191 °] Tracy: WhiU Markings in Birds. 297 



directions around and through hushes without call-notes. The 

 belter-skelter disappearance of the flock certainly puzzles me 

 and leaves me wondering where any one of the birds may be 

 relocated. The eye gets no single permanent impression." 



The behavior here cited is found when analyzed to be the 

 exact opposite of what occurs in a typical foraging Hock of birds 

 having white rear-markings. In a Hock of pipits the birds do not 

 move independently, as a rule, in changing feeding grounds; 

 they move in a body when alarmed, in one general direction. 

 with great uniformity; not through bushes, always with call- 

 notes, thus using every reasonable means of keeping the flock 

 together. Under stress of alarm, growing darkness, or accidental 

 scattering through considerable distance, the bright moving rear- 

 marks must at least lie of appreciable value to the bird in keeping 

 others of the flock in sight. Whether or not that is the main 

 reason for their existence is, of course, open to question. 



THE PROBLEM DISCUSSED FOR ARBOREAL SPECIES. 



We have been considering up to this point only the white 

 markings characteristic of birds of the open. Among arboreal 

 species the problem becomes more complicated both as to 

 variety and distribution of the patterns and their possible signifi- 

 cance. A form of "top-white" which can be shown to have the 

 effect of making the wearer conspicuous is the basal patch of 

 white upon flight feathers, usually the bird's primaries. It need 

 not have this effect, however, when regarded as a fixed pattern 

 against a foliage background. The latter gives it a disruptive 

 value, as Thayer (1909) has shown for similar designs. In order 

 to test the concealing values of this particular wing marking I 

 mounted the green-hacked goldfinch (Astragalinus psaltria 

 hesperophilus) and the black-headed grosbeak (Zamelodia melan- 

 ocephala) with spread wings and photographed them against 

 sunlit foliage and backgrounds of leaves with spaces of sky 

 showing through them. The birds were difficult to find in the 

 resulting prints. Undoubtedly the photographs by their lack 

 of relief exaggerated the concealing effect ; yet that there is such 

 an effect, in general, it is safe to admit. 



