Divoky and Morton 



Chapter 7 



Dispersal, Habitat Loss, and Implications 



pairs (Petersen 1981). For Ancient Murrelets, reoccupancy 

 rates of burrows that supported successful breeding the 

 preceding year was 80 percent, and only about 50 percent for 

 unsuccessful burrows (Gaston 1992). Nest changes caused 

 by simple breeding failure typically result in small scale 

 movements (usually tens of meters) to nearby sites (Divoky, 

 unpubl. data; Petersen 1981). 



Chronic disturbance at the nest site can cause estab- 

 lished breeders to move to a new breeding location thousands 

 of meters away. A Pigeon Guillemot that experienced 

 persistent disturbance at its nest site was found breeding on 

 an island 7.7 km away 3 years later (Drent 1965). At a Black 

 Guillemot colony where any movement of established 

 breeders is typically to an adjacent nest site (<10 m), one 

 bird moved approximately 1 km and another over 5 km, 

 after Horned Puffins (Fratercula corniculata) using the 

 same nest site had repeatedly disrupted nesting (Divoky 

 1982 and unpubl. data). 



Essentially all information on breeding dispersal in alcids 

 has been obtained through the banding and resighting of 

 individuals. The difficulty of capturing and observing Marbled 

 Murrelets at the nest site has prevented the collection of 

 similar information for this species. The old-growth nesting 

 habitat of the Marbled Murrelet is relatively stable. Natural 

 destruction of old growth forests through fire or wind storms 

 is rare enough, and the degradation of nest trees is slow 

 enough, that high site fidelity could have evolved. 



Observations of murrelets engaging in "occupied behavior," 

 strongly suggesting nesting (Ralph and others 1993), indicate 

 that Marbled Murrelets, as a species, exhibit high fidelity to 

 a nesting area. Marbled Murrelets have been recorded in the 

 same forest stands for a minimum of 20 years in northern 

 California (Strachan, pers. comm.; Miller, pers. comm.), 18 

 years in central California (S.W. Singer, pers. comm.), 7 

 years in Oregon (Nelson, pers. comm.), and 3 years in 

 Washington (Hamer, pers. comm.). These results are in part 

 a function of the duration of survey effort. While these 

 observations indicate that the species exhibits high fidelity 

 to forest stands, no direct information is available on stand 

 or nest-site fidelity of individual birds. 



For species having high annual survival and site fidelity, 

 the occupation of the same nest site in consecutive years is 

 strongly suggestive of individual nest-site fidelity. Re- 

 occupation of the same nest site has occurred only once in 

 the 13 instances where Marbled Murrelet nests have been 

 examined in the breeding season following a year of known 

 occupancy (P. Jones, pers. comm.) and nesting occurred in 

 the same tree four times (P. Jones, pers. comm.; Naslund, 

 pers. comm.; Nelson, pers. comm.; Singer, in press). 

 Additional evidence of fidelity to a nest tree is provided by 

 Nelson's (pers. comm.) finding of three nest cups on three 

 platforms in a single tree, although we do not know if it was 

 the same individuals. While the sample size is small, the 

 observed fidelity to the same nest depression in consecutive 

 years appears to be lower than for other alcids. This could be 

 related to the high rate of predation recorded for murrelet 



nests (Nelson and Hamer, this volume b). It also indicates 

 that while breeding habitat for this species is reduced (Perry, 

 this volume), and may be limiting, the number of nest 

 platforms apparently is not. If the high predation rate is a 

 recent phenomenon, nest-site fidelity may have been higher 

 in the past. As previously mentioned, breeding dispersal 

 increases with increased rates of nesting failure (Greenwood 

 and Harvey 1982). The high rates of observed nest failure 

 (Nelson and Hamer, this volume b) may explain murrelets 

 not reoccupying a nest site in subsequent years. 



Natal Dispersal 



The primary benefit that a bird derives from breeding at 

 its natal colony may be that the natal area is a known 

 location where conspecifics of a similar genetic background 

 successfully bred in the past (Ashmole 1962). However if a 

 breeding location is near those of related individuals, there is 

 the possibility of kin selection occurring and a moderate 

 level of inbreeding (Shields 1983). 



Philopatry (chicks returning to their natal colony or 

 nesting location to breed) is more difficult to study than the 

 fidelity of breeders to a nest site. It had been assumed that 

 the majority of alcids surviving to breeding are recruited 

 into their natal nesting area (Hudson 1985). More recent 

 information, however, shows that prospecting by prebreeders 

 at non-natal colonies is a regular occurrence in Common 

 Murres (Halley and Harris 1993) and Atlantic Puffins (Harris 

 1983, Kress and Nettleship 1988). Until recently, the instances 

 of banded birds initiating breeding at a non-natal colony 

 were limited (Asbirk 1979, Lloyd and Perrins 1977). However, 

 recent information indicates that, at least in the Atlantic 

 Puffin, half the chicks that survive to breeding emigrate to a 

 new colony (Harris and Wanless 1991). 



Other evidence of natal dispersal is provided by the 

 establishment of new colonies and growth rate of existing 

 colonies that could only be explained by immigration (Divoky, 

 unpubl. data; Gaston 1992; Petersen 1981). The frequency 

 with which new alcid colonies have formed on the west 

 coast of North America in the short period that systematic 

 censusing has been conducted (table 1) proves that natal 

 dispersal is common in the alcidae. 



The distance that birds will breed from their natal site 

 can be great. Banding returns show that the distance dispersed 

 can be as great as 420 km (by sea) for the Common Murre 

 (Halley and Harris 1993) and over 450 km for the Atlantic 

 Puffin (Harris and Wanless 1991). The rate of increase of 

 some breeding populations, and the establishment of new 

 colonies, indicates that Ancient Murrelets are being recruited 

 into breeding populations at least 30 km from their natal site 

 (Gaston 1992), Black Guillemots from over 500 km, and 

 Horned Puffins from over 200 km (Divoky, unpubl. data). 



Because of the difficulties of marking and subsequently 

 resighting Marbled Murrelets, any direct evidence of natal 

 dispersal would have to come from observations of range 

 expansion, occupation of previously unoccupied breeding 



84 



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



