Migration and Distribution of Nortli American Birds. 19 o 



cealed larvae and small insects from under the edges of the 

 shaggy bark. They were seen to frequent the terminal 

 branches, and seek their food amidst the starting foliage, 

 after the only throng of their brother warblers began to 

 arrive and winge insects were becoming numerous. 



This matter of being able to obtain sufficient insect food 

 in various ways, must give this warbler an advantage over 

 its allies, which I think may possibly account for its im- 

 mense numbers as compared with some of the other species. 



Does not this rule apply to other birds as well, and in part 

 explain the reason why our earlier species are among our 

 most abundant birds? 



Birds in districts of homogeneous characters are restricted 

 to certain areas by isotherms, but in country not entirely 

 suitable for certain representative species, would not the 

 isotherm be only a minor consideration as affecting their 

 distribution ? 



Can we properly judge of the extent of the faunal areas 

 in any specified section, until the whole section is 

 practically inhabitable for them during the warmer months ? 

 In other words, as civilization progresses and timbered dis- 

 tricts and swampy tracts become converted into meadows 

 and farming lands, may not the avian fauna of an adjoin- 

 ing southern district extend northward until checked by the 

 isotherm that constitutes the actual boundary of these divisi- 

 ons ? Brown County at the lower terminus of the Fox 

 River Valley is the last of a series of counties connecting 

 with the warmer sections of southern Wisconsin. and north- 

 ern Illinois, that are throughout habitable for such birds as 

 prefer meadows and cultivated fields. 



As single field sparrows, orchard orioles, and wood thrushes 

 are found here only at irregular intervals, must we not feel 

 confident of the close proximity of the northern limit of the 

 AUeghanian fauna, of which they are representative 

 members ? 



Shiocton, Outagamie Co., the place of Mr. F. L. Grund- 

 tvig's diligent and thorough observations is almost directly 

 west from us at De Pere, yet notwithstanding this, he has 

 never on any of his varied excursions found the lark finch. 



