Cavity-nesting birds are those that make their nests inside holes in trees (Fig. 1). These holes, or cavities, are usually built by woodpeckers (Fig. 2) and occasionally by birds such as nuthatches and chickadees (Figs. 3 & 4), which have the ability to do so. Other birds use these cavities because they cannot construct their own, including wrens, swallows, bluebirds, and some small owls (Figs. 5-10).

Quaking aspen is the tree of choice for cavity-nesting birds in Rocky Mountain National Park (Fig. 11), perhaps because the wood is relatively soft and easier to excavate. Aspen provide nest sites for 19 different bird species that regularly breed in the Park. Many trees have multiple cavities (Fig. 12) and it is common to find more than one pair of birds using the same tree. Over the course of this project, as many as five different pairs of birds have been found nesting in different cavities in the same tree at the same time.

Aspen is also relished as food by the large population of elk in the Park (Fig. 13). Elk eat the tips of new aspen shoots, which makes the aspen grow into small, stunted “bushes” rather than sizable trees (Fig. 14). Elk also eat the bark from the trunks of grown aspen, creating entry points for insects and diseases that weaken and may eventually kill the trees (Figs. 15 & 16).

Research conducted on aspen groves within the elk winter range of the Park since 1975 has shown significant aspen declines primarily due to elk browsing. Trees that die and fall (Fig. 17) are not being replaced by new trees. Live, healthy trees fall as well, compounding the problem; trunks are snapped and whole trees are uprooted by the frequent strong winds occurring in the Park (Figs. 18 & 19). The concern for cavity-nesting birds is that eventually there will not be enough trees of sufficient size to support nest cavities – most of the aspen cavity trees in the Park are about 12” in diameter. The likely result will be a decline in cavity-nesting birds in the Park.

This project was originally begun in 1997, when 550 cavity trees were located in 21 aspen stands within the elk winter range on the eastern side of the Park. Since 2002 when the project resumed, the number of cavity trees surveyed has grown to over 1400. Surveys are conducted during the breeding season of cavity-nesting birds, from May to July (Fig. 20). All trees are mapped with coordinates that allow them to be easily found with the map and a GPS device. Information collected for each tree includes the condition of the tree (live or dead), tree height, DBH (diameter-at-breast-height; Fig. 21), number and height of each cavity, diameter of each cavity entrance, the direction each cavity faces, and any birds using the cavities for nesting. Any changes in these features are documented when the trees are revisited in subsequent years, in particular, if a tree dies or falls, if it is still being used for nesting, if new cavities are built, and if existing cavities are still be being used.

Many of the cavity trees found in 1997 are still being used, but over 20% of these are known to have fallen and are no longer available to cavity-nesting birds. Some of these trees were never found again and are presumed to have fallen as well. New cavity trees are found during surveys each year, many of them selected by sapsuckers and other woodpeckers for building new nest cavities (Fig. 22). These cavity builders appear to choose live trees more often than dead ones. From 2002 to 2007, 71% of all woodpecker nests found during surveys were in live trees. This preference emphasizes how important it is for aspen to be able to reproduce to larger, healthy trees.

Most cavities are believed to be built by Red-naped and Williamson’s Sapsuckers (Figs. 23 & 24), because of direct observation of new excavations, or because most existing cavities have the hole size and height normally created by these two species. Sapsuckers may use the same cavities in subsequent years, or they may build new ones. Northern Flickers (Fig. 25), another type of woodpecker, will often enlarge existing sapsucker holes for their own use. The most common species found in cavities annually, however, are the “secondary” cavity-nesting House Wren (Fig. 26), followed by Tree Swallow and Violet-green Swallow (Fig.27). Cavity occupancy rates, or the proportion of cavities used to cavities available, have varied among all the stands surveyed, from 5.6 % to 31.8%. The average rate across all stands is 15%.

Another aspect of this project involves capturing and color-banding adult Red-naped Sapsuckers and Williamson’s Sapsuckers at their nest cavities. The idea is to try to determine how important different stands of trees and individual cavity trees are to the sapsuckers that return to the Park to breed. Mist nets erected at the cavity trees (Fig. 28) are used to capture the birds as they come and go from the nest (Fig. 29). Birds are fitted with a unique combination of colored leg bands along with the federally required, aluminum leg band with a unique 9-digit number (Fig. 30). This allows for returning birds to be identified as they are seen, without having to recapture them.

From 2003 to 2007, 21 Red-naped and 24 Williamson’s Sapsuckers have been color-banded (Fig. 31). Of these, six Red-naped Sapsuckers and three Williamson’s Sapsuckers have been seen in successive years after being banded, either in the same cavity tree or in a different tree nearby. One male Red-naped Sapsucker, dubbed “Jet-Yellow” for his jet black and yellow leg bands (Fig. 32), has returned every year since his capture in 2003, through 2008. His banded mate returned through 2005, but he has been seen with a new mate since 2006.

Tracking aspen cavity trees on the elk winter range to determine their longevity and their continued use or disuse by cavity-nesting birds is a primary objective of this project. This information will be tied to the impact of elk browsing on aspen and the potential long-term consequences for cavity-nesting birds. Field work for this project is expected to conclude during 2008, after which the data will be analyzed and conclusions drawn. Results will assist Park managers in the implementation of the 2008 Elk and Vegetation Management Plan ( http://www.nps.gov/romo/parkmgmt/elkvegetation.htm). It will also be useful to land managers of aspen habitats across Colorado and elsewhere.

For more information, contact:

Nancy Gobris, Field Biologist, Rocky Mountain Bird Observatory, nancy.gobris@rmbo.org

Jeff Connor, Natural Resources Specialist, Rocky Mountain National Park, 970-586-1296, jeff_connor@nps.gov

Regularly breeding, cavity-nesting bird species that use aspen for nesting in Rocky Mountain National Park.
Common Name	Scientific Name
Northern Saw-whet Owl	Aegolius acadicus
Flammulated Owl	Otus flammeolus
Northern Pygmy-Owl	Glaucidium gnoma
Williamson’s Sapsucker	Sphyrapicus thyroideus
Red-naped Sapsucker	Sphyrapicus nuchalis
Downy Woodpecker	Picoides pubescens
Hairy Woodpecker	Picoides villosus
American Three-toed Woodpecker	Picoides dorsalis
Northern Flicker	Colaptes auratus
Violet-green Swallow	Tachycineta thalassina
Tree Swallow	Tachycineta bicolor
Mountain Chickadee	Poecile gambeli
Red-breasted Nuthatch	Sitta canadensis
White-breasted Nuthatch	Sitta carolinensis
Pygmy Nuthatch	Sitta pygmaea
House Wren	Trogolodytes aedon
Mountain Bluebird	Sialia currocoides
Western Bluebird	Sialia mexicana
European Starling	Sturnus vulgaris