Chapter 9 



Molts and Plumages in the Annual Cycle of the 

 Marbled Murrelet 



Harry R. Carter 1 



Janet L. Stein 2 



Abstract: Marbled Murrelets have distinct basic, alternate and 

 juvenal plumages. In after-hatching-year (adult) birds, the incom- 

 plete pre-altemate molt occurs rapidly over a period of about one 

 month per individual between late February and mid-May. The 

 complete pre-basic molt occurs between mid- July and December. 

 At this time, individuals are flightless for about two months. In late 

 summer, it is difficult to distinguish adult birds undergoing pre- 

 basic molt from juveniles at sea. Field methods for separating 

 these age categories at sea at this time of the year are presented. By 

 early fall, older juveniles are not distinguishable in the field from 

 after-hatching-year birds in basic plumage. The timing of pre- 

 basic and pre-altemate molts were closely related to the timing of 

 breeding, movements and other aspects of the annual cycle of 

 Marbled Murrelets in Barkley Sound, British Columbia. 



Little has been published on the plumages and molts of 

 the Marbled Murrelet (Brachyramphus marmoratus). 

 Although the general pattern of molt and plumages has been 

 documented, many details that are important for interpreting 

 aspects of the biology of this enigmatic species have remained 

 undescribed. Adults, also referred to as after-hatching-year 

 birds (i.e.. breeding adults and subadults. including first- 

 year birds in their second calendar year), have distinct alternate 

 versus basic plumages that they wear during summer and 

 winter periods, respectively. Subadults have not attained full 

 maturity and have not yet bred. The mottled-brown alternate 

 plumage is certainly responsible for the English name 

 "Marbled" Murrelet. In addition, juveniles less than 6 months 

 old, also known as hatching-year birds, wear a distinct juvenal 

 plumage during late summer. Murrelets replace their alternate 

 plumage with a basic plumage during a complete pre-basic 

 molt (involving flight and body feathers) in the late summer 

 and fall. Similarly, during an incomplete pre-alternate molt 

 (involving only body feathers), they replace their basic 

 plumage with the alternate plumage in spring. These general 

 plumage stages and molts are similar for many other seabirds 

 and birds in general (Welty and Baptista 1988). The juvenal. 

 alternate, and basic plumages of the Marbled Murrelet are 

 illustrated in many reputable bird identification field guides 

 (e.g.. Harrison 1983. National Geographic Society 1983). 



Many past studies of Marbled Murrelets have not required 

 a detailed knowledge of the stages of molts and plumages. 

 Workers quantifying distribution and abundance of murrelets 



1 Wildlife Biologist. National Biological Service, U.S. Department of 

 Interior. California Pacific Science Center, 6924 Tremont Rd. Dixon. CA 

 95620 



: Wildlife Biologist, Washington Department of Fish and Wildlife, 

 16018 Mill Creek Blvd.. Mill Creek. WA 98012 



at sea have usually lumped all murrelets together regardless 

 of plumage, or they conducted their studies in summer or 

 winter when most or all birds were in the same plumage. 

 Plumages of birds observed at inland nesting areas have not 

 been distinguished because individuals fly high overhead 

 under low light conditions or darkness during censuses. 

 Interest in the relationship of plumage and molt to other 

 aspects of the murrelet's life history has grown rapidly since 

 1992. Researchers in Alaska. British Columbia, Washington, 

 Oregon, and California have recently attempted to census 

 juveniles at sea in the late summer and early fall to indirectly 

 determine breeding success. These efforts were prompted by 

 concerns that the very low numbers of juveniles compared to 

 adults (1-5 percent) observed during recent surveys in Oregon 

 and California represent very low breeding success (Nelson, 

 pers. comm.: Hardin. pers. comm.; Ralph and others, this 

 volume; Strong and others 1993). Such low levels of breeding 

 success could indicate that murrelet populations in 

 Washington. Oregon and California can no longer maintain 

 themselves. However, surveys at this time of the year have 

 difficulties that can lead to undercounting or o\ ercounting 

 juveniles in relation to adult birds from the same breeding 

 population, including: (1) the degree that researchers can 

 accurately separate the plumages of juveniles and adult buds 

 in the field, even under adequate viewing conditions; (2) 

 possible post-breeding season movements of adults, juveniles, 

 or both into or out of the area studied; (3) differential use of 

 at-sea habitats by various age classes and during different 

 stages in the annual cycle; and (4) the timing and degree of 

 natural and anthropogenic mortality of juveniles and adult 

 birds. Thus, the adult:ju\ enile ratio is complex and must be 

 interpreted with caution. 



To address these difficulties, especially the first, we 

 reviewed available information on plumages and molt from 

 published and unpublished sources with three main objectives 

 in mind. First, we summarized information on plumages and 

 molt and identified gaps. Second, we summarized some 

 other aspects of murrelet biology during the molt period that 

 may be important for assessing the adult:juvenile ratio. Third, 

 we developed field criteria for separating juveniles from 

 adult birds at sea during the late summer and early fall. This 

 method, based on current knowledge, will require modification 

 as new results are obtained. Our goal has been to provide 

 workers with sufficient information to gather more data to 

 confirm and expand on known patterns. This summary is not 

 complete and we refer the reader to other chapters in this 

 volume for additional information on murrelet biology during 

 the breeding and non-breeding seasons. 



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



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