesting material.  The  
Swallow was sitting high in a nearby tree.  
 
Last year perhaps the same two birds fought over the old box and when the  
Tree Swallow won, I installed another box and the Prothonotary quickly made a  
nest in it.  I wondered why this year they didn't just pick a box a piece  and 
avoid the conflict.
 
Yesterday PM the question was answered.  I saw a female Bluebird enter  the 
new house and not come out.  My assumption was that she was brooding  eggs.  I 
confirmed that today.  I also opened the other box and found  evidence of both 
the Tree Swallow and the Protonotary trying to nest.  Both  male and female 
Warblers were agitated nearby.  I did not see the Swallows,  but there were 
several there yesterday, including a male I saw enter the old  (left hand) box.
 
The upshot is that I installed yet another house adjacent to the first  two.  
Now there is a possibility of a future triple quadruple.  That  is, three 
species fledging four young apiece, all in the same  neighborhood!
 
BTW, has anyone ever witnessed, or heard of, Bluebirds nesting over  water?  
First time for me.  First time in any of my Warbler  boxes.
 
Bob Mumford
Darnestown