RESIDENCE IN THE ARCTIC 297 



observations. All records of the shortest interval covering the ar- 

 rival reports of each species in any 3 years are therefore plotted to- 

 gether. They comprise 51 species and the range of first recorded ar- 

 rival dates extends from one to 11 days. The median range is between 

 3 and 4 days. Among 30 species the range of arrival dates did not 

 differ more than ±2 days. Since there are few records from 1948, the 

 period comprised is principally from 1949-53. 



The range of first-arrival records in 4 years, shown in figure 13 

 for 37 species, extends from 4 to 12 days. In 23 of the species 4 annual 

 arrivals were recorded within a period of 7 days. In a longer series 

 of years the arrival of the birds can vary more than in a shorter 

 period, and errors of observation likely to prolong the span of arrival 

 records can also enter more frequently. It is my belief that in the 

 4-year series, owing to the influence of erroneous observation, the 

 variations in date of arrival are more apparent than real, and that 

 the first birds of any one species will be found to arrive at close to the 

 same date each year. For example, during 3 years out of 5, with 

 1948 eliminated, 30 species were first seen within 2 days of the mean 

 date of the species and 45 species arrived within 5 days of the mean 

 date. As shown by the records of 37 species during 4 of the 5 years, 

 27 were first seen within 31/^ days of the mean date characteristic 

 of the species and all were reported within 6 days of their mean date 

 of arrival. 



As a seasonal event, migration must be related to annual changes 

 in the sun's heat. In weather records this seasonal variation is in- 

 dictated by measurements of air temperature. These are, by present 

 convention of the U. S. Weather Bureau, measured as daily maximum 

 and minimum, and their mean is designated daily mean temperature. 

 For a longer period of time, the daily mean temperatures are averaged 

 as the mean temperature of the period. C. E. Watson and Robert F. 

 Dale, U. S. Weather Bureau climatologists, kindly made available to 

 me the mean temperature records for 3 Alaskan localities and for 3 

 localities in northern states representative of western, Rocky Mountain 

 and east-central areas through which the northbound migration of 

 American birds must pass. I have plotted in figure 14 winter to sum- 

 mer progress of the monthly mean temperatures in these 3 localities. 

 It shows that in the Arctic the change of air temperature in spring 

 is pronounced, and that in general both rate and amount of rise of 

 spring temperature increases in higher latitudes, while the seasonal 

 change diminishes southward and is imperceptible in the low tropics. 



A sequence of official records of mean temperatures is not yet avail- 

 able at Anaktuvuk, for only in July 1953 was an observaiton station 

 established there by the Weather Bureau. The temperatures shown 

 in figure 5, from Simon Paneak's journal records and from our oc- 



