Coloration of the Mouths and Eggs of Birds. 273 



legitimate attack — mimicry for shelter, one might say. (2) A. 

 less-kuown unpleasant species may mimic an abundant, well- 

 known species, and so share in its relative immunity from 

 mistaken attack — mimicry for due notoriety. Or, (3) con- 

 ceivedly, two abundant and unpleasant species may develop 

 points o£ resemblance to one another, such as will associate 

 them, to their mutual advantage, in an enemy's mind — 

 mimicry for increased notoriety. 



Mimicry is best regarded, perhaps, not as of different 

 kinds but simply as mimicry, with the above as factors 

 contributing to each particular instance in varying and not 

 always easily-ascertainable proportions. For most mimics 

 have some unpleasantness of their own, and there are 

 probably few models that are not to some extent more 

 unpleasant than their mimic, as well as more abundant. 

 But, however they are built up, the function of many 

 common groups to-day is, I believe, largely a matter of 

 memory and simplification. 



3. On the Mouth-colours of Nestlings. 



Towards the end of 1908, I was much struck by the 

 mouth-markings of a brood of nestling Estrilda astrild. 

 The possibility that both the pattern and the distinctive 

 hissing sound uttered by the young birds might be of a 

 " warning " nature — a reminder to enemies of the presence 

 of some degree of nauseousness — at once occurred to me. 

 I therefore made a coloured drawing of a mouth of one 

 of the nestlings (PI. YII. fig. 7), and resolved to follow up 

 the observation by others. Prinia mystacea (fig. 8), Colius 

 striatus minor (fig. 17), and Pijcnonotus layardi (figs. 15, 38), 

 were noted soon afterwards ; but I shortly became absorbed 

 in other directions, and it was not until late in the breeding- 

 season 1912-13 tliat the distinctive and striking mouths 

 of some nestling Macronyx croceus (figs. 19, 20), Chloropeta 

 nataleiisis (figs. 9, 33), and Centropus burchelli (fig. 21), 

 which I was rearing, recalled me to the subject. 



The study is an extraordinarily interesting one. The 



SER. X. — VOL. IV. T 



