286 BULLETIN 191, UNITED STATES NATIONAL MUSEUM 



he was left in peace. The third instance was a Httle more uncertain — 

 but was of a similar nature to the first." 



The subject of these mobbings deserves further study. If Yeates's 

 conclusion is correct — and his evidence cannot be lightly dismissed — 

 it would indicate a very singular state of affairs and one presenting 

 an intriguing problem to students of bird behavior and psychology, 

 since it is certainly not to be explained by attributing a sense of morality 

 to the birds! It should not be overlooked, however, that in various other 

 species of birds the sight of a pair engaged in the sexual act has been 

 observed to have a very provocative effect on other individuals, which 

 are stimulated thereby to interfere in very much the same way that has 

 been described in rooks — and this quite irrespective of the "legitimacy" 

 or otherwise of the nuptial behavior. 



What may be regarded as a kind of courtship — or display flight, 

 though its full significance is not altogether clear, may sometimes be 

 noticed, several birds following one another with peculiar slow wing 

 beats. Headlong dives from a height and other aerial evolutions may 

 sometimes be observed at the beginning of the breeding season, but 

 such performances have apparently no sexual significance (except 

 indirectly as an expression of excitement) and are much more charac- 

 teristic of the fall, under which heading they are further described. 



Nesting. — Rooks are c.olonial birds, normally building their nests in 

 tall trees, and it appears to be a matter of indifference to them 

 whether the trees are in compact groups or scattered. Rookeries may 

 be built on the one hand in trees dispersed over open parkland, fields, 

 or lawns, irregularly spaced along hedgerows or in closer rows forming 

 windbreaks or avenues, or on the other hand may occupy more compact 

 groups of trees forming copses, spinneys, plantations, or small woods 

 or in the shrubberies of gardens. More rarely a rookery may be 

 situated on the borders of a large wood, but it is probably safe to say 

 that no rookery is ever found far inside any extensive and uninterrupted 

 tract of woodland. A great many rookeries are near farms or buildings 

 or in the grounds of country houses. No doubt the tendency to nest near 

 farms may be partially accounted for by the fact that very often a farm 

 has a spinney or a windbreak planted near it, as well as by the better 

 food supply near such settlements, and no doubt the well-timbered 

 character of the grounds of so many country houses is sufficient to 

 account for the occupation of many such sites. But the observer who 

 critically notes the surroundings of a large number of rookeries will 

 probably find it difficult to resist the impression that the birds have 

 often deliberately selected a site near a building though others were 

 available. 



