540 



THE COMMUNITY 



spring breeding grounds, rear their young 

 in the summer, and return to the wintering 

 grounds in the autumn. Here belong such 

 birds as the numerous warblers, vireos, and 

 thrushes. Winter residents are those birds 

 that move from their breeding grounds in 

 the north to spend the winter in a more 

 southern locaUty where food is more abun- 

 dant. In the northern United States, the 

 junco is a common winter resident. Tran- 

 sient visitants are those birds that pass 

 through a locaUty, generally twice each 

 year, first in the spring, en route to the 

 northerly breeding area, and again in the 

 autumn, en route to the winter habitat. 

 Obviously, this last group is interpreted 

 upon a geographic point of reference, since 

 many summer residents of one locality 

 would be transients in other areas. Partial 

 migrants are those species which include 

 members that are migratory in the northern 

 portion of their range, and are more or less 

 permanent residents further south. In this 

 last category it is difficult to state whether 

 such a species is undergoing a southward 

 shift of the entire population, or whether 

 two population fragments are represented 

 within the species, a migratory northern 

 group and a sedentary southern group. 

 There are also erratic wanderers, that move 

 about erratically except during the breed- 

 ing season. Certain herons belong in this 

 group. These categories, well known to 

 students of North American birds, are gen- 

 erally applicable. 



From the community point of view, all 

 these truly migratory animals are temporary 

 residents, integrating into a series of dis- 

 parate community food webs upon a sea- 

 sonal basis. Since their period of inte- 

 gration for any given community varies 

 with the local weather, amount and Idnd 

 of foods available, and the endogenous 

 stimuli that control such phenomena as 

 breeding, oviposition, and rearing of young, 

 the eflFect upon the food web is variable. 

 In any event, the eflFect of summer residents 

 on communities that lie along the route of 

 the migration, and at the ends of such def- 

 inite paths of migration, is important and 

 tangible. 



Birds, especially, follow rather circum- 

 scribed paths of migration or -ftifwai/s, in 

 correlation with the seasonal advance of 

 certain isotherms (Fig. 186), The majority 

 of birds migrate at night. Such birds travel 



from 200 to 400 miles per night, and 

 recuperate and feed during the day. It is 

 during this daytime interval that the vernal 

 waves of warblers and other passerines im- 

 pinge upon the food webs of communities 

 along their line of migration. At these rela- 

 tively briefs intervals they prey upon the 

 then increasing populations of insects, and 

 are in turn preyed upon by resident preda- 

 tors. 



As a general tendency, the effect of 

 northward migrating birds upon the com- 

 munities along the flyway is inversely pro- 

 portional to the distance from their south- 

 ern wintering grounds. This strange rela- 

 tionship is a consequence of many factors. 

 The rate of progress of an individual may 

 be much greater than the progress of the 

 species as a whole, and the advance of a 

 migrating population appears to be cor- 

 related with the climatic conditions along 

 the flyway. Thus, although a particular bird 

 may cover several hundred miles in one 

 night, the average daily advance from New 

 Orleans to southern Minnesota, up the 

 much-used Mississippi flyway, is about 25 

 miles a day for all species populations. As 

 the species continue to move northward, 

 the average rate of travel increases with the 

 rapidly advancing vernal season. The daily 

 average between southern Minnesota and 

 southern Manitoba is 40 miles; from south- 

 em Manitoba to Lake Athabasca, 72 miles; 

 and from here to Great Slave Lake, 116 

 miles. Five species continue on to Alaska, 

 on an average of 150 miles daily rate of 

 migration. As the species move northward, 

 their rate of travel increases, and their con- 

 sequent effect upon the communities along 

 the migration path becomes less and less, 

 until the breeding grounds for any partic- 

 ular species is reached, where, as noted, the 

 impact is prolonged and important. 



On the other hand, some birds migrate 

 during the daylight hours; for example, 

 robins, kingbirds, and swallows. Such birds 

 feed as they travel, and traverse con- 

 siderable territory in search of food. 



The migration route may be long; con- 

 sequently migrants come in contact with a 

 proportionallv great many communities. 

 The bam swallow, for example, winters as 

 far south as Argentina, and breeds as far 

 north as the Alaskan Yukon, a distance of 

 some 7000 miles. 



In this brief survey of migration as it 



