Reprinted from Wildlife Society BulletinVolume 29, Number 1, Spring 2001

Initial and long-term use of inserts byred-cockaded woodpeckers

Daniel Saenz, Richard N. Conner, Christopher S. Collins,and D. Craig Rudolph

Abstract Artificial cavities have become a standard management technique for red-cockadedwoodpeckers (Picoides borealis). Seventy cavity inserts were installed in our study siteson the Angelina National Forest in eastern Texas from 1990 to 1995. Eighty-two percentof the inserts were used for at least one year. It is still too early to make a direct com-parison, but it is likely that inserts will remain usable as long as natural cavities do. Insertsinstalled in 1990 and 1991 were 20.5 cm in height, whereas inserts installed from 1992to 1995 were 25.5 cm in height. Larger inserts (25.5 cm) appear to remain usable for alonger time than smaller inserts (20.5 cm). Newer unused inserts are more likely tobecome active for the first time than older unused inserts. Similar to unused inserts,active cavities (naturally excavated and inserts) that have become inactive are less likelyto be reactivated the longer they are inactive. Newness and recency of cavity use andred-cockaded woodpecker activity appear to be important factors in the attractiveness ofinserts and naturally excavated cavities.

Key words active cavity, artificial cavity, cavity tree, insert cavity, Picoides borealis, red-cockadedwoodpecker

Red-cockaded woodpeckers excavate their roostand nest cavities in a “cluster” of living pines (Pinusspp.) that are generally close to each other. Duringthe 1980s red-cockaded woodpecker populationswere declining in many parts of the species’ range(‘James 1995). Conner and Rudolph (1995~2) report-ed that cavity losses were exceeding new cavityexcavation by the woodpeckers in eastern Texasand that this might be a factor in causing clusterabandonment and subsequent population declines.Techniques to provide artificial cavities were devel-oped in the late 1980s to offset the presumed cavi-ty deficit. Copeyon (1990) developed one type ofartificial red-cockaded woodpecker cavity bydrilling a cavity directly into the heartwood of a liv-ing pine tree. Allen (1991) followed with an insertdesign that consisted of a nest box made of west-ern redcedar (Thuja plicata) that is inserted into aliving pine tree with the aide of a chainsaw. The

insert had an advantage over the drilled cavity inthat’ it could be used in trees with smaller heart-wood diameters (typical of younger trees) as longas the overall diameter of the tree was large enoughto house the insert (>38 cm). The insert was cho-sen as the only artificial cavity type to be used inTexas because it was easier to install.

Artificial cavities were first installed in the nation-al forests inTexas in1990. By this time red-cockad-ed woodpecker populations in Texas were declin-ing (Ortego et al. 1988, Conner and Rudolph 1989)and cavities were thought to be a limiting factor(Conner and Rudolph 1995a). Since 1990, red-cockaded woodpecker populations in Texas nation-al forests have stabilized and some have shownmoderate increases (Conner et al. 1997b). A largeproportion (67%) of the current population on theAngelina National Forest is now roosting and nest-ing in artificial cavities.

A u t h o r s ’ a d d r e s s : W i l d l i f e H a b i t a t a n d S i l v i c u l t u r e L a b o r a t o r y , S o u t h e r n R e s e a r c h S t a t i o n , U n i t e d S t a t e s F o r e s t S e r v i c e , Nacog-doches, T X 7 5 9 6 2 , U S A ; e - m a i l f o r S a e n z : c-saenzd@titan.sfasu.edu.

Our objective was to determine the frequency ofuse of inserts by red-cockaded woodpeckers. Wealso compared the usable life spans of inserts withthose of natural cavities. Finally, we determined theusable life span of inserts and how initial use andperiods of inactivity of insert and naturally excavat-ed cavities affect the probability of subsequent use.

Study areaWe conducted the study on the southern por-

tion of the Angelina National Forest in easternTexas. We worked in longleaf pine (Pinus palus-h-is) stands because longleaf is much more resist-ant to southern pine beetle (Dendroctonusfrontalis) infestation than are loblolly (P taeda)and shortleaf (P echinata) pines (Conner et al.1991, Conner and Rudolph 1995a, Conner et al.1998a), resulting in a lower incidence of cavity-tree turnover. This allowed us to focus on cavityselection by red-cockaded woodpeckers withoutthe confounding effects of a high incidence ofcavity-tree mortality.

MethodsWe surveyed active clusters (those with at least

one tree in which a red-cockaded woodpecker wasroosting at night) annually on the southern portionof the Angelina National Forest for red-cockadedwoodpecker activity from 1983 to 1999. Freshexcavation at resin wells was used to distinguishbetween active and inactive cavities (Jackson1978). We conducted surveys immediately prior tothe nesting season to obtain a prebreeding seasonestimate of woodpecker cavity use. We did notdetermine the internal status of the cavities, and wenever cleaned debris from cavities that may influ-ence use rates. However, a program to removesouthern flying squirrels (Glaucomys volans) wasimplemented in active clusters on the AngelinaNational Forest beginning in 1998 (Ronald Mize,personal communication).

Seventy inserts were installed into 12 active clus-ters over a 6-year period from 1990 to 1995. Insertsinstalled in 1990 and 1991 were smaller (20.5 cm inheight) than inserts installed from 1992 to 1995(25.5 cm in height). We surveyed the clusters forwoodpecker activity to determine insert use rateand the long-term usability of inserts. The sameclusters also were surveyed to determine thelength of time that naturally excavated cavities

were used by red-cockaded woodpeckers between1983 and 1999. The survey was expanded toinclude cavities that were excavated prior to theimplementation of inserts to obtain a sufficientsample size.

We combined all data on inserts to determine theproportion of use the following year after installa-tion. Inserts that were not used in their first yearwere combined to determine the proportion of usein the second year following installation. We con-tinued to combine data for remaining inserts todetermine the proportion of inserts initially used ina given age class for a total of 9 years. We used alogistic regression analysis to determine whetherthe probability of a cavity insert being used for thefirst time was related to insert age. Our sample sizewas too small within year and variable among yearsto evaluate the effect of year.

Naturally excavated cavities and inserts that wereactive at any time during the study and laterbecame inactive (n=78 and 23, respectively) wereexamined from 1983 to 1999 to determine the pro-portion that would become active again after aperiod of inactivity. We combined the unused cavi-ties from each age cohort and determined the pro-portion of cavities that became active after a givenperiod of inactivity, up to 5 years. We used a logis-tic regression analysis to determine whether theprobability of a cavity becoming active after a peri-od of inactivity was related to the length of time thecavity was inactive. Our sample size also was toosmall within year and variable among years in thismodel to evaluate the effect of year.

ResultsRed-cockaded woodpeckers used 82% of the cav-

ity inserts installed in the southern portion of theAngelina National Forest for at least 1 year between1990 and 1999. Inserts also were used by severalother species of wildlife (Conner et al. 1996, Con-ner et al. 1997~) possibly precluding use by red-cockaded woodpeckers in some instances. A slight-ly larger proportion of the older, smaller insertsreceived use by red-cockaded woodpeckers initial-ly, but most of the use occurred when cavities werein short supply and the inserts were new. Use ofthese inserts dropped markedly as they aged (Fig-ure 1). A slightly lesser proportion of the newer,larger inserts were used, but amount of use did notdecrease at the same rate as that of the older, small-er inserts (Figure 1).

100 Table I. Probability of cavity use by red-cockaded woodpeck-ers as a function of insert age and length of time a cavity was

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Figure 1. Percentage of small and large inserts used by red-cockaded woodpeckers (active) by insert age. Small inserts(20.5 cm in height) were installed in 1990 and 1991. Largei n s e r t s ( 2 5 . 5 c m i n h e i g h t ) w e r e i n s t a l l e d f r o m 1 9 9 2 t o 1 9 9 5 .

Thirty-two cavities were naturally excavated byred-cockaded woodpeckers in the study site from1983 to 1999, ranging in age from one to 16 years.Natural cavities were used a minimum of 1 year anda maximum of 12 years. To date, inserts have beenused for 0 years to 8 years.

Logistic regression analysis indicated that theprobability of an insert being used for the first timedeclined as the insert aged (Table 1). Sixty percent(42 inserts) of the 70 inserts in the longleaf habitatwere used in their first year after installation. Ourmodel predicted a 350% decrease in odds of aninsert cavity becoming active for the first time witheach additional year of age. No unused inserts over5 years of age have ever become active in our studysite (Figure 2).

Logistic regression analysis indicated that theprobability of a cavity becoming active after a peri-od of inactivity declined the longer a cavity was

1 2 3 4 5 6 7 6 9 10

A g e o f i n s e r t

Figure 2. Percentage of cavities receiving initial use by red-cockaded woodpeckers by insert age. All inserts (n = 70) werec o m b i n e d a n d u s e d c a v i t i e s w e r e r e m o v e d f r o m t h e f o l l o w i n gy e a r ’ s s a m p l e s o t h a t t h e p r o p o r t i o n o f c a v i t i e s u s e d f o r t h e f i r s tt i m e i n e a c h a g e g r o u p c a n b e c a l c u l a t e d .

a Estimate of explanatory slope (px).b S t a n d a r d e r r o r o f s l o p e e s t i m a t e .c Percentage of variation explained by the explanatory vari-

a b l e i n t h e m o d e l ( M e n a r d 1 9 9 5 ) .d ~2 statistic testing He: slope estimate = 0.e Probability to reject H,.

inactive (Table 1). Forty-five percent of cavities(naturally excavated and inserts combined) becameactive again after one year of inactivity. Our modelpredicted a 36% decrease in odds of a cavitybecoming active with each additional year of inac-tivity. No originally active cavity that had becomeinactive for more than 4 years has yet to be used byred-cockaded woodpeckers in this study (Figure 3).

DiscussionA large proportion of inserts installed in the

southern portion of Angelina National Forest wereused by red-cockaded woodpeckers, regardless of

0 1 2 3 4 5 6Y e a r s o f i n a c t i v i t y

Figure 3. Percentage of reactivation by red-cockaded wood-peckers after a period of inactivity by the number of years ofi n a c t i v i t y . C a v i t i e s t h a t h a d b e c o m e a c t i v e a g a i n a f t e r a p e r i o do f i n a c t i v i t y w e r e r e m o v e d f r o m t h e f o l l o w i n g y e a r ’ s s a m p l e s ot h a t t h e p r o p o r t i o n o f c a v i t i e s r e a c t i v a t i n g a f t e r a g i v e n p e r i o dof inactivity could be calculated. The peak that occurred iny e a r 4 o f i n a c t i v i t y i s d u e t o t h e s m a l l s a m p l e o f a r t i f i c i a l c a v i -t i e s i n t h a t a g e g r o u p . N a t u r a l c a v i t i e s (n = 7 8 ) a n d i n s e r t s ( n =2 3 ) a r e g r a p h e d s e p a r a t e l y a n d c o m b i n e d .

Original-style cavity insert (20.5 cm in height) on left comparedto new, larger insert (25.5 cm in height) on right.

the size of the insert. The smaller inserts that wereinitially installed received immediate use by thewoodpeckers, probably because there was a veryhigh demand at that time for cavities (Conner andRudolph 1995). After the newer, larger cavitieswere installed, use of the smaller insert cavitiesdropped because the birds apparently preferredthe larger and newer cavities. The continued highuse of the larger inserts (Figure 1) suggests a pref-erence over the smaller ones.

No naturally excavated cavity in our study sitehas been active more than 12 years, whereas 82% ofthe 7-year-old inserts were still being used in 1999.Natural cavities are often excavated in pines thatare infected with red-heart fungus (Phellinus pini,Jackson 1977, Conner and Locke 1982, Hooper1988, Hooper et al. 1991, Conner et al. 1994), whichfacilitates cavity excavation. The same fungus thatfacilitates excavation may reduce the useful lifespan of a cavity because the bottoms of cavitiesoften rot to a condition that is unacceptable to red-cockaded woodpeckers. Inserts, which are made ofwestern redcedar, are very resistant to decay andmay last longer than naturally excavated cavitieswith heart rot. Thus, it appears that in this popula-tion inserts may remain usable for at least as long asnatural cavities.

Long-term usability of inserts might be correlatedwith initial use. The chance of an insert being useddrops considerably if the insert is not used withinthe first couple of years. But if it is used, it appearsto remain usable for years. It is unknown whyunused inserts lose their attractiveness over timewhile active inserts remain attractive. One possi-bility is that some cavity trees chosen by the biolo-gist for insert installation may not be attractive tored-cockaded woodpeckers while others are attrac-tive. For example, trees that produce large amountsof resin are probably preferred by red-cockadedwoodpeckers over trees that produce less resin(Conner et al. 1998b), so trees that produce smallamounts of resin may never be used. Alternatively,woodpeckers may maintain their cavities in a con-dition that is similar to new inserts, whereas unusedinserts may somehow degrade in quality. Unusedcavities are often used by southern flying squirrels(Rudolph et al. 1990, Loeb 1993, Conner et al.1996). Their feces and urine may render cavitiesless attractive to red-cockaded woodpeckers. Ifthere is a cavity shortage or if a biologist judges thata replacement insert is needed, then it could beplaced higher in a tree with an existing insert, anadditional tree, or reinserted in the old insert treewhere the old insert had been extracted.

Cross-section of a red-cockaded woodpecker cavity insert with-in a pine tree.

A cavity insert recently installed in a living pine tree, exhibitingnewly excavated resin wells indicating current use by a wood-pecker.

Conner et al. (1998b) demonstrated that breedingmale red-cockaded woodpeckers prefer newer cav-ities. The insides of new inserts appear to be lighterin color than old cavities (Saenz et al., personalobservations). The insides of active cavities (includ-ing inserts) are often pecked by red-cockadedwoodpeckers, giving them a lighter color than thatof unused cavities. Perhaps the brightness of theinterior portion of cavities is a factor in determiningtheir attractiveness to red-cockaded woodpeckers.

In Texas, the bark of the insert tree is usuallyscraped smooth with a drawknife to facilitate insertinstallation. Scraped bark gives the tree the samesmooth, reddish appearance that characterizes anactive cavity tree (Saenz et al., personal observa-tions). Mock resin wells also are painted on the treeswith gray spray paint to attract woodpeckers (SteveBest, personal communication). Over time, if a treeis not used by a red-cockaded woodpecker, it beginsto look inactive because the bark roughens and losesits reddish appearance and the mock resin wellsfade. The external appearance of an insert tree may

be as important to attract red-cockaded woodpeck-ers as condition of the interior of the inserts.

Active natural cavities and inserts that becomeinactive show a similar trend in the probability ofbecoming active again as unused inserts show forinitially becoming active. The longer a cavity isinactive, the lower the probability of its everbecoming active again. In our study, natural cavitiesand cavity inserts follow this trend, which reducesthe possibility that poor tree selection by biologistswill cause the observed phenomenon. Interesting-ly, Doerr et al. (1989) noted that recently aban-doned red-cockaded woodpecker clusters in NorthCarolina were reoccupied at a greater rate thanlong-abandoned clusters.

Copeyon et al. (1991) and Walters et al. (1992)demonstrated that the use of artificial (drilled) cav-ities, in addition to providing cavities for existinggroups, can be an effective way to induce red-cock-aded woodpeckers to occupy previously unoccu-pied habitat and form new social groups. Insertsare used readily by red-cockaded woodpeckers inTexas and also should provide excellent opportuni-ties to induce birds to occupy new habitats and toform new social groups as long as good inserts areavailable. Based on our findings, it is possible thatolder inserts may not be as attractive to woodpeck-ers and that the absence of new inserts may limitthe success of new group formation with artificialcavities.

Inserts are proving to be an excellent techniquein the red-cockaded woodpecker recovery effort.Research is still needed to determine whetherinsert maintenance, routine cleaning, and activitiessuch as bark scraping and painting can prolong theinitial period of attractiveness to red-cockadedwoodpeckers. Additionally, inserts also may play aresearch role in understanding certain natural his-tory attributes important to red-cockaded wood-pecker recovery. Studies of cavity inserts also maybe important in developing an understanding ofnatural cavity excavation rates in woodpecker clus-ters with high and low cavity densities. Researchusing inserts also may help to answer questionsabout cavity-tree selection and cavity quality.

Acknowledgments. We thank J. R. Walters, E C.James,W. G. Montague, J. C. Neal, G. Bukenhofer, andI! D. Doerr for providing helpful comments on ear-lier drafts of this manuscript. We also thank NancyKoerth for statistical advice pertaining to this man-uscript.

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Daniel (Dan) Saenz is a wildlife biologist with the United StatesForest Service, Southern Research Station. Dan received hisB.S. and M.S. degrees from the Biology Department at StephenF. Austin State University and is currently a doctoral student inthe Wildlife and Fisheries Sciences Department at Texas A&MUniversity. Richard N. (Dick) Conner is a research wildlifebiologist with the Southern Research Station. Dick received aB.A. from Rutgers University in biological sciences and an M.S.in wildlife management and a Ph.D. in zoology from VirginiaPolytechnic Institute and State University. His interests includethe behavior and ecology of woodpeckers and other cavitynesters, fungi associated with woodpecker nest trees and snags,and forest bird community ecology and habitat requirements.Christopher S. (Chris) Collins is currently employed as a bio-logical sciences technician with the Southern Research Station,where he is studying alligator snapping turtles, timber rat-tlesnakes, and butterflies. Chris received his B.S. and M.S.degrees from the Biology Department at Stephen F. Austin StateUniversity. D. Craig Rudolph, a research ecologist with theSouthern Research Station, has a Ph.D. from Texas Tech Univer-sity. His major interest is vertebrate ecology. Current researchconcerns are red-cockaded woodpeckers, timber rattlesnakes,and Louisiana pine snakes.

Associate editor: Thompson