THE ROOK AND JACKDAW 41 



are in excess of the sites available. This is certainly at times the 

 case. One rookery of two trees is known to have contained thirty- 

 five nests and two hundred birds, another rookery two hundred nests 

 and one thousand birds. But at present there is no evidence to 

 show how many of the non-nesting birds are immature or sterile. 1 



The shortage of sites may also account for the attempts occa- 

 sionally made by individual pairs to build nests in trees outside the 

 main rookery. These attempts are not always successful, the nests 

 being ruthlessly pulled to pieces and their builders savagely attacked 

 and driven away by members of the parent colony. When outlying 

 birds that have been thus attacked do succeed in rearing a brood, it 

 will generally be owing either to their having defended their nest 

 successfully day after day till the aggressors became too busy with 

 their family duties to continue the persecution, or to their deferring 

 nest-building till after the busy season in the main rookery had 

 commenced. But the nests that escape in this way are sometimes, 

 if not always, pulled down at the beginning of the following season. 

 In thus forbidding the foundation of a new colony near the ancestral 

 one, the rooks are perhaps giving collective expression to the same 

 instinct that causes non-gregarious birds to resent so strongly any 

 intrusion by others of their own species into the area in which they 

 have made, or are preparing to make, a home for their young. This 

 explanation renders it, however, still harder to understand what 

 enabled the rooks themselves, in the first place, to overcome this 

 primitive instinct to the extent of submitting to nest, as they do, side 

 by side, sometimes as many as thirty pairs or more in the same 

 tree, when there are other trees available in the immediate vicinity. 2 



It is usually not till the second week in March that the rooks 

 begin to make ready their nests. If the old nest has been completely 

 destroyed by the winter gales, they have no option but to build anew. 



1 Irish Naturalist, 1903, pp. 160-1 (C. B. Moffat) ; Field, 1902, May 4, p. 729 ; J. A. Sowerby, 

 Rooks and their Neighbours, 2nd edit., 1895, p. 25. 



- Irish Naturalist, 1903, pp. 160-1 (C. B. Moffat) ; Zoologist, 1904, p. 191 (R. H. Rambotham, 

 from whom I have received additional information) ; Field, 1865, xxv. p. 230 ; 1882, Ix. p. 569 ; 

 1896, Ixxxvii. p. 653. 



