<) I THE SPECIES ITS T.iXOXOMY. RA.\GE. BIOLOC). & ECOXOMIC I MI'OKTANCE 



Agk Differences 



Pvormally, a grouse chick at birth weighs nearly !/•> oz. (] 1 to 13 grams), though this varies 

 slightly from year to year. Because of the difficulties involved, few youngsters have been 

 banded and subsequently recaptured and weighed in the wild. Weekly weights of birds raised 

 in captivity, however, have been kept (table 11). Comparing these data with those in figure 

 10 it will be noted that while the pen-reared females attained somewhat higher weights by 

 early fall than the average for birds collected in the field, the reverse was true among the 

 males. 



From September's frosty nights, the birds of the year seem to follow much the same weight 

 pattern as do their elders (figure 10). Although they are then some 2y-j, oz. (60 to 70 grams) 

 lighter on the average, they are probably even more active foragers, for they rapidly nar- 

 row the gap. Too, the gain is continued later, for apparently they do not reach their peak 

 until sometime in January, although they still average upwards of 1 oz. (20 to 30 grams) 

 less at this time. Also, birds of the year seem to lose a little less throughout the rest of 

 the winter, though they are still some 8 to 16 grams lighter as late as the first of March, 

 the greater difference being recorded among the males. Beyond this time they usually be- 

 come indistinguishable from older birds, due to the disappearance of the bursa and the moult- 

 ing of their two juvenile outer primaries. 



TABLE 11. AVERAGE WEEKLY WEIGHTS. IN GRAMS, OF 8 MALE AND 7 FEMALE 



HAND-REARED GROUSE CHICKS 



By the middle of September, the weight difference between the sexes among these younger 

 birds has apparently reached its maximum, for the female is nearly 4 oz. (110 grams) the 

 lighter of the two, the average being 1 lb. (450 grams) and 1 lb. 4 oz. (560 grams) re- 

 spectively. Even at their January peak there is a difference of close to 3 oz. (75 grams) 

 between them, a spread, however, not much greater than that between their elders at this 

 point. 



Differences in weights of grouse shot during the hunting season may often be ascribed to 

 age differentials for even then the birds of the year average 2 oz. less in weight than do old 

 birds of the same sex. 



Kegional and Yearly Differences 



To secure an accurate picture of regional and ycuil) differences, it is necessary to analyze 

 the weights of an adequate number of individuals collected from each region each year. Un- 

 fortunately, full grown birds collected prior to 1939 were not separated into adults and 

 birds of the year. At that time the reasonably accurate means of identifying the latter by 

 the presence of the bursa of Fabricii or an examination of the tips of the two outer primaries, 

 had not been recognized. A careful analysis of the resultant weight data for each age group 

 for 1939-41 reveals no significant differences cither between the three major regions of the 



