FLUCTUATIONS IN A RED GROUSE POPULATION 105 



birds ignore them. We use a different coloured tab for each sex and age- 

 group (bird of the year or older) and identify individuals by a letter and 

 number written on the tab. These figures can be seen clearly through 

 binocular or telescope up to about 100 m. 



We know of only five tabs (about i per cent) that have come off the bird 

 through the leather rotting and we think that this occurs rather seldom. 

 When the leather rots, it breaks first on one side and the tab may hang 

 lopsided for weeks or months before it comes off altogether and we are thus 

 forewarned of the loss of the tab. We know of three instances where the 

 plastic snapped through and in these cases the remnant could be seen; careful 

 examination of many grouse in the tabbing area suggests that this rarely 

 happens and is not an important factor influencing our conclusions. One 

 bird managed to get its head back under the transverse thong so that the tab 

 hung down its belly. All these events occurred after the tabs had been on 

 the grouse a year or longer and most of the events in which we are interested 

 occur within a few months of the birds being tabbed. We are therefore 

 satisfied that errors of technique are not influencing the data obtained. 



Grouse can only be tabbed when they are full-grown, and we have not 

 yet devised a satisfactory means for catching them except on stubbles. The 

 period in which both red grouse and black grouse (Lyrurus tetrix (L.)) will 

 visit stubbles is rather hmited, varying from about five or six weeks to more 

 than three months in different years. We do not know if these annual 

 differences are correlated with the behaviour of the grouse or with the 

 supply of other sources of food or with the palatabihty of the waste oats on 

 the stubble, or with something else, perhaps the weather. 



The sample obtained by trapping was probably typical of the grouse in 

 the immediate vicinity of the stubble fields though not all these birds were 

 necessarily resident there during the previous summer. Some may have been 

 transient visitors. When we counted the moor it was always clear that grouse 

 were either on the heather or on the stubbles (i.e. they were living on the 

 study area and were not visitors to the stubbles from a distance). The birds 

 entering the traps were therefore part of the adjacent population, which 

 sometimes did not change in numbers during the trapping period; they were 

 not additional to the surrounding grouse stock. Each year tabbed birds were 

 numerous within about a quarter-mile of the stubbles and scarce further 

 away; we think that this was the usual distance that grouse moved to the 

 tabbing field. 



The sample trapped sometimes included many more first-winter (young) 

 hens than other age-sex groups (Table IV), though in the one year, 1959, 

 when we could trap in September we caught twelve young cocks that 

 month compared with only two young hens. These differences probably 



