302 The Ruffed Grouse 



practically all of the nests that failed were destroyed by predators. 



Most of the other nest losses are due to man (farmers, lumber- 

 men, etc., in their various work operations), with some losses result- 

 ing from fire (which may assume great importance locally), deser- 

 tion, flooding, etc. 



The time of destruction is of considerable importance since it has 

 a direct bearing on the probability of renesting, hence on the ulti- 

 mate success in nesting. Very few nests are destroyed during the egg- 

 laying period— the last half of April and first week or ten days of 

 May in southern New York— probably not over five per cent of the 

 total nest losses occurring at this time in normal years. As soon as in- 

 cubation begins the rate of destruction increases, slowly at first, then 

 quite rapidly after the first week. Most of the nests are broken up 

 during the last ten days of incubation. 



Now there is an inverse relationship between tlie length of time 

 that incubation has progressed and the likelihood of the bird's re- 

 nesting. The longer the incubation has proceeded, that is, the nearer 

 the nest is to hatching, the smaller is the probability of renesting. 

 If a nest is destroyed before incubation has begun, the hen will 

 probably make another nest. It is even possible that some of these 

 second nests may occasionally be mistaken for a late first nest. Up 

 until about two weeks after setting there is some chance, though a 

 constantly reduced one, of the bird renesting if broken up. Hens 

 whose nests are taken during the last ten days of incubation rarely 

 make another nest that year. And since most of the nest losses occur 

 during this last ten days, most broken-up birds do not make another 

 attempt. 



Ratio of Females Unsuccessful in Nesting and in Rearing Broods. 



We have seen that the mortality of grouse nests averages about forty 

 per cent. This means that about sixty birds out of every hundred 

 succeed in hatching their first clutch. We also noted that a rather 

 small but significant proportion of birds that lose their first nest will 

 try again, and some of these will succeed in hatching. Thus the ratio 

 of unsuccessful nesting is somewhat lower than forty per cent. 



The exact ratio of renesting is difficult to gauge on the relatively 

 sparse data available. It is likely below twenty-five per cent. Using 

 this figure, about ten of each forty birds that fail in their first nest- 

 ing attempt (out of an average hundred birds), will nest again. The 



