We just returned from a two-week shoot which included shorebirds and migratory birds in Tule Lake, CA and the Klamath basin in OR. The only lens my husband used nearly the entire time was his 600mm f/4 II for birds in flight, birds standing in glass calm water, and even small animals such as a mink on the bank of a river. The detailing is amazing. From 40 ft. the mink's head shows the skin pores on its nose and every lock of water-drenched hair on its head! I drove the RV while he shot out the window on a bumpy gravel road. This lens brings your subject up close and really personal. Birds in flight were so detailed we could see the scales on their feet tucked in while flying. It's a pricy lens but surprisingly light and will out-do your expectations. Have your wife get you one for your birthday like I did.
You don't happen to have a twin sister who is available? Do you?