Strong and others 



Chapter 32 



Distribution and Population Estimates in Oregon 



Population Versus Breeding Population 



Correcting estimates to account for birds tending the 

 nest, or those not part of the breeding population, is valid. 

 Our knowledge of nest-tending behavior and breeding status, 

 however, is so limited that applying factors from other 

 studies or species may only be misleading at present. Nesting 

 site limitations have been shown to also limit breeding 

 populations of other alcids (Ainley 1990, Divoky and others 

 1974, Manuwal 1974, Nelson 1987, Preston 1968). If loss 

 of old-growth and ancient forest nesting habitat is the major 

 factor affecting populations of Marbled Murrelets from 

 California to Washington (Carter and Erickson 1992, 

 Leschner and Cummins 1992a, Marshall 1988a), then we 

 would expect the 'floating' proportion of non-breeding adults 

 to be very high, probably over 50 percent. Members of the 

 alcid family are long lived, in the range of 20 - 40 years 

 (Ainley 1990, Sealy 1975a), so the possibility of a 'remnant' 

 population is realistic. If only a small proportion of the 

 measured population is nesting then the low number of 

 fledglings observed on the water may be explained. Given 

 this, we would expect total populations, as estimated from 

 vessel survey data, to decline in coming years due to lack of 

 recruitment. Population monitoring and measurements of 

 productivity are crucial to evaluating this concern. 

 Information on the life history and longevity of the bird will 

 also be important in interpreting results of population and 

 productivity monitoring. 



Future Research 



This report establishes the feasibility and preferences 

 of using vessel surveys for population assessment on the 



Oregon coast. Population monitoring and more refined 

 population estimates are attainable objectives using methods 

 outlined in this paper. Other areas of at-sea research which 

 may be essential to developing effective management and 

 protection strategies for Marbled Murrelets are relating at- 

 sea habitat use and distribution to forest nesting habitats, 

 finding a means of assessing yearly productivity and 

 population demographics, and more developing knowledge 

 of prey species' composition and availability in relation to 

 oceanographic parameters and location of nesting habitat. 



Acknowledgments 



We are grateful to S. Kim Nelson for setting this pro- 

 ject in motion. This work would not have been possible 

 without the logistical support of the U.S. Fish and Wildlife 

 Cooperative Research Unit at Corvallis, and Robin Brown, 

 region coordinator of the Wildlife Diversity Program, Oregon 

 Department of Fish and Wildlife. 



John G. Gilardi and Janice M. Cruz were invaluable 

 field observers in 1992. W. Breck Tyler assisted during 

 initial aerial surveys. Dr. Christine Ribic (U.S. Environmental 

 Protection Agency), Dr. Nadav Nur (Point Reyes Bird 

 Observatory), and Dr. Steven Beissinger (Yale University) 

 each provided important comments on statistical treatment 

 of data. We thank Dr. Jeffrey Laake and the other authors of 

 the program DISTANCE for providing the program and 

 manual. C. John Ralph and Dan Varoujean provided helpful 

 comments on methodology. MacKenzie Flight services and 

 Craig Johnson (Gasquet Aviation) provided excellent piloting 

 and reliable aircraft. 



352 



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



