1154 U.S. NATIONAL MUSEUM BULLETIN 237 part 2 



grubs and larvae. Hamilton (1933) found small stoneflies in the 

 stomachs of foiw out of five tree sparrows collected in midwinter near 

 a small stream near Ithaca, N.Y. He suggests that any warm day 

 throughout the winter will bring thousands of transformed imagos to 

 the vegetation surrounding such streams, and may account for the 

 little flocks of birds that haunt such situations. 



Knappen (1934), in summarizing the material in the Biological 

 Survey files, states that in the 14 midwinter stomachs contain- 

 ing animal food, proportions varied from 1 to 90 percent of the 

 total stomach content. Insects and spiders were found in adult form, 

 as eggs, larvae, and pupae. Of a total of 550 stomachs on file, 38 

 contained animal food. During October animal matter comprised 

 a third of the total. From November through March it ranged well 

 below 10 percent. By April it had increased to almost a third again, 

 and a single May specimen from North Dakota contained 100 percent 

 animal food. 



"When they arrive on their northern breeding grounds, however, 

 tree sparrows find conditions nearer winter than summer. Seeds con- 

 tinue to form the greater part of their diet until past the middle of 

 June, when insects first become active (Baumgartner, 1937c). By 

 mid-August the brief nesting season is over, and the proportion of 

 seeds consumed again approaches the 98 percent maintained during 

 the winter. 



Young birds up to 3 or 4 weeks old are fed almost 100 percent 

 animal matter, with a trace of Rosaceae leaves for their greens. 

 When they begin to forage for themselves about the first of August, 

 seeds and fruit are taken to a limited extent, their proportions gradu- 

 ally increasing. The last stomachs examined, on August 19, still 

 showed a considerably larger proportion of animal food than did those 

 of the adults of the same date. 



A trace of gravel is first found in young birds of 3 days. At 5 days 

 and thereafter gravel may compose from 10 to 15 percent of the 

 stomach content. In August when they are feeding independently 

 the gravel content increases to adult proportions, ranging from 15 

 to 70 percent, the majority about 35 percent. 



In summarizing the summer food, it was noted that the outstanding 

 vegetable item was the seeds of the genus Carex. These were found 

 throughout the summer in all adults and juvenals that contained any 

 vegetable matter, though they were not found in the nestlings. At 

 least six species of Carex were distinguished in a single bird. In late 

 July and August seeds of the crowberry (Empetrum nigrum), cran- 

 berry (Vaccinium vitus-idaea) , and bulb lets of the alpine knotweed 

 {Polygonum viviparum), became an important part of the menu. 

 Grass seeds, which form such a large proportion of the winter food, 



