He achieved worldwide attention for his rescue operations of the whooping crane (Grus Americana) in the 1940s and 1950s.Allen helped save the roseate spoonbill from extinction.
Allen was a pioneer in early field biology and led large conservation efforts around the world to save the whooping crane (Grus americana), roseate spoonbill (Platalea ajaja), and the flamingo (Phoenicopterus ruber).
He discovered that the whooping crane migrates to Aransas Refuge during the winter, because of viable food sources and mild climate.
Allen discovered that unlike the flamingos the whooping crane can reproduce in small numbers and live a long time.
Later Allen began three years of fieldwork in the Caribbean, and his studies focused on the entire range of the American flamingo (Phoenicopterus ruber).
This evidence proves that the flamingos are low on the evolutionary scale, and their breeding practices a complicated and can confuse the birds.
Allen discovered that flamingos unlike other birds need over three hundred individuals to successfully carry out the mating ritual.
Gunshots, planes flying overhead, hurricanes, flooding, damming and diking bodies of water all caused the population to decline.
Allen gained national popularity and news coverage when he spent eight years looking for the last remain nesting site for whooping cranes.
Allen concluded that overdevelopment; habitat loss and unregulated hunting were the main causes for low numbers of whooping cranes, roseate spoonbills, and flamingos.