HARRIS' SPARROW 1267 



Life expectancy. — As the Harris' sparrow lays an average of 4 eggs 

 for the single brood it raises each year and has comparatively few- 

 natural enemies, it may be assumed that when the birds leave the 

 nesting grounds the ratio of young to adults is approximately 2 to 1, 

 and the population is composed of roughly 66 percent immatures and 

 33 percent adults. An analysis of my banding records for 12 years at 

 Stillwater, Okla., in the heart of the winter range, shows the proportion 

 of young to adults fluctuating through the non-breeding season as 



follows : 



Percent Percent 



Month Adults Immatures Totals adults immatures 



Totals 608 820 1,426** 42 58 



*By the end of April all birds are in adult plumage. Only birds of known age 

 are included here. 



**Total includes each return tabulated in month it returned. 



Thus during the autumn the young of the year outnumber the 

 adults by the anticipated two to one, and the ratio decreases only 

 slightly in December. By January the effects of the higher mortality 

 rate among the immatures begin to be evident; the proportion of 

 young to old becomes nearly equal and apparently remains so through 

 March. The April increase of immatures to 73 percent is based on a 

 small sample of only 41 birds, which may well lack statistical signifi- 

 cance. Far more likely, it suggests that the older birds tend to leave 

 the wintering grounds for the north some weeks earlier than the first- 

 year individuals. Also these ratios may be biased against the adults 

 by the tendency of immature birds to enter the banding traps more 

 readily in the spring as Stevens (in litt.) has suggested. 



The higher mortality in the young birds is also demonstrated by 

 shrike predation at the banding traps, which occurs at Stillwater 

 almost entirely during mid-winter when the age ratio is approximately 

 balanced. During the 18 years of banding there, shrikes killed in the 

 traps a total of 36 immature Harris' sparrows and only 6 adults, one 

 of which was somewhat crippled by a leg injury. The percentage of 

 first year mortality cannot be determined from the data available, 

 but it is probably in the neighborhood of 70 to 80 percent. 



Annual mortality and longevity in the birds that survive the first 

 year may be estimated from the numbers of wintering birds that return 



