That's something you should know and Depends if the landscapers uses chemicals, insecticides, herbicides.... to treat your lawn or not.
I never afforded someone to cut my lawn, but as I get older it might become an option.
As far as a good clean grass itself. the birds would enjoy SOME of it I am sure. But the excess grass pile (if large piles/clumps) were left for a few days, would most likely start to mold as it dried. Mold is not your friend.
Bottom line: if the grass is not chemically treated, a small amount would be better than huge piles. I have heard that some grasses can be rather toxic, but cannot cite any specific grass.