MOVEMENTS OF BIRDS IN RELATION TO THE WEATHER. 389 



panying map (fig. 30) the first of the kind ever published in this 

 country giving the spring advance of the season, as shown by 

 isotherms, in comparison with the corresponding equal flight lines or 

 isochronal lines of the summer warbler. This bird was selected be- 

 cause it winters entirely south of the United States and during migra- 

 tion occurs from ocean to ocean and from the Gulf of Mexico to 

 Canada. By April 10 the warbler is noted across the whole country 

 from South Carolina to California. At this date the foremost rank 

 of birds the equal flight line of April 10 is closely coincident with 

 the isotherm of 62 at the Mississippi River, slightly in advance in 

 South Carolina and Arizona, and still farther north on the Pacific 

 slope. These differences in the West constantly increase as the season 

 advances. 



During the 10 days from April 10 to April 20 the isochronal line 

 of the summer warbler moves to Virginia, southern Illinois, and north- 

 ern California, which brings it on April 20 approximately along the 

 isotherm of 58. The birds have moved north faster than the season. 

 During the whole trip from the Gulf of Mexico to Canada, for each 

 10 days of the spring flight there is a remarkably uniform drop of 

 4 in the average temperature at which the van of migration is mov- 

 ing, and while the earliest migrants reach the United States when 

 the daily mean temperature is about 60 F., those which nest in north- 

 ern Canada reach their northern summer home when the daily mean 

 temperature is below 45 F. 



On April 30 the earliest summer warblers have reached northeast- 

 ern Nebraska, while to the westward the van is 350 miles in the rear 

 and is just appearing in southeastern Colorado. This retardation 

 of migration is due to the increasing elevation of the land from the 

 Missouri westward, which causes a decrease of temperature. On the 

 great western plains, where the slope is about 6 feet to the mile, bird 

 migration is retarded on the average one day for each 300 feet in- 

 crease in altitude. For steeper slopes there is still greater retarda- 

 tion of migration relatively to the increase in altitude. 



On the Pacific coast, from April 10 to 20, the summer warblers ad- 

 vance about as fast as the spring, but within the next 10 days they 

 appear in southwestern British Columbia, having averaged 75 miles 

 a day two and one-half times the speed of those on the Atlantic 

 slope; also in these 10 days they have gone from a temperature of 

 58 F. to one of 48 F., while the eastern birds were dropping from 

 58 F. only to 54 F. 



The map shows also some interesting facts as to the route of the 

 migration flight. All the numerous records of the summer warbler's 

 arrival in southern Texas from San Antonio to Brownsville are later 

 than those of northeastern Texas, showing that the early migrants 

 reach the northeastern part of that State by a direct flight over the 



