ON INCUBATION. 141 



Heron {A. cinerea), for young birds in the same nest 

 exhibit considerable variation in size, and judging from 

 this, it seems probable, in some instances at all events, 

 that an interval of several days occurs between the laying 

 of the eggs. This species rears two broods in the season. 

 The first clutch is hatched in March or the beginning of 

 April. It seems certain that if the eggs were not covered 

 by the birds a large number would be spoilt by frost and 

 snow. Of course, in the earlier as well as the later laying, 

 the risk of losing the eggs through the attacks of the 

 Corvidce is much reduced by the Herons being constantly 

 on them. 



I have known one or two cases in which Ravens {C. 

 corax) commenced incubation before the clutch had been 

 completed, but cannot say whether this is always the case. 

 Young Ravens were found during March of the past 

 spring (this, of course, is not unusual) in a nest in the face 

 of a crag situated high on the fell, and very much exposed 

 to the elements. Anyone who has experienced the snow- 

 storms and the occasional severity of the cold at these 

 altitudes during the early months of the year, will readily 

 realise that the eggs in a nest so situated would frequently 

 be rendered infertile if not covered by the bird as soon as 

 they were laid. 



In this connection it is interesting to note that Naumann 

 states that the Crossbill {L. curvirostra) begins incubation 

 as soon as she has laid her first egg. 



During a visit to a well-known guUery I was able to 

 make observations on the incubation of the Black-headed 

 Gull {L. ridibundus), Common Tern {8. fluviatilis), Sand- 

 wich Tern {S. caMiaca), and Little Tern {S. minuta). 

 Some of the results obtained are appended : — 



Hatching of L. ridibundus. 



I. — Two eggs. June 23rd, first hatched. 

 June 24th, second hatched. 



II. — Three eggs. June 25th, first egg hatched. 

 June 26th, second egg hatched. 

 June 27th, no change. 

 June 28th, third egg hatched. 



