18 MIGRATION OF WARBLERS 



on foot can easily keep up with the shifting flocks, but in the aggre- 

 gate it amounts to quite a portion of the whole distance to be traversed. 

 The northward or southward migration of Warblers is not a 

 constant, uniform movement, but rather a succession of waves. 

 Yesterday the woods were deserted, to-day almost every tree is alive 

 with a flitting host of bright-hued migrants ; in a few hours they have 

 passed, to be followed, at longer or shorter periods, by similar com- 

 panies. 



Warblers have the peculiar habit, during migration, of collecting 

 in mixed flocks composed of 'many different species. These com- 

 bined flocks may be large or small, but during the height of the 

 migrating season, it is rather unusual to find a flock composed of a 

 single species. No other group or family of birds presents such com- 

 posite flocks as the Warblers. In northern Minnesota, twenty-three 

 different species, most of them in large numbers, were seen during 

 one forenoon in a single spot in the woods through which they were 

 passing in practically a continuous flock. 



The Warblers, as a whole, are among the later Spring migrants. 

 Feeding on insects, they remain in their southern homes until Spring 

 is well advanced and their food abundant. Their northward move- 

 ment is more rapid than the advance of the season. Thus some Yellow 

 Warblers arrive in the Great Slave region when the average daily 

 temperature is only 47 F. But these same Warblers remain so late 

 in South and Central America, that when they reach New Orleans, 

 about April 5, an average daily temperature of 65 F. awaits them. 

 Thence northward they hasten, covering one thousand miles in a 

 month, and, moving faster than the advance of Spring, find in southern 

 Minnesota a temperature of 55 F., and when they arrive, late in May, 

 at Great Slave Lake, they have gained 8 more on the season. During 

 the whole trip from New Orleans to Great Slave Lake, these birds 

 are continually meeting colder weather. The last fifteen days they 

 traverse a district that Spring requires thirty-five days to cross. Late 

 and rapid journeys of this kind offer certain advantages; fewer 

 storms are encountered and food is more plentiful along the way. 



The mortality of birds during the time of migration is very great 

 and probably no other family suffers so severely as the Warblers. 

 Small in size, with loose feathers ill adapted to withstand storm or 

 rain, they nevertheless cross and recross the Gulf of Mexico, which 

 doubtless becomes each year the watery grave of untold thousands. 

 Warblers are peculiarly susceptible to the attraction of a bright light, 

 and on stormy or dark nights during the period of migration, many 



