Good afternoon!

The active ingredient that you would use to treat the kermes scale in the fall is a pyrethroid - those are shown to be non-toxic to birds. 

Here is more information on pyrethroids and their affect on different species. I would still suggest not allowing the chickens near the tree for 24 hours after you spray. The label is the law on pesticides so make sure to use the correct ratio for the tree!

Thanks for reaching out!

Rachel