Divoky and Morton 



Chapter 7 



Dispersal, Habitat Loss, and Implications 



with a bird that owns a nest site increases the chances that a 

 bird will pair with an experienced breeder. 



Nonbreeding birds, with no previous experience, also 

 probably form pairs near the nest site. Observations of Black 

 Guillemots in northern Alaska (Divoky, unpubl. data) show 

 that nonbreeders are present at the colony throughout the 

 breeding season, and many display a high level of mate and 

 site fidelity. Although nonbreeders form pairs with each other, 

 when one member of an established nest site owning pair dies, 

 the vacancy is typically filled by a nonbreeder of the appropriate 

 sex. Nonbreeding pairs can be recruited as a unit should a new 

 site be created or should two vacancies occur at an established 

 site. However, the low annual mortality rates of breeding 

 alcids indicates that most recruitment occurs through a single 

 vacancy in an established pair. With recruitment occurring at 

 or near the nest site, the established breeder and the individual 

 being recruited, can pair with a familiar bird. Recruitment in 

 murrelets could occur in the same manner. Those birds 

 prospecting new nesting areas could pair on the water before 

 prospecting potential nest sites. 



Implications of Habitat Loss and 

 Fragmentation of Populations 



The final rule listing Marbled Murrelets as threatened 

 (U.S. Fish and Wildlife Service 1992) regards loss of older 

 forests and associated nest sites as the main cause of decline 

 in murrelet populations. When nest sites are limiting, the 

 loss of nesting habitat has both immediate and long term 

 impacts on the reproductive potential of a murrelet population. 

 While alcid populations have been shown to recover in a 

 relatively short period from episodic anthropogenic mortality 

 events, such as gill net and oil spill mortality (Piatt and 

 others 1991; Carter and others 1992), loss of nesting habitat 

 directly affects the long term reproductive potential of a 

 population. This is especially true for tree-nesting Marbled 

 Murrelet populations where the creation of nesting habitat is 

 extremely time-consuming, perhaps 200 years. 



Fragmentation of old-growth also has the potential of 

 reducing murrelet breeding success by increasing the densities 

 of predator populations. Corvids are "edge species" that 

 have been found to increase in numbers with increased 

 forest fragmentation (Andren and others 1985, Wilcove 

 1985, Small and Hunter 1988). Similar findings have been 

 reported in central Oregon regarding Great Horned Owls 

 (Johnson 1992). In addition, corvid predation on small bird 

 nests has been found to increase with increased forest 

 fragmentation, decreased distance of nests from a forest 

 edge or both (Gates and Gysel 1978, Andren and others 

 1985, Small and Hunter 1988, Yahner and Scott 1988). 

 Factors that increase fragmentation, such as a wildfire or 

 timber harvest, could reduce murrelet breeding success both 

 through the reduction of cover and the increase in predator 

 densities. This reduced breeding success could be expected 

 to increase the rate, and possibly the distance, of breeding 



dispersal. The distances moved would probably relate to the 

 level of disturbance and the threat that the predators pose to 

 adult birds. The reduction and fragmentation of habitat 

 would also act to increase the distance prospecting prebreeders 

 would have to travel to find a suitable nest site. 



Habitat loss could be expected to result in the 

 displacement of breeding birds, while fragmentation could 

 lead to both displacement and decreased breeding success. 

 In cases where stands used for nesting are destroyed, the 

 birds previously breeding in the stand would have to locate 

 a new nesting area. If all available nest sites in adjacent 

 habitats are occupied, the displaced birds could attempt to 

 breed in suboptimal sites with a decreased chance of 

 successful reproduction, prospect more distant areas, or not 

 breed at all. There are no conclusive indications of higher 

 densities of murrelet nesting in stands remaining after timber 

 harvests (Ralph and others, this volume). The ease and 

 rapidity with which displaced murrelets seek out new 

 breeding areas could be expected to be related to how 

 frequently murrelets normally change sites. If the level of 

 individual nest-site fidelity is as low as observations indicate, 

 then murrelets may be able to readily move at least short 

 distances to new nest sites. The fidelity birds show to a 

 previously used breeding area or site that no longer can 

 support breeding, should be related to the rate and magnitude 

 of habitat destruction. There is evidence of murrelets visiting 

 remnants of newly harvested stands before disappearing 

 from the area (Folliard, pers. comm.), thus indicating that 

 murrelets might not immediately abandon the unsuitable 

 nest stand. This is consistent with observations in other 

 alcid species. Pairs have shown fidelity to previously 

 occupied, and recently destroyed, nest sites for two years in 

 the Black Guillemot (Divoky, unpubl. data), and a minimum 

 of two years in the Least Auklet (Aethia pusilla) (I. Jones, 

 pers. comm.). This type of nest loss would be similar to the 

 loss of a previously used murrelet nest platform branch and 

 not the removal of a nesting stand. 



Management Implications of Dispersal 



High levels and extensive distances of natal dispersal 

 could result in source areas with high productivity producing 

 young that will be incorporated into sink regions with low 

 productivity, or high adult mortality, or both. This could 

 result in populations in sink areas showing little change in 

 numbers. Without monitoring breeding success, the inability 

 of the sink population to produce enough young to balance 

 adult mortality would not be evident. The maintenance of 

 such a population would be dependent on the continued 

 production of a surplus of young by the source population. 

 The true reproductive status of the sink population would be 

 masked until immigration declines. Such immigration could 

 explain the ability of the central California murrelet population 

 to lose an estimated 150 to 300 birds in the early 1980s 

 (Carter and Erickson 1988) and not show any signs of decline 

 (Carter and others 1992). 



86 



USDA Forest Service Gen. Tech. Rep. PS W- 152. 1995. 



