292 University of California Publications in Zoology. C"^°^- ^ 



would fly across a road into the neighboring field, where they 

 would presently be followed by others. Their white tail-borders 

 were often conspicuous during the entire flight of the birds, the 

 tail remaining partly spread. As the meadowlarks on being 

 flushed rose to the height of a man's head or higher, they must 

 have seen the retreating forms from a similar view-point, i.e., 

 against a dark background. They would not commonly see them 

 against the sky. For the birds themselves, white tail-borders 

 would serve no purpose if not a directive one. Common observa- 

 tion does not seem to be at fault here, nor the term "white 

 guides" ill- chosen. 



It will be seen that the common assumption to the effect that 

 white upon an object makes it conspicuous is well grounded in 

 this ease, for the reason that there is nothing corresponding to 

 it in the usual background; further, because it does not in the 

 least efface the outlines of the bird's contour, and finally because 

 the bird is in motion at the time when the marking is displayed. 

 We take note, therefore, of the distinction between a fl.ight- 

 exposed marking and one that appears at its full value when the 

 bird is at rest. The former acquires added conspicuousness from 

 the fact that a moving object fixes and holds the attention, indeed 

 a white object moving across a dull background is the best 

 mechanism that can well be devised for signalling at long 

 distances. 



Mr. A. H. Thayer, in the article to which we have just 

 referred, indicates a supposed correlation between white rear- 

 markings and the habit of nesting on the ground and flying 

 from the nest when disturbed, stating that the markings are 

 absent from birds that habitually run from it to escape a furred 

 enemy. This would point to an obliterative function in the rear- 

 mark, which shows white against the sky as seen from the level 

 of a quadruped. This statement and conclusion seem to have 

 been hastily made, for they apply only in the case of water birds 

 such as rails, coots and gallinules which the author of the state- 

 ment had in mind. The horned lark (Otocoris alpestris, all 

 subspecies), the meadowlark {Sturnella magna and S. neglecta), 

 and the vesper sparrow {Pooecetes gramineus) , are among the 



