382 YEARBOOK OF THE DEPARTMENT OF AGRICULTURE. 



tion and the weather is found in the fact that migration does not 

 progress uniformly, but is a series of advances interspersed with pe- 

 riods of rest or inactivity. The average daily advance of migration 

 northward in spring in the Mississippi Valley is only 23 miles, or 

 scarcely more than a half hour's flight; but since there are many 

 stop-over nights on the migration journey, it follows that on each 

 night of flight a correspondingly greater distance must be covered. 

 Probably few night migrants make less than 100 miles at a flight, while 

 spurts of 200 miles or more must be very commo'n. The records of 

 migration near the Mississippi River during one spring indicate that 

 the purple martin made nearly the whole trip from southern 

 Louisiana to southern Manitoba in 12 nights (120 miles a night), 

 although 70 days were spent between the two places. A study of 

 the weather map and of the records of the migration observers makes 

 it probable that the fox sparrow, brown creeper, ruby-crowned king- 

 let, yellow-bellied woodpecker, field sparrow, and purple martin, 

 which were noted at Lanesboro, Minn., the morning of April 1, 1888, 

 had traveled the night before from at least as far south as Daven- 

 port and probably from Keokuk, Iowa. 



It must frequently happen that migrating birds pass from aus- 

 picious into adverse weather or are caught by sudden storms and 

 forced to alight. Next day, when they are noted as arrivals, they 

 are recorded as having migrated during unfavorable weather, such 

 records going to swell the percentage of exceptions to the supposed 

 rule that birds prefer good weather for migrating. 



During spring migration it is probable that birds do not start in 

 the evening except under favorable conditions, and when these 

 conditions hold throughout the night the flight northward is 

 greatly prolonged. This probably explains a hitherto unnoted fact 

 brought to light by a study of the migration data in the Biological 

 Survey. After the northward movement has been checked and then 

 resumed, the birds do not stop when they have made up for lost time, 

 but keep on until they are in advance of their normal position. Sev- 

 eral striking examples of this appear in the. Lanesboro notes in fact, 

 examples are numerous enough to warrant the assertion that in every 

 great migratory movement there are numerous individual birds which 

 are ahead of their normal time. 



MIGRATION AND TEMPERATURE. 



On the night of March 13-14, 1904, an innumerable host of Lap- 

 land longspurs, migrating northward in southern Minnesota, en- 

 countered a heavy fall of soft, damp snow. Weighted by the clinging 

 flakes, the birds dropped to earth, and a large proportion perished. 

 The death toll on the hard, icy surface of two small lakes was 



