Would a motion detecting sprinkler, set low, work without disturbing the
birds?

lewer the cat in, take him to the vet get his balls cut out

if anyone asks
deny the whole thing

A motion detector sprinkler might work, as cybercat said.  Here are two:
Also
Scarecrow:
http://www.biconet.com/critter/sprinkler.html
Havahart Spray Away:
http://tinyurl.com/nxrjg

Also, do you have a fence around your yard?  If so, it can be "cat-proofed"
by adding guards at the top.  One person posted pictures of coiled aluminum
(fairly wide) mounted to the top of the fence.  He used his to keep his cats
in, so your would have to be mounted on the outside of the fence to keep
cats out.  Some people use wire "mesh" mounted to the top of the fence and
slanted -- slanted in to keep your cats in or slanted out to keep other cats
out.  I would try the motion detector first -- cheaper, easier, and simpler.

MaryL

Photos of Duffy and Holly:      >'o'<
Duffy:  http://tinyurl.com/cslwf
Holly:  http://tinyurl.com/9t68o
Duffy and Holly together:  http://tinyurl.com/8b47e
Recent pics:  http://tinyurl.com/clal7