No. 412.] SMALLER NORTH-AMERICAN SHRIKES. 285 
or, in other words, that relatively to the tail, the wing is longer in 
migrans than in ludovicianus, as his theory would require. 
Palmer’s explanation of length of wing in migrans as the 
result of habit is plausible. However, it has not been proved 
that shrikes which migrate northward actually fly any more 
than those remaining in Florida; moreover, it seems to me 
that there are other possible explanations. I am inclined to 
think that we have here a case of the condition already noted 
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Depth in mm. 8.4 8.6 8.8 9o 9.3 9.4 9.6 9.8 10.0 10.2 
Fie. 6.— Frequency polygons for depth of bill. 4, 126 males from entire region; B, 54 males from 
ustroriparian subprovince; C, 21 males from Appalachian subprovince. 
by Allen — a tendency towards greater length of tail in tropical 
birds. Unfortunately I cannot prove this, — any more than 
Palmer does his hypothesis, — for it has not been possible to 
determine the actual ratios of tail length and wing length with 
reference to the general size of the bird. 
2. Length and Depth of Bill. — In Fig. 5 are given frequency 
Polygons of variations in length of bill for the entire region and 
