Carter and Stein 



Chapter 9 



Molts and Plumages 



five main criteria for use as an efficient, standardized method 

 in identifying juveniles versus adult murrelets in the late 

 breeding and early post-breeding season from June to 

 November. The criteria examined included: (1) relative size; 

 (2) dark:light coloration ratio; (3) ventral coloration and 

 patterning; (4) dorsal coloration; and (5) primary molt and 

 wing shape. 



Recently-fledged juveniles are smaller than adult birds 

 (70 percent adult mass at fledging) but become more similar 

 to adult birds in size after foraging at sea during the first few 

 months after fledging. Total length differed significantly 

 between adult and juvenile birds (t = 7.52, P < 0.001) but 

 some juveniles collected in August and September were as 

 long as adults collected during these months. In the field, 

 size may be a useful criterion for differentiating juveniles 

 from adults when mixed flocks are encountered early in the 

 post-breeding season, but will be less useful during August 

 and September when many juveniles have already grown at 

 sea for at least a month. 



Recently-fledged juveniles are lighter in color than adults 

 in alternate plumage. In alternate plumage, a murrelet was 

 estimated to have a 90:10 dark:light (D:L) ratio. In basic 

 plumage, the ratio changed to 55:45 D:L. One molting adult 

 collected on 19 June 1985 was 75:25 D:L. By August, more 

 molting adults were found in the collections. Flightless adults 

 collected on 11, 22, and 31 August measured 70:30, 65:35, 

 and 79:21 D:L, respectively. By September, there was a 

 larger range of color ratios. Some birds were close to 

 completing the pre-basic molt while others were still in 

 alternate plumage. Birds collected on 1 1, 16 (two specimens), 

 20, and 30 September varied from 53:47, 99: 1, 47:53, 85:15, 

 and 52:48 D:L, respectively. Juvenile specimens were also 

 examined. Generally, older juveniles appeared to be lighter 

 overall than recently-fledged juveniles. The coloration ratio 

 for juveniles with egg teeth averaged 65:35 D:L (n = 15), 

 whereas juveniles without egg teeth averaged 58:42 D:L (n 

 = 14). Due to protracted breeding, juveniles varying in age 

 by 2-3 months may be present on the water in August and 

 September and be easily confused with molting adults that 

 may have similar color ratios. 



Recently-fledged juveniles possess a "speckled" 

 appearance, resulting from many of the feathers on the sides 

 of the head, neck, breast and abdomen being edged with thin 

 dark margins (fig. 3). However, older juveniles without egg 

 teeth appear lighter overall as many of the dark margins are 

 lost. In contrast to the thin dark margins of juvenal feathers, 

 the dark margins of the feathers of adult birds are much wider 

 and have a "blotchy" appearance (fig. 3). As adult pre-basic 

 molt progresses from anterior to posterior in ventral tracts, 

 the density of blotchy feathers decreases. During boat survey 

 work in Washington, "blotchy" feathers were often visible 

 on the posterior ventral surface of molting adults as they 

 dove in front of the boat (Stein, unpubl. data). This criterion 

 was often helpful in separating adult birds that were more 

 advanced in their pre-basic molt from juveniles. Remnant 

 blotches or "speckled" feathers were noted on some specimens 



as late as November and December, but they probably would 

 not have been noticeable in the field (fig. 1). 



As pre-basic molt progresses in adult birds, the rust- 

 edged back feathers are gradually replaced by the grey- 

 edged, dark back feathers, typical of basic plumage. Some 

 grey-tipped back feathers were observed on adult specimens 

 collected as early as June although specimens in which all 

 orange-tipped feathers had been replaced had not been 

 collected until July. Although recently-fledged juveniles are 

 uniformly dark brown to almost black above, the upperparts 

 of older juveniles possess grey margins similar to adult 

 birds. Grey-tipped feathers were not noted on juvenile 

 specimens collected earlier than September, but were more 

 common after this time. Dorsal surface coloration appears to 

 be an unreliable criterion for separating older juveniles from 

 adult birds during the pre-basic molt. 



About 35 percent (n = 20) and 78 percent (n = 9) of adult 

 birds collected in August and September, respectively, 

 appeared flightless. Except for their flightless condition, 

 these specimens were similar in appearance to juveniles. 

 During August 1993 boat surveys in Washington, the condition 

 of the molted primaries was the most reliable criterion for 

 differentiating confusing birds (Stein, unpubl. data). Many 

 adult birds were advanced in their pre-basic body molt by 

 this time and could not be differentiated from older juveniles 

 on the basis of the other four criteria mentioned above. 



In summary, the number of criteria useful for 

 differentiating juveniles from adult murrelets decreases as 

 the post-breeding season progresses. Of the five criteria 

 evaluated, the first four would be useful in June and July. 

 Recently-fledged juveniles are smaller than adults (70 percent 

 body weight at fledging), lighter overall, appear speckled on 

 the throat, breast, abdomen, and are uniformly dark brown to 

 black on the upper body parts. In comparison, most adults in 

 June and July are still in their alternate plumage and are 

 much darker overall with rust-edged back feathers still 

 apparent. By August and September, most adult birds are 

 undergoing pre-basic molt, have lost the dorsal rust coloration, 

 have replaced many of the dark-edged ventral body feathers 

 with totally white feathers, and appear much lighter overall. 

 Many juveniles, which have grown at sea for at least a month 

 by this time, lose many of the characteristic speckled feathers, 

 either by wear or replacement. During these months, the 

 most reliable criterion for differentiating juvenile from adult 

 birds is the condition of the molted primaries that can be best 

 assessed when birds flap their wings while sitting on the 

 water. Molting adults have "stubby" wings, if all primaries 

 have recently been lost. Later, wings appear more rounded 

 and have a "paddle-shaped" appearance when the new inner 

 primaries become fully grown before the outer primaries 

 (fig. 5). Juveniles lack gaps and have more pointed wing tips 

 than molting birds at all times. In October and November, it 

 is not practical to separate juveniles from adult birds in the 

 field (fig. 1), although some late breeders, late molters and 

 late fledglings may still be encountered and differentiated on 

 the basis of all criteria. 



108 



USDA Forest Service Gen. Tech. Rep. PSW-152. 1995. 



