Widmann A Preliminary Catalog of the Birds of Missouri. 9 



tical value for the observer or hunter, as it affords him oppor- 

 tunity to find for a longer time and in greater numbers birds 

 which under other, for them more favorable, conditions would 

 have passed on at once or with little delay. The abundance and 

 scarcity of migrants in transit through our state is therefore 

 largely dependent on the time at which prolonged cold or warm 

 spells strike our region. Should the cold spell set in at the time 

 when the bulk of ducks is present, the hunter will have cause to 

 rejoice; but should their arrival be delayed and then be followed 

 by a decided and extensive warm period, the bulk will pass on, 

 proceeding on their way to the northern breeding grounds, and 

 the hunters will find the season a poor one. This is the case with 

 all transients and is the reason why we find certain birds common 

 in one year and rare in another; it is especially noticeable in 

 May when the presence of north-bound warblers, thrushes, and 

 others, is greatly influenced, shortened or lengthened, by these 

 warm and cold waves or spells. 



A great diversity is also found in the seasonal distribution of 

 precipitation which in a year amounts to thirty-four inches in the 

 northwest and forty-six in the southeast. May and June are the 

 months of greatest precipitation, and five inches of rain fall in 

 each of these months throughout the state. This rainy season 

 is generally followed by dry periods in July and August, when 

 droughts of several weeks duration are not rare. But there are 

 no fixed rules: while in some years no appreciable precipitation 

 takes place from early July to September, in other years rainy 

 periods occur almost every week throughout summer. State- 

 ments of average precipitation, as of average temperature, give 

 no insight into the weather conditions of a region. Four inches 

 of rain may fall within twenty-four hours and not a drop fall for 

 a whole month, or the four inches may come down in install- 

 ments of half an inch distributed over the same period. 



The effect of such different conditions on bird life is remark- 

 able. Heavy storms with copious downpours in the height of 

 the breeding season destroy immense numbers of broods, and 

 long droughts make insect life so scarce that some species of 

 birds find it impossible to provide enough food for their young. 

 The increase or decrease in the number of individuals of a species 

 is therefore often the direct result of favorable or unfavorable 

 weather of the preceding summer. 



While spring migration is chiefly influenced by temperature, 

 fall migration is controlled in a large measure by precipitation. 



