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THE AMERICAN NATURALIST [Vol. XLII 



determined from the data of this trip for the period of 

 the fall migrations, with the mid-summer average for 

 the same section of the state, as determined last July, 

 shows an interesting difference which leads us to con- 

 sider the effect of the autumnal movement to the south on 

 the numbers of the local bird population. On the above 

 trip across the state, made between August 28 and 

 October 17, 1906, a general average of 579 native birds to 

 the square mile was found, while the corresponding mid- 

 summer average for the present year is 537 native birds 

 to the square mile— a difference of 42 birds to the mile, or 

 nearly 8 per cent., in favor of the fall population. 



Native Birds per Square Mile, Fall (1906), Summer (1907) 



Was this difference due to the fact that the fall migra- 

 tion was in progress when last year's observations were 

 made? That is, does the migration movement begin first 

 at the north and result in a local wave of increased num- 

 bers, birds coming in from the north earlier and faster 

 than the resident species leave for the south? It is 

 possible to answer this question by reference to the data 

 of the paper just cited. 



An analysis of the list of species identified on last 

 year's autumnal trip shows that 481 per square mile of 

 these birds were summer residents, still remaining, and 

 that 98 per square mile belonged to migrant species, on 

 their way to the south. The summer residents still pres- 

 ent in this autumnal period were thus 56 per square mile 

 fewer than the resident birds of the present summer. 

 That is, 56 summer residents for each square mile of 

 central Illinois had gone south, on an average, and 98 fall 

 migrants had, on the other hand, come in to take their 

 place, the difference between these numbers giving us 



