Research Project Summary: Effects of understory fire on cavity-nesting birds in Arizona pine forests

Smith, Jane Kapler, compiler. 2008. Research Project Summary: Effects of understory fire on cavity-nesting birds in Arizona pine forests. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: [].

Unless otherwise indicated, the information in this Research Project Summary comes from the following paper:

Horton, Scott P.; Mannan, R. William. 1988. Effects of prescribed fire on snags and cavity-nesting birds in southeastern Arizona pine forests. Wildlife Society Bulletin. 16: 37-44. [2]

Common names are used throughout this summary. For a complete list of the common and scientific names of species discussed in this summary and for links to FEIS species reviews, see the Appendix.

The study was conducted in the Santa Catalina Mountains on the Coronado National Forest, southeastern Arizona.

Elevation at the 3 study sites ranged from 2,075 to 2,410 m. Two sites were in southwest-draining canyons, and the third was on a west-facing mountainside. Area of the study sites ranged from 22 to 45 ha.

Before treatment, Arizona pine snags constituted 62% of snags and showed more evidence of use by cavity-nesting birds than snags of other species; Arizona pine snags contained 94% of cavities and 82% of active nests. Arizona pine snags >15 cm DBH are referred to in this Summary as "large snags", and those <15 cm DBH are referred to as "small snags".

Small snags in unburned stands showed little evidence of decay and contained no cavities. Most large snags in unburned stands showed some evidence of decay:

Density and condition of Arizona pine snags in unburned stands [2]

Decay class Number of snags/ha* Description of decay class
Limbs Sapwood Bark cover (%)
I <0.1 Foliage still on twigs Sound 100
II 4.0 Fine twigs remaining Sound to rotting 75-100
III 1.9 Secondary branches remaining Intact to rotting 25-85
IV 3.0 Branch stubs remaining Some sloughed 10-50
V 2.5 None Much sloughed 0-50
VI 0.5 Secondary branches remaining Sound to rotting <10
*Average of density in control stands and prefire treatment stands

Densities of cavity-nesting birds before treatment are shown below with posttreatment data (see Fire Effects on Cavity-nesting Birds). In unburned stands, cavity nesters generally preferred snags >50 cm DBH in decay classes II or III, although nest-tree preferences varied for the 8 most common species:

Nest tree preferences* of 8 cavity-nesting bird species in unburned stands [2]

Bird species Preferred size of large snag (cm DBH) Preferred decay class
House wren no preference no preference
Mountain chickadee >30 II
Pygmy nuthatch >50 III
Violet-green swallow 15 to <30 III
Western bluebird no preference III
Woodpecker species** >50 II
*Preferences shown in this table are all statistically significant (P<0.10), except for mountain chickadee (for which all 5 nests occurred in the same size and decay class)
**Data were combined for 3 species: acorn woodpecker, hairy woodpecker, and northern flicker

In the 65 years prior to this study, no wildfires in the study area were larger than 2 ha. The authors note that, prior to the 20th century, fires were probably frequent and extensive in southwestern ponderosa pine forests (including Arizona pine), burning every 5 to 12 years [2]. A recent synthesis of fire regime information for southwestern ponderosa pine forests [4] provides this description of the historical fire regime:

Fire regime information on the vegetation community studied in this Research Project Summary. Fire regime characteristics are taken from the LANDFIRE Rapid Assessment Vegetation Model [4]. This vegetation model was developed by local experts using available literature, local data, and expert opinion as documented in the PDF file linked from the Potential Natural Vegetation Group listed below. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
Minimum interval
Maximum interval
Southwest mixed conifer (warm, dry with aspen) Replacement 7% 300    
Mixed 13% 150 80 200
Surface or low 80% 25 2 70
*Fire Severities:
Replacement=Any fire that causes greater than 75% top removal of a vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not cause a lethal effect on the plants.
Mixed=Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects.
Surface or low=Any fire that causes less than 25% upper layer replacement and/or removal in a vegetation-fuel class but burns 5% or more of the area [1,3].

Plant Phenology was not described in the study. The November fires probably occurred after tree growth had ceased, and the May burn probably occurred while trees were growing.


Fire Management Objectives: Study sites were underburned to improve regeneration and growth of Arizona pines by exposing mineral soil and reducing density of small trees, and to reduce fuels as follows:

Woody fuel reduction planned for prescribed burns in the study [2]

Woody fuel size class Planned % reduction in fuel load
<0.6 cm diameter 70
0.6-7.6 cm diameter 55
>7.6 cm diameter 25

Research Objectives: This study compared 3 characteristics of Arizona pine forests before and after prescribed fire: number and characteristics of Arizona pine snags, availability of snags used by cavity-nesting birds, and breeding populations of cavity-nesting birds.

Fires were broadcast underburns that backed downslope and into the wind. Two stands were burned in mid-November 1984; the third was burned in early May 1985. Burns were conducted within the prescribed weather conditions listed below, and fire behavior generally stayed within the prescribed limits.

Range of burning conditions and fire behavior acceptable for prescribed burns [2]
Air temperature 4 to 24 C
Relative Humidity 25% to 40%
Wind speed 1-16 km/hr
Flame height 0.3-1.0 m
Flame length 0.3-1.2 m
Rate of spread 40-120 m/hr

Fire effects on Arizona pine snags were measured the first summer after prescribed burning (summer 1985). Treatments reduced both large and small snags by about 50%. Density of the kind of snags generally preferred by cavity nesters (>50 cm DBH in decay classes II or III) were reduced from 1.5 to 1.0 snags/ha. Snags with greater decay were significantly (P<0.001) more vulnerable to fire damage; 73% of snags in decay class IV burned down or were reduced more than 50% in height, in comparison with less than 20% of snags in decay classes II and VI. Snags >30 cm DBH were more likely to burn than smaller snags, probably because larger snags had more advanced decay. Snags with large amounts of undecayed twigs, branches, and sloughed bark at the base ignited more easily than those with less fuel or mostly decayed fuel at the base. Few large snags were created by the burns, but small snags increased 20-fold:

Effects of understory prescribed burns on density of Arizona pine snags [2]

Size class Prefire density (number/ha) Postfire density (number/ha)
Burned* Created** Total after fire
Large (>15 cm DBH) 11.1 5.5 0.5 6.1
Small (<15 cm DBH) 10.9 6.0 216.1 221.0
*Burned down, were reduced >50% in height, or had most of sapwood burned away
**Killed by prescribed burns

Cavity-nesting birds were surveyed the first breeding season after prescribed burning (May-July 1985). Only 3 species showed significant (P<0.10) differences in density between burned and unburned sites (which included both control sites and preburn treatment sites): Mountain chickadee density was greater, and northern flicker and violet-green swallow  densities decreased. The authors attributed the differences to changes in prey populations, shifts in foraging areas after burning, or both.

Density (number/40 ha) of cavity-nesting birds before and after understory fire [2]


Unburned (control) sites

Burned sites

Species 1984 1985 1984
(breeding season after treatments)
Acorn woodpecker 1.1 1.5 4.1 2.8
Ash-throated flycatcher     P*  
Bridled titmouse P P P P
Bewick's wren P P P P
Brown creeper 5.8 7.3 9.8 7.9
Cordilleran flycatcher (western flycatcher) 62.4 48.0 39.2 67.0
Dusky-capped flycatcher P P P P
Eastern bluebird P      
Hairy woodpecker 0.7 0.8 0.8 1.0
House wren 53.2 91.8 15.0 46.2
Mountain chickadee 7.2 7.0 5.6 12.2**
Northern flicker 1.7 2.3 2.6 2.1**
Pygmy nuthatch 26.2 15.8 24.4 14.2
Violet-green swallow 31.7 34.0 37.4 17.6**
Western bluebird 11.3 6.1 12.5 6.2
White-breasted nuthatch 4.4 2.9 7.0 4.2
*P=present at low density. Blank cell indicates not present.
**Significantly different from density on unburned sites (P<0.10)

Management objectives for the burns, to improve regeneration and growth of Arizona pines and reduce woody fuels, were met.

The authors recommend that protection of large snags (by constructing fuel breaks around the trees) be considered before conducting prescribed underburns in forests where large snags have been reduced by logging. It is most important to protect snags >50 cm DBH in decay classes II and III, which can provide nest sites for most cavity-nesting birds, and snags in decay class I, which may provide nest sites in the future.

This Research Project Summary contains fire effects and/or fire response information on the following species. For further information, follow the highlighted links to the FEIS reviews for those species.


Common name Scientific name
Arizona pine Pinus ponderosa var. arizonica
Bridled titmouse Baeolophus wollweberi (Parus wollweberi)*
Brown creeper Certhia americana
Northern flicker Colaptes auratus
Cordilleran flycatcher (western flycatcher) Empidonax occidentalis  (Empidonax difficilis)
Acorn woodpecker Melanerpes formicivorus
Ash-throated flycatcher Myiarchus cinerascens
Dusky-capped flycatcher Myiarchus tuberculifer
Hairy woodpecker Picoides villosus
Mountain chickadee Poecile gambeli (Parus gambeli)
Eastern bluebird Sialia sialis
Western bluebird Sialia mexicana
White-breasted nuthatch Sitta carolinensis
Pygmy nuthatch Sitta pygmaea
Violet-green swallow Tachycineta thalassina
Bewick's wren Thryomanes bewickii
House wren Troglodytes aedon
*For species that have undergone scientific name changes, names in parentheses are those used in the research paper.


1. Hann, Wendel; Havlina, Doug; Shlisky, Ayn; [and others]. 2005. Interagency fire regime condition class guidebook. Version 1.2, [Online]. In: Interagency fire regime condition class website. U.S. Department of Agriculture, Forest Service; U.S. Department of the Interior; The Nature Conservancy; Systems for Environmental Management (Producer). Variously paginated [+ appendices]. Available: [2007, May 23]. [66734]
2. Horton, Scott P.; Mannan, R. William. 1988. Effects of prescribed fire on snags and cavity-nesting birds in southeastern Arizona pine forests. Wildlife Society Bulletin. 16: 37-44. [5549]
3. LANDFIRE Rapid Assessment. 2005. Reference condition modeling manual (Version 2.1), [Online]. In: LANDFIRE. Cooperative Agreement 04-CA-11132543-189. Boulder, CO: The Nature Conservancy; U.S. Department of Agriculture, Forest Service; U.S. Department of the Interior (Producers). 72 p. Available: [2007, May 24]. [66741]
4. LANDFIRE Rapid Assessment. 2007. Rapid assessment reference condition models. In: LANDFIRE. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Lab; U.S. Geological Survey; The Nature Conservancy (Producers). Available: [66533]

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