FLUCTUATIONS IN A RED GROUSE POPULATION 97 



subsequent spring. When one cock hears another it will respond by singing 

 and may make a song-flight in the direction of the other bird. Sometimes 

 two cocks come close in this way; they will then threaten and occasionally 

 fight. We regard this as territorial behaviour. 



Hen grouse stay in parties and do not, as a rule, show territorial behaviour 

 during the autumn. These flocks vary in composition and size (from two or 

 three to twenty-five or more) from day to day, and individual hens often 

 wander considerable distances, the furthest autumn recovery being 22 J km 

 (14 miles) away. On bright mornings, when cocks are singing loudly, hens 

 may associate with them so that they appear to be paired. Such pair associa- 

 tions are usually temporary and a cock has been seen in company with two 

 difierent hens on the same morning as well as with a succession of different 

 hens on different days. Despite the movements of individual hens and of 

 some cocks (presumably those that fail to secure territories), grouse numbers 

 may stay constant throughout the autumn and early winter. 



Some hen grouse start singing in October or November but few sexual 

 chases have been seen at this time. Much of the display at this season is 

 between birds mated previously that often stay on their old territory. There 

 is usually Httle sexual activity between cocks and hens in the autumn. 



During the winter grouse behaviour depends on the severity of the 

 weather. When it is mild, as in 1956-7, the autumn pattern may continue 

 during December and January. Territorial display is usually confined to 

 bright, sunny mornings, or to mild days following a colder spell, and 

 territory-owners may join together at other times. In exposed places or 

 during snow the birds eventually gather into packs, more readily at higher 

 altitudes. Hens pack first and in fme snowy weather cocks may show vigorous 

 territorial display. If the snow continues, all the birds on a moor will join 

 into packs hundreds strong. These readily break into smaller parties which 

 are continually on the move from ridge to ridge, to black ground swept 

 clean of snow by the wind. Such grouse are very wild and we have often 

 seen packs moving up to one mile; when disturbed by an eagle (Aquila 

 chrysaetos (L.)), they may fly high and over ridges out of sight. They soon 

 return, however, and we have found no evidence of emigration from our 

 study areas owing to winter storms in 1956-9. 



Sexual activity starts quite suddenly with the return of fme weather, 

 usually in February below 300 m (1,000 ft) but later at higher altitudes where 

 the snow hes longer. Singing, threat, fighting, courtship display and chases 

 are all at their maxima at these times. 



The first eggs are laid in April or May and the peak hatch is in early June. 

 The largest clutches are sometimes as many as twelve or thirteen eggs, more 

 usually seven or eight. Only the hen incubates. In 1957, the only year when 



