-2 2 6 Recent Literature. \\v\ 



Butterfield comments briefly, considering the breadth of the subject, on a 

 few of the many theories respecting the origin, manner, and causes of tbe 

 migration of birds. His criticisms are mainly destructive rather than 

 constructive, and he offers us little in the way of a clear and sharp formu- 

 lary of even his own views on the several points discussed. His remarks 

 are grouped under the subheadings (i) ' Incentives to Migration,' (2) ' Mi- 

 gration Routes,' (3) 'How do Birds find their Way ?' and (4) 'Origin of 

 Bird Migration.' 



Taking the last topic first, the author seems to accept, in a general way, 

 the hypothesis that "the changes of climate induced in the northern hemi- 

 sphere by the decline of the Glacial Period as the ultimate cause of migra- 

 tion in this part of the globe"; although, under present conditions, "the 

 migratory impulse tends to strengthen in some forms and to weaken in 

 others." He cites the case of the varying degrees of migration presented 

 bv different forms of the Horned Larks ; but almost any widely dispersed 

 group of closely related birds offer equally pertinent illustrations of this 

 rather obvious condition. 



Under ' Incentives to Migration ' he believes that too much stress has 

 been laid upon "scarcity of food" as the impelling cause of the autumn 

 migration, inasmuch as in species of wide latitudinal dispersion, the 

 places of individuals that live in the middle districts are taken, for a time, 

 later in the season, by individuals of the same species from further north. 

 "While admitting," he says, "that want of sustenance may prompt the 

 autumn migration in some cases, it may be doubted whether it is so 

 important a factor as is generally supposed." He believes that the com- 

 pletion of the moult and (in adults) "the decline of the stimulus of repro- 

 duction " are also factors. The first is undoubtedly an important one in 

 the case of certain groups of birds, as the Anserine series and some 

 others, which suffer simultaneous loss of the wing-quills during moult 

 and are thus for a time almost flightless, and it doubtless affects others less 

 effectually crippled by the annual moult. It is also true that many migra- 

 tory species of birds lead a less sedentary life after the close of the breed- 

 ing season, and in many cases become wanderers, quite deserting their 

 immediate breeding grounds, and in some cases even depart for more 

 southern latitudes long before the decline in either temperature or (appar- 

 ently) the food supply would necessitate such a movement. Yet, sooner 

 or later, migration from these causes would become compulsory, and 

 while under present conditions few migratory species wait for a crisis 

 from such conditions before moving from their breeding stations, it is 

 hardly to be doubted that far back in the history of bird migration they 

 were the impelling factors. They may even still be accounted as the pri- 

 mary cause, and that for reasons not at present quite clear to us many 

 species anticipate the ultimate necessity by a movement somewhat in 

 advance of compulsory conditions, they being free to roam at large as 

 soon as the restraining duties of reproduction are fully past. 



Regarding the incentive to the spring migration, Mr. Butterfield hardly 



