288 BULLETIN 191, UNITED STATES NATIONAL MUSEUM 



It should be observed that while some rookeries are compact others 

 may be dispersed in irregular groups over a considerable area, and in 

 dealing with these latter the student is soon confronted with the question, 

 What constitutes "a rookery"? Groups situated close to one another 

 are obviously essentially parts of a single colony, but what degree of 

 separation justifies us in speaking of two groups as two distinct rook- 

 eries? Many fac.tors have a bearing on the problem, which is too 

 complex to discuss here, but it may be said that in the present state 

 of knowledge any criteria adopted must necessarily be somewhat arbi- 

 trary, and the point is mentioned mainly in order to stress that state- 

 ments as to the dimensions attained by individual rookeries will be 

 affected in a certain proportion of cases by the criteria employed. But 

 for the most part the large colonies mentioned would be reckoned as 

 single rookeries on any reasonable scheme. 



At the opposite extreme to these giant colonies, isolated nests are 

 not very rare, but as a rule they have little permanency, and it has 

 been repeatedly stated that isolated nests situated at all close to a 

 regular rookery are liable to be raided by the ocQupants of the latter, 

 such individualistic tendencies being regarded with disfavor in rook 

 society. But more critical observation on this point seems desirable. 



Rooks are early breeders, but, although the rookeries are visited 

 from time to time in the winter and more frequently as the season 

 advances, the serious business of nest building or repair rarely begins 

 before late February. The nests are generally built in the topmost, 

 slender branches of trees, often several or even a considerable number 

 in one tree, and are frequently very difficult or almost impossible 

 for even a good climber to reach, though an enterprising egg collector 

 or photographer will not as a rule have great difficulty in finding a 

 rookery where the nests are more accessible. They are built of sticks, 

 solidified with earth and lined with grasses, dead leaves, moss, roots, 

 straw, and the like. Wool and hair, beloved of the carrion crow, are 

 only quite exceptionally employed and then only in small quantities. 

 Exceptionally the use of feathers in quantity has recently been re- 

 corded, where a plentiful supply happened to be available close at hand. 

 Both sexes build, the cock doing most of the collecting and the hen most 

 of the arranging. The sticks are broken off from trees or pillaged 

 from other nests and are not collected from the ground. Even a stick 

 accidentally dropped is not ordinarily retrieved, though rarely one may 

 be picked up again. Eggs are usually found in the latter half of March 

 or early April. The task of incubation is performed solely by the hen, 

 to whom the cock brings food at the nest, and sometimes, but not al- 

 ways, begins with the first tgg laid. As an exception a case of a male 



