377 



suits of a trip across central Illinois made by Gross and Ray 

 during the fall of 1906. A comparison of the general average of 

 the bird population, determined from the data of this trip for the 

 period of the fall migrations, with the midsummer average for the 

 same section of the state, as determined July, 1907, shows an inter- 

 esting difference which leads us to consider the effect of the autum- 

 nal movement to the south on the numbers of the local bird popu- 

 lation. 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 midsummer 

 average for the year 1907, 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 migration was 

 in progress when the observations for 1906 were made? That is, does 

 the migration movement begin first at the north and result in a local 

 wave of increased numbers, birds coming in from the north earlier 

 and faster than the resident species leave for the south? It is pos- 

 sible to answer this question by reference tO' the data of the paper 

 just cited. 



.An analysis of the list of species identified on the autumnal trip 

 of 1906, 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 present in this autumnal period were thus 56 per square mile 

 fewer than the resident birds of the summer of 1907. 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 the excess of 42 birds per square mile of fall over sum- 

 mer. This temporary increase of 8 per cent, in autumn in the 

 average number of our birds is thus evidence of a wave of conden- 

 sation running southward in consequence of the earlier beginning 



