Joe Paulson wrote in a message to All:

 JP>  
 JP> I enjoy watching the birds in my back yard but the sparrows have
 JP> monopolized the feeders.I get some chickadees,titmouse,cardinals,
 JP> and a Downy woodpecker.
 JP>    I opened a birdhouse and found 4 dead chickadee babies.Those 
 JP> damm sparrows.
 JP>  
 JP>   I started wondering if ,in these days of electronics,if it was
 JP> possible to make a bird box that would be selective .If a box would
 JP> only allow birds in with the color blue,bluebirds only could  come
 JP> in if the entrance hole was small.Bluejays would be too large.  
 JP> Similiarly,a barrier that would prevent brown birds from entering
 JP> would keep out sparrows,and allow chickadees in.
 JP>   
 JP>   A box could be powered with a small battery and photovoltaic
 JP> cells  on the side of the box.A color detector at the entrance
 JP> would  make choices.I'm assuming daytime conditions.
 JP> The only problem is HOW to activate a barrier for going IN and  for
 JP> going OUT.Maybe some kind of fiberoptic sensor.
 JP> And HOW to design a mechanical barrier that stops a bird.
 JP>  
 JP>   I'm sure a computer program could be written for different 
 JP> situations.  I'll have to think more about this some more.

From what I understand,  the size of the hole and the length of the outside
perch go a long way toward controlling this...

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