191 ] Tracy: White Markings in Birds. 293 



rear-marked birds that run from the nest, habitually, when 

 disturbed. 



In the foregoing paragraphs, the evidence for the revealing 

 function of white rear-markings exposed in flight has been given 

 as common observation, supplemented by psychological analysis 

 of the facts ; and the supposed instance of a concealing function 

 in the case of certain birds has been shown to be quite incon- 

 clusive as regards perching birds. These considerations alone 

 would point strongly to the other interpretation of the markings 

 as advanced by Todd (1888), and rather generally accepted 

 since. They are not, however, the only ones bearing on the 

 question. 



In giving recognition to the theory of directive markings of 

 this type Wallace (1889, p. 222) alluded to their prevalent 

 occurrence among flocking birds, as strengthening the theory, 

 since it was just here that they would be of the greatest use. He 

 did not, however, test the facts regarding such occurrence, or 

 inquire into the question of a significant absence of white mark- 

 ings among non-flocking birds; nor has anyone else done so, 

 heretofore. It is a test that is easily made by segregating all the 

 birds bearing the white rear-marks, and making an inventory of 

 those that remain. In order to judge of the significance of the 

 resulting division it is necessary to know to what extent the birds 

 thus arbitrarily separated differ in habits, especially as regards 

 flocking ; and this is not as easy as might be supposed, among the 

 perching birds. 



The accompanying list gives all the species of open-ground 

 passeriform birds native to North America. Although the white 

 tail-borders occur also on a few arboreal birds, these may be 

 regarded as coming in the class of ' ' top-patterns, ' ' which will be 

 taken up in connection with birds of the forests and thickets. 



