This behavior is technically called digital fluttering. i am also starting a study on this behavior. It has been found in many spp. Southern toads, Phyllomedusas, American hylids- when they are froglets, etc... Nobody has been able to narrow it down to the real cause or why they do it, but it has a strong correlation with feeding and hunting. I'll post when the results of my study are complete.
Enjoy
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Justin
stk18119@loki.stockton.edu
D.auratus
D.leucomelas
D.tinctorius (lorenzo, yellowback, citro, pb, oyopak,etc.)
D.azureus
D.ventrimaculatus
D.pumilio (blue jeans, red, bastimentos)
P.aurotaenia
P.bicolor
E.tricolor
blah
blah
blah
too many treefrogs and toads to list!