Age Determination and Plumage Observations of Blue Grouse

The primary purpose of this study was to obtain data on the molt pattern and growth rate of captive known-age young blue grouse (Dendragapus obscurus) upon which to base a system of age determination. Secondarily, observations were made on the molt pattern of adult and subadult blue grouse raised in captivity. The average weight (measured at weekly intervals) of the young birds in 1960 was conspicuously different from the average weight obtained in 1961. This variation in weight was attributed to a difference in diet during the first 4 weeks of life. The average weight of young wild blue grouse closely approximated the average weight of the 1960 captive juveniles at a corresponding age, whereas the wild birds were heavier than the 1961 captive birds at a corresponding age. The growth rates of post-juvenal primaries and rectrices, rather than sequence of feather loss, provided the principal means for placing young blue grouse in weekly categories from the third to the eleventh week. The loss of the primary and secondary remiges in subadult male blue grouse (about a year of age) commenced earlier than did the loss in adult males, remaining in advance of the adults until October. Techniques for determining age of game birds are used widely for analyzing and comparing age structure of populations by time and area. A means for determining age of young blue grouse to the nearest week had not been conclusively determined when our study began. Bendell (1955: 354) used the sequence of feather loss over the entire bird for placing young grouse in 2-week categories. The ages of the wild b rds used in his study were determined from the peak of hatching, and therefore were not known exactly. The research upon which this paper is based was conThis content downloaded from 157.55.39.203 on Sat, 27 Aug 2016 05:47:03 UTC All use subject to http://about.jstor.org/terms BLUE GROUSE AGING* Smith and Buss 567 ducted in 1960 and 1961 and was designed: (1) to obtain, from young blue grouse, characteristics which are precise enough to allow age determination to the nearest week, and (2) to record observations on molt patterns of subadult and adult grouse. The data for this study were taken from a thesis submitted by the senior author in partial fulfillment of the requirements for an M.S. degree in Wildlife Biology at Washington State University. The investigation was supported in part by funds provided for biological and medical research by the State of Washington Initiative Measure No. 171. Acknowledgment is made to the State of Washington Department of Game and the Washington Game Bird Club for providing funds to construct experimental pens. Acknowledgment is also made to Oscar W. Johnson, Department of Zoology, Washington State University, for critical reading of the manuscript. Appreciation is expressed to Leo S. Jensen, Department of Poultry Science, Washington State University, for technical assistance in developing the bird diets used in this investigation. MATERIALS AND METHODS Three broods were hatched from captive grouse, Broods A and B in 1960 and C in 1961. Brood A consisted of three birds. Four birds were present originally in Brood B, but two died at 3 and 7 weeks respectively. Four of the eight chicks hatched in 1961 had died by 5 days of age; the remaining four birds survived until October 26, 1961, when field observations ended and the birds were utilized for study skins. All chicks were taken from the hen as soon after hatching as possible and placed in a small brooder at a temperature from 85 F to 95 F for the first week. Thereafter the brooder was utilized only at night and during cool days until the chicks were 5 weeks of age. At this age, their feather development was adequate to protect them against climatic contingencies, and they were placed in large pens like those occupied by the adult grouse. Food and feeding techniques varied between 1960 and 1961. In 1960, the young birds were started on mealworms (larvae of Tenebrio molitor), turkey starter mash, chopped dandelion greens, and hard-boiled egg. Mealworms were the chief food until the second or third week, at which time the birds were shifted to the other foods. During the first 4 weeks, while they were confined to the brooder, the chicks were r leased daily for about an hour on a mowed lawn, where they consumed insects, grass, and weeds. Between the fifth and s xth week of age, they were gradually shifted to whole-grain wheat, cracked corn, crushed oyster shells, crushed rock, and a pellet food formulated by the Poultry Science Department at Washington State University (for ingredients see Table 1, Nelson et al. 1962:270). In 1961, the newly hatched blue grouse were placed in a brooder with a supply of turkey starter mash. At the same time, a newly hatched domestic chicken was obtained and placed with the grouse chicks. The young chicken began pecking at the mash at once, and the blue grouse followed its actions. Thus the grouse were started on mash, and despite a daily ration of mealworms, the mash was the chief food. Alfalfa was provided daily and was devoured quickly by the young birds. As in the previous year, the birds were placed on a mowed lawn for about an hour each day. The diet was shifted gradually to the adult diet soon after the young grouse were placed in one of the large pens. This pen contained a bush of honeysuckle (Lonicera japonica), the berries of which were aten promptly by the young grouse. This content downloaded from 157.55.39.203 on Sat, 27 Aug 2016 05:47:03 UTC All use subject to http://about.jstor.org/terms 568 Journal of Wildlife Management, Vol. 27, No. 4, October 1963 Medication (an antibiotic and blackhead preventative) was added to the drinking water in 1961. The following data were recorded weekly for the young grouse: weight; the last primary dropped; the primaries that were found, on gross examination, to contain blood (also recorded for secondaries and rectrices in 1961); and the length of primaries, secondaries, and rectrices (1961 only). The average weekly growth rates of post-juvenal primaries were obtained by subtracting the length observed when first measured from the final length and dividing by the number of weeks required to complete development. In 1960, weekly photographs were taken of the young grouse against a grid of 2-inch squares, with the birds placed as close to the grid as possible to reduce the effect of parallax. These photographs disclosed subtle, but definite, week-to-week differences among males which would be very difficult to convey in words. Until the grouse were 9 weeks of age, photographs were taken of both sexes; thereafter, photographs were taken of males only. All linear measurements were recorded to the nearest millimeter and all weights were recorded to the nearest gram. In 1960, the eight captive adults and four subadults were maintained exclusively for breeding purposes and for the development of a larger colony of experimental grouse. Since handling might have jeopardized breeding success, these grouse were not used for study purposes. In 1961, two subadults, three adult males, and three adult females were present in the experimental flock. Observational procedures were generally the same as for the young birds. The grouse were examined weekly, commencing at the time the first dropped feather was observed, on May 22. At this time, only the males were examined, as the nesting season was not yet complete and disturbance of the females was deemed undesirable. The first female was examined on May 29; the other two females were exam ned initially on June 6. Observations on all grouse were terminated on October 26. The following records were kept for the dult and subadult grouse: the last primary, secondary, and rectrix which had been dropped; the primaries, secondaries, and rectrices which contained blood; the lengths of the incomplete primaries, secondaries, and rectrices; and the development of feathers in the various feather tracts. Development of the contour feathers was observed by stroking the feathers toward the head and noting the area of each feather tract which contained incomplete feathers. Measurements of the flight feathers were obtained by placing a rule against the patagium on the distal side of the feather being measured. Rectrices were measured with a rule from tip to point of insertion. RESULTS AND DISCUSSION There was a conspicuous difference in average weights of young birds between the 2 years of this study. This difference is presented graphically in Figs. I and 2.