Divoky and Morton 



Chapter? 



Dispersal, Habitat Loss, and Implications 



The secretive nature of murrelet nesting has precluded 

 the determination of breeding areas solely by the discovery 

 of nests, eggs or chicks. Biologists and managers have had 

 to identify breeding areas based on the birds engaged in 

 activities included in "occupied behavior" as strongly 

 indicative of nesting (Ralph and others 1994). Relying on 

 instances of occupied behavior as an indication of the 

 importance of a stand to Marbled Murrelets has a number of 

 potential weaknesses. 



First, recently matured forests that are able to support 

 nesting could not be expected to be immediately discovered 

 and occupied by prospecting murrelets. The ability of alcids 

 to occupy areas where suitable breeding habitat is made 

 available is evident from the rapid colonization of islands in 

 the Aleutian Islands where fox have been eliminated (Bailey 

 and Kaiser 1993). The occupation of newly available suitable 

 habitat by Marbled Murrelets in Washington, Oregon, and 

 California may be delayed by the small stand size, high 

 fragmentation and disjunct distribution of the old growth 

 forest. The small size and apparently low breeding success 

 (Nelson and Hamer, this volume b^ of the population can be 

 expected to further slow occupation ( -f newly available habitats. 

 Because almost all prospecting of currently unoccupied suitable 

 habitat would occur through natal dispersal, low productivity 

 would reduce the potential of a population to disperse. This 

 would result in a lack of detections in stands that have the 

 potential of supporting murrelet breeding, but have not yet 

 been discovered by murrelets. The importance of this 

 apparently suitable but currently unoccupied habitat to the 

 future of the species needs to be recognized. 



In regions where a large nonbreeding population is 

 prevented from breeding by lack of nest sites, prospecting 

 birds might investigate areas and habitats that do not support 

 breeding. This could result in "occupied" behavior being 

 recorded in areas where nesting is not occurring. Prospecting 

 alcids can be present in apparently suitable habitat (Divoky 

 1982, unpubl. data; Kress and Nettleship 1988; Carter and 

 others 1992), although no breeding is occurring. If loss of 

 old-growth habitat has both increased the level of dispersal 

 and limited potential nest sites, substantial numbers of 

 murrelets could be displaying "occupied behavior" in habitats 

 where breeding is not currently being attempted or where 

 successful breeding could not occur. Such could be the case 

 in central California where Carter and Erickson (1988) 

 believed that all remaining nesting habitat is occupied and 

 because the population is nest site limited, nonbreeding 

 birds may be present over land and sea in a greater percentage 

 than elsewhere. While this may result in overestimating the 

 use of stands, it is unlikely that murrelets would be repeatedly 

 encountered in stands that do not have some present or 

 future potential for supporting successful breeding. 



Discussion 



The coastal old-growth forest utilized for breeding by 

 Marbled Murrelets would have selected for relatively high 

 rates of breeding and natal dispersal. Based on the behavior 

 and cryptic coloration of the breeding adults and chicks, and 

 the high rate of nest predation for observed nests (Nelson 

 and Hamer, this volume b), the risk of nest predation appears 

 to be higher than for other alcids. The assumed high rate of 

 nest predation would have selected for frequent short distance 

 movements, while the extensive time required for old growth 

 stands to be destroyed or degraded under natural conditions 

 would have selected for individual fidelity to a nesting stand. 

 There is no indication that the distance that breeding murrelets 

 typically disperse would be any greater than the conservative 

 movements (usually <1 km) that have been observed for 

 other alcids. 



Most dispersal in alcids is probably due to natal dispersal, 

 and Marbled Murrelets appear to have the capacity for extensive 

 natal dispersal given the extent of the breeding range, the 

 overlap between the wintering and breeding areas, and the 

 distance individuals are known to move from breeding areas 

 in winter. It would not be unreasonable to assume the percentage 

 of birds that initiate breeding at a non-natal locality (natal 

 dispersal) is as high or higher than has been reported for other 

 alcids (approximately 50 percent) (Harris and Wanless 1991). 

 The ability to prospect for breeding localities should be well 

 developed in Marbled Murrelets. Unlike the potential breeding 

 area of most alcids, which is linearly distributed in a narrow 

 band on the shoreline, murrelet nesting habitat is found in a 

 wide (as much as 60 km) band adjacent to the coast. 



Breeding habitat fragmentation and loss can be expected 

 to have affected the rate and extent of murrelet dispersal. 

 In Washington, Oregon, and California, high predation 

 rates apparently associated with fragmentation would select 

 for increasing the rate and extent of breeding dispersal. 

 However, the small size and highly fragmented and disjunct 

 nature of the old-growth remaining in this area can be 

 assumed to have decreased the potential distance for breeding 

 dispersal (at least in areas where stand size is small). Natal 

 dispersal rates and extent may have been increased as 

 habitat in the natal locality was reduced and the distance to 

 the location of suitable habitat is increased. These changes 

 in dispersal may have the overall effect of depressing 

 reproductive output. 



Acknowledgments 



We thank George Hunt, Linda Long, Phil Detreich, 

 and Edward Murphy for helpful comments and work on 

 this manuscript. 



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



87 



