Chapter 8 



Nest Success and the Effects of Predation on 

 Marbled Murrelets 



S. Kim Nelson 1 



Thomas E. Hamer 2 



Abstract: We summarize available information on Marbled Murrelet 

 (Brachyromphus marmoratus) productivity and sources of mortality 

 compiled from known tree nests in North America. We found that 

 72 percent (23 of 32) of nests were unsuccessful. Known causes of 

 nest failure included predation of eggs and chicks (n = 10), nest 

 abandonment by adults (n - 4), chicks falling from nests (n = 3), 

 and nestlings dying (n = 1). The major cause of nest failure was 

 predation (56 percent; 10 of 18). Predators of murrelet nests in- 

 cluded Common Ravens (Corvus corax) and Steller's Jays (Cyanocitta 

 stelleri); predation of a nest by a Great Homed Owl (Bubo virginianus) 

 was also suspected. We believe that changes in the forested habitat, 

 such as increased amounts of edge, are affecting murrelet produc- 

 tivity. Successful nests were significantly further from edges ( x = 

 155.4 versus 27.4 m) and were better concealed ( x = 87.2 versus 

 67.5 percent cover) than unsuccessful nests. The rate of predation 

 on Marbled Murrelet nests in this stud appear higher than for many 

 seabirds and forest birds. If these pre< ation rates are representative 

 of rates throughout the murrelet's range, then the impacts on 

 murrelet nesting success will be significant. We hypothesize that 

 because this seabird has a low reproductive rate (one egg clutch), 

 small increases in predation will have deleterious effects on popu- 

 lation viability. Rigorous studies, including testing the effects of 

 various habitat features on recruitment and demography, should be 

 developed to investigate the effects of predation on Marbled Murrelet 

 nesting success. 



Nesting success in seabirds is influenced by a variety of 

 physical and biological factors, including food availability, 

 habitat quality, energetics, predation, and climatic conditions 

 (Croxall 1987, Nettleship and Birkhead 1985, Vermeer and 

 others 1993). Because the effects of these factors can vary 

 spatially and temporally, seabird nesting success can be 

 highly variable among years (Birkhead and Harris 1985; 

 Boekelheide and others 1990; De Santo and Nelson, this 

 volume). For example, in some years, anomalous warm 

 oceanographic conditions (El Nino) cause a decrease in prey 

 availability, thus impacting nesting attempts and nest success 

 (Ainley and Boekelheide 1990, Hodder and Greybill 1985, 

 Vermeer and others 1979). In addition, disturbance to nesting 

 habitat (e.g., habitat loss, modification) and associated 

 cumulative impacts can affect the ability of seabirds to 

 successfully reproduce (Evans and Nettleship 1985; Gaston 

 1992. Reville and others 1990). 



1 Research Wildlife Biologist. Oregon Cooperative Wildlife Research 

 Unit. Oregon State University. Nash 104, Corvallis, OR 97331-3803 



: Research Biologist, Hamer Environmental. 2001 Highway 9, Mt. 

 Vemon,WA 98273 



The influence of these biological and physical factors 

 on the nesting success of Marbled Murrelets (Brachyramphus 

 marmoratus) is not fully known. In order to completely 

 address this issue, well designed studies investigating the 

 conditions that directly influence murrelet reproduction are 

 needed. However, data are available on murrelet nesting 

 success from tree nests that have been located and monitored 

 in North America. In this paper, we summarize this 

 information on murrelet productivity and sources of mortality. 

 In addition, because predation was the major cause of nest 

 failure, we discuss the implications of predation on this 

 threatened, forest-nesting seabird. 



Methods 



We compiled information on nest success and failure 

 from published and unpublished records of 65 Marbled 

 Murrelet tree nests found in North America between 1974 

 and 1993. The sample size of tree nests were distributed by 

 state and province as follows: Alaska (n = 18), British 

 Columbia (n = 9), Washington (n = 6), Oregon (n = 22), 

 and California (n = 10) (table 1). Success and failure of 

 nests were determined through intensive monitoring of 

 nesting activity, or evidence collected at the nest. The 

 outcomes of nests were compared between regions (Alaska 

 versus British Columbia, the Pacific Northwest and northern 

 California). Nests were considered to fail if: (1) the chick 

 or egg disappeared, fell out of the nest, or was abandoned; 

 (2) the chick died; (3) unfaded eggshell fragments were 

 found during the breeding season in nest cups without 

 fecal rings; or (4) predation was^ documented. Nests were 

 considered or assumed to be destroyed by a predator based 

 on one or more of the following: ( 1 ) predation was observed, 

 (2) the egg or chick disappeared prematurely between nest 

 observations and neither were located on the ground after a 

 thorough search of the area, and (3) evidence, such as 

 puncture marks on eggs, or albumen or blood on eggshell 

 fragments, was discovered and predators were aware of the 

 nest location or seen in the immediate area. In addition to 

 data from active nests, information on eggs, nestlings, and 

 hatch-year birds found on the ground were compiled from 

 published and unpublished records between 1900 and the 

 present. 



We used a Mann-Whitney U-test to compare the 

 characteristics of nests that were successful with those of 

 nests that failed because of predation. Variables used in the 

 analysis were those that could have an effect on nest exposure 

 or concealment: distance to edge, canopy cover, stand size, 

 percent cover above the nest cup, nest height, distance of the 



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



89 



