298 University of California Publications in Zoology. [VOL. 6 



When, however, the bird takes wing a wholly different prin- 

 ciple comes into play. Suppose it be the goldfinch that has left 

 its perch. What we actually see is a pair of wings opened and 

 closed, alternately revealing and concealing a pattern which finds 

 no background to blend with, because it consists of intermittent 

 flashes of white, not haphazard like the ruffling of leaves, but 

 rhythmical, emphasizing the essential features of the flying bird. 

 This is not speculation but a description of facts it is what one 

 sees in the field. Those w T ho are unfamiliar with the bird named 

 may recall a similar flight-effect in the black-throated blue 

 warbler (Dendroica caerulescens) , and even more striking pat- 

 terns in the common shrike (Lanius ludovicianus)., the mocking- 

 bird (Mimus polyglottos) and the "black mocker" (Phainopepla 

 nit ens). Concealed wing patterns are not the only ones that 

 become conspicuous in flight. Obviously any white pattern 

 located on primaries, secondaries, tertials, or wing coverts, will 

 be expanded to the greatest extent during the motions of flight. 

 It happens that even the comparatively obscure wing bars of the 

 lazuli bunting (Passerina amoena) are easily distinguished as 

 white bands on the flying bird. The white of scapular feathers, 

 as in the magpie, is emphasized in the same way. Tail blotches 

 on some warblers reveal their whereabouts as they flit from twig 

 to twig, rather than conceal them. I have not, therefore, 

 attempted to distinguish various classes of top markings among 

 arboreal birds with a view to finding special functions for each. 

 It is precisely such attempts that have discredited theories of 

 the functions of these markings. Arbitrary distinctions do not 

 occur in nature. 



In order to give the reader an opportunity to review the 

 entire series of white-patterned birds and to compare it with the 

 complete series of those that lack top-white, I have prepared the 

 accompanying lists. One family, the Mniotiltidae, has been 

 reserved for more detailed study, and is given in a separate list. 

 The ground-frequenting birds of the open have already been 

 given. These lists, therefore, are fully representative of arboreal 

 perching birds of the temperate zone, and include all species 

 of the order Passeriformes regularly found in the United States. 



