140 BRITISH BIRDS. 



from these eggs would inherit the tendency to sit as soon 

 as the first egg had been deposited, and in course of time 

 through the process of natural selection the species as 

 a whole would be affected. The very real benefit gained 

 would in all probability cause the time needed to bring about 

 the change to be short, that is, comparatively speaking. 



It may be objected that the Jackdaw breeds in colonies 

 and is also an egg-stealer, but yet does not incubate until 

 the full clutch has been laid. The case, however, is 

 very different, for the eggs are not exposed, the nests 

 being in holes. The Jackdaw is a wary bird, and will 

 not venture off-hand into a hole that may be occupied 

 by one of the rightful tenants, and the risk of being taken 

 in the rear is also, without doubt, a deterrent. It is 

 significant that when the nest is found in the branches 

 of trees, it is frequently domed, if not already protected 

 by impending foliage. 



It is well known that such birds as the Sparrow-Hawk 

 (A. nisus), Kestrel [F. tinnunculus), Long-eared Owl 

 {A. otus), and Short-eared Owl {A. accipitrinus) commence 

 incubation with the first egg. It is also well known that 

 an interval of a day or more occurs between the laying 

 of each egg. All these birds deposit their clutches 

 with no attempt at concealment, and to any of the Crows 

 passing overhead they would appear conspicuous objects 

 and would, of course, pay the penalty. Also, as the eggs 

 are not laid on consecutive days, the danger of being 

 discovered extends over a considerable period. Thus, it 

 is evident that birds of these species which incubate as 

 soon as the first egg is laid, are much more likely to 

 rear young than those which do not do so. 



The case of the Barn-Owl (aS'. flammea) is rather different, 

 for the eggs are not exposed as are those of the birds 

 last mentioned, but the habit of covering the eggs is un- 

 doubtedly of great advantage to the species by protecting 

 the eggs from the attacks of rodents. 



Another bird that certainly does not wait for the clutch 

 to be completed before commencing incubation is the 



