From: (Michael C. Smith)

I'd like to rent a Canon EF long lens to photograph birds at Ding
Darling National Wildlife Refuge, Sanibel Island, Florida, USA (300mm,
400mm, 500mm, or 600mm).

Does anyone know a shop in southern Florida that rents?

This list is now four years old so maybe things have changed, but worth a phone
call. One of the companies mentioned is in Miami, but maybe no better than the
ones you already mentioned.

http://www.birdsasart.com/b31.html