Hamer and Nelson 



Chapter 6 



Characteristics of Nest Trees and Nesting Stands 



from 50-200 cm. No increase in the mean number of platforms 

 was evident for larger trees that ranged from 220-300 cm in 

 diameter. These results explain why all the nest trees found 

 in the Pacific Northwest were >88 cm in diameter, although 

 mistletoe brooms on smaller trees may also provide habitat. 

 In southcentral Alaska, the minimum d.b.h. associated with 

 a tree having at least one platform ranged from 29-37 cm 

 (Naslund and others, in press). 



In a study completed in 1993, nest tree and stand 

 characteristics in Washington and Oregon were compared 

 between 15 murrelet nests and randomly located dominant 

 trees and plots within the same nest stand (Nelson and others 

 in press). Nest sites were similar to the forest stands in which 

 they were located, except that a significantly higher number 

 of potential nest platforms were recorded at nest trees, than 

 at random trees. They also found that Marbled Murrelets 

 selected trees at nest sites that had >4 potential nest platforms, 

 and trees with <3 platforms were avoided. In Alaska (Naslund 

 and others, in press), one study compared nest tree char- 

 acteristics (n = 14) to a sample of rardom trees surrounding 

 each nest tree, and found nest trees /ere larger in diameter, 

 had more potential nest platforms, and had greater epiphyte 

 cover. This study also concluded that Sitka spruce appeared 

 to be the most suitable tree for nesting when compared to 

 western hemlock and mountain hemlock, because of its high 

 number of platforms and greater epiphyte cover. They also 

 found that nest and landing trees tended to be larger in 

 diameter than surrounding trees, and nest trees were more 

 likely to contain at least one potential nest platform with 

 moderate to heavy epiphyte cover when compared to nearby 

 trees. Stands with high potential nest platform densities may 

 reduce competition for nest branches and provide a high 

 diversity of nest site choices. 



Nests located high in the canopy may provide better 

 access by adults to the nest site in dense, old-growth stands. 

 Nesting as high in the canopy as possible may also help in 

 avoiding predation. Although positioning the nest as high 

 off the ground as possible would likely reduce the incidence 

 of mammalian predators, we have also observed that the 

 Steller's Jays (Cyanocitta stelleri), predators of nestlings 

 and eggs, often forage in the lower portions of the canopy. 

 Better horizontal and vertical cover is available in the top 

 portions of the tree crown which may help reduce predation. 

 Data needs to be collected on the positioning of nests 

 within the live crown of the tree, not just the tree bole, to 

 determine if murrelets prefer certain areas of the tree crown 

 foliage for nesting. 



Murrelets may choose to place nests near the trunk of 

 the tree for a variety of reasons. First, overhead and horizontal 

 cover is higher around the nest cup due to the position of the 

 tree crown directly overhead. Second, the tree trunk itself 

 provides a large amount of cover and visual screening and 

 branches are typically larger in diameter near the tree bole. 

 Also, more duff and litter, which often form the nest substrate, 

 is trapped near the tree bole, and the percent cover of moss 

 on the limbs of trees is higher, often forms a more complete 



coverage, and forms a deeper layer near the tree bole. Some 

 conifer species typically have little or no moss available on 

 their limbs, so that platforms created by accumulations of 

 duff and debris are the only nest choices available for murrelets 

 in these forest types. 



Murrelets nest on large limbs. The smallest limb used at 

 the nest cup throughout the range of the murrelet was 10 cm 

 in diameter, which is likely the smallest diameter branch 

 that could support a successful nest. Nests located on smaller 

 limbs would probably have a higher likelihood of losing 

 chicks or eggs from accidental falls, an occurrence that is 

 well documented (Hamer and Nelson, this volume a). Nests 

 located on limbs <16 cm diameter all had moss as a nest 

 substrate, except in one instances where a 13 cm nest branch 

 had litter and lichen as a substrate. Small limb diameters 

 without a moss covering may be avoided by nesting birds 

 because the hazards of raising eggs and young are increased 

 without the moss to help stabilize and insulate the egg on 

 the limb, increase the diameter of the nest limb/platform, 

 and provide a substrate on which to create a nest cup 

 (depression). In addition, moss and litter may help insulate 

 eggs and chicks during cold weather and may help drain 

 water from eggs and chicks helping thermoregulation 

 (Naslund and others, in press). An abundance of mosses 

 creates a multitude of nest platform choices by providing 

 substrate on many locations throughout a single limb. In 

 addition, the presence of dwarf mistletoe in stands can 

 increase the number of nesting opportunities for murrelets 

 and may be important in providing nest platforms in areas 

 with low moss abundance and dryer conditions. 



The nest site selection of the Marbled Murrelet may 

 have evolved primarily to reduce predation. Selection of 

 nest sites away from the coast, in dense old-growth and 

 mature forests with multi-layered canopies, high in the forest 

 canopy, on limbs with high overhead and horizontal cover, 

 and near the tree bole where the tree bole itself provides a 

 large degree of cover, may help reduce nest predation. Results 

 from studies of murrelet habitat use to date have been derived 

 from comparisons of stands occupied by murrelets to 

 unoccupied stands, comparisons of stands receiving high 

 use versus low use, or comparisons of nest trees and nest 

 plots to random trees and plots. Although these can provide 

 extremely useful descriptions and definitions of suitable 

 habitat, they do not provide information on the habitat 

 characteristics associated with successful nests. Information 

 on the landscape and within-stand habitat characteristics 

 that influence reproductive success is needed to fully 

 understand murrelet nesting ecology and to model optimum 

 habitat suitability for this species. Such studies may find that 

 stand size analyzed in conjunction with the number of nesting 

 and hiding opportunities within the stand (habitat quality), 

 may greatly influence reproductive success because of 

 predation pressures at the nest site. Habitat factors that could 

 influence reproductive success may include stand fragmen- 

 tation, stand canopy closure, and the amount of overhead 

 and horizontal cover surrounding the nest. 



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



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