J. Field Ornithol. 79(2):193–197, 2008 DOI: 10.1111/j.1557-9263.2008.00162.x

Artificial nest site preferencesof Black-capped Chickadees

Caren Cooper1 and David Bonter

Laboratory of Ornithology, Cornell University, 159 Sapsucker Woods Road, Ithaca, New York 14850, USA

Received 2 January 2008; accepted 22 February 2008

ABSTRACT. To facilitate study of the breeding biology of parids, Grubb and Bronson (1995; Condor 97: 1067–1070) designed artificial “snags” made from polyvinyl chloride (PVC) tubes. Because the cost of artificial snags isgreater than that of traditional wooden boxes, we examined alternatives to PVC snags for attracting chickadees toartificial nesting sites. From 2005 to 2007, we compared the use of PVC snags and wooden nest boxes by Black-capped Chickadees (Poecile atricapillus) in Sapsucker Woods Sanctuary in Ithaca, New York. We also quantifiedthe use of cavities with and without wood shavings. The probability of chickadee excavation was greater (60–70%per yr) in filled snags (with wood shavings) than in filled boxes (40–50%; logistic mixed model, P = 0.01), andchickadees initiated more nests in filled snags (25–30%) than filled boxes (15%; P = 0.03). Chickadees also initiatedsignificantly more nests (P = 0.03) in filled than unfilled boxes. Although wooden boxes filled with wood shavingswere used more often by Black-capped Chickadees than unfilled wooden boxes, artificial snags filled with woodshavings were used most, were no more likely than boxes to be usurped by House Wrens (Troglodytes aedon), andwere less likely than boxes to be occupied by mice. Thus, artificial snags may be the better option for investigatorsstudying the breeding biology of chickadees.

SINOPSIS. Preferencias para lugares artificiales de nidificacion de Poecile atricapillusPara facilitar estudios sobre la biologıa reproductiva de los paridos, Grubb and Bronson (1995; Condor 97: 1067–

1070) disenaron “tocones” artificiales hechos de tubos de policloruro de vinilo (PVC). Debido a que el costo de lostocones artificiales es mayor que el de las cajas de madera tradicionales, examinamos las alternativas a los toconesde PVC para atraer a Poecile atricapillus a lugares artificiales de anidacion. Entre el 2005 y el 2007, comparamos eluso de tocones de PVC y cajas de madera por P. atricapillus en el santuario de Sapsucker Woods en Ithaca, NuevaYork. Tambien cuantificamos el uso de cavidades con y sin aserrın de madera. La probabilidad de excavacion porP. atricapillus fue mayor (60–70% por ano) en tocones con aserrın que en cajas con aserrın (40–50%; modelologıstico mixto, P = 0.01), y los individuos comenzaron mas nidos en tocones con aserrın (25–30%) que en cajascon aserrın (15%; P = 0.03). P. atricapillus comenzo significativamente mas nidos (P = 0.03) en cajas con aserrınque en cajas sin aserrın. Aunque las cajas de madera con aserrın en su interior fueron usadas mas frecuentemente porP. atricapillus que cajas de madera vacıas, los tocones artificiales con aserrın en su interior fueron mayormente usadasy no tuvieron una mayor probabilidad de ser usurpadas por Troglodytes aedon y tambien tuvieron una probabilidadmas baja de ser ocupadas por ratones que las cajas. Ası, los tocones artificiales pueden ser una mejor opcion parainvestigadores quienes estudian la biologıa reproductiva de P. atricapillus.

Key words: artificial snag, Black-capped Chickadee, cavity excavation, field experiment, nest box

Artificial nest sites, such as the use of nestboxes by Eastern Bluebirds (Sialia sialis; Gowatyand Plissner 1998) or platforms by Osprey (Pan-dion haliaetus; Poole et al. 2002), have aidedthe recovery and management of several speciesof birds. Artificial nest sites can also facilitateresearch on various aspects of reproductive biol-ogy. However, using artificial nest sites may alsointroduce biases (Møller 1989, 1992, Koeniget al. 1992). Parid species readily use nest boxesin Europe. However, North American parids

1Corresponding author. Email: caren.cooper@cornell.edu

use nest boxes less often, probably due to aninnate tendency to excavate a cavity (Kluyver1961, Smith 1993) that is common amongmost parids, but not all, for example, MountainChickadee (Poecile gambeli; McCallum et al.1999). To facilitate research on parids in NorthAmerica, Grubb and Bronson (1995) designedartificial snags using 7.8-cm interior-diameterpolyvinyl chloride (PVC) pipe. They hypoth-esized that filling the snags with sawdust wouldallow chickadees to “excavate” and lead to greateroccupancy rates.

Given the greater expense of constructingsnags versus traditional wooden boxes, we soughtto test alternatives to PVC snags for attracting

C©2008 The Author(s). Journal compilation C©2008 Association of Field Ornithologists

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194 C. Cooper and D. Bonter J. Field Ornithol.

chickadees to artificial nesting sites. Further, be-cause snags are made of materials that chickadeesdo not encounter in forests (i.e., plastic), wehypothesized that they may prefer wooden nestboxes over plastic snags if both options allowedexcavation of cavities. Although Black-cappedChickadees (Poecile atricapillus) do not readilyuse nest boxes, investigators have noted that theyare more likely to use artificial cavities in forestswith few natural cavities and when nest boxesare filled with material for chickadees to excavate(Kluyver 1961, Smith 1993).

We examined the nest cavity preferences ofBlack-capped Chickadees by comparing the re-sponse of chickadees to artificial snags filledwith wood shavings, wooden nest boxes filledwith wood shavings, and empty wooden nestboxes. Our objectives were to determine if (1)chickadees were more likely to excavate andnest in artificial snags than boxes, (2) chickadeeswere more likely to nest in boxes that requiredexcavation than those that were empty, and(3) snags and boxes were equally susceptible tousurpation by competitors, such as House Wrens(Troglodytes aedon) and mice (Peromyscus spp.).

METHODS

Our study was conducted in the SapsuckerWoods Sanctuary (42◦28′N, 76◦27′W), a 56-haswamp forest in Ithaca, New York. In 2005, weestablished 20 sites at least 75 m apart within theSanctuary. At each site, we installed two artificialnesting cavities within 6 m of each other: awooden nest box filled to the entrance hole withwood shavings (filled box) and an artificial snagfilled to the entrance hole with wood shavings(filled snag). Eight of these sites also includedan empty wooden nest box (unfilled box) within6 m of the filled pairs.

For the filled boxes, we used front-opening,wooden nest boxes made from 2.54-cm rough-cut unpainted wood, with overhanging roofs,2.8-cm diameter entrance holes, and mountedat a height of approximately 1.5 m on metalposts. We stapled a strip of burlap, the heightmatching the distance from the nest box floorto the entrance hole, under the front open-ing of each box prior to filling each withwood shavings to keep wood shavings in theboxes when opened for monitoring. Modifyingthe design of Grubb and Bronson (1995), wemade artificial snags using plastic PVC pipes

(10.16-cm inside diameter) with a 2.8-cm di-ameter entrance hole approximately 8 cm belowthe top of the pipes, and inserted a cavity floorapproximately 25 cm below the entrance hole.We filled the cavities from the cavity floor to theentrance hole with wood shavings, camouflagedthe pipes with green, brown, and black paint,and attached them to 1.5-m metal posts. Weused a larger diameter tube (10.16-cm internaldiameter) than Grubb and Bronson (1995) sothat the interior volume of the snags was com-parable to that of the wooden nest boxes. Afterinstallation, the entrance holes on the snags wereapproximately 0.9 m higher than those of thewooden boxes. The entrance holes of all artificialcavities at each site faced the same direction.

In 2005, 2006, and 2007, prior to the nestingseason, the filled boxes and snags were cleanedof any old material and filled with new woodshavings up to the level of the entrance hole. Wevisited all sites 2 to 3 times per week from lateApril through mid-July or early August each year(14 to 20 visits per site). We classified cavitiesas “excavated” if at least half the shavings wereremoved. We classified cavities as a chickadeenest initiation if we observed a moss nest in thecavity. We assume that Black-capped Chickadeescarried out all excavations, even though HouseWrens ultimately nested in many of the cavities.Male House Wrens are known to build nests inempty cavities and are not known to excavate(Johnson 1998).

We compared cavity excavation and nest ini-tiation of the three cavity types using logisticrandom effects models. Because the number ofnesting sites per replicate differed across the20 sites, with 12 sites containing filled boxesand filled snags and eight sites containing filledboxes, filled snags, and unfilled boxes, we in-cluded this variable (number of artificial cavitiesper replicate) as a covariate to assess its potentialimportance. Thus, we modeled the probabilityof excavation and nest initiation as a functionof cavity type, number of artificial cavities (2or 3), and year with “site” as a random effectto account for the repeated observations at sitesover the 3 yr. We used the GLIMMIX macrowith REML (restricted maximum likelihood)variance component estimates in SAS (Littellet al. 1996). First, all models included a yearby cavity type interaction and an interactionbetween cavity type and number of artificial cav-ities per replicate. We removed these interaction

Vol. 79, No. 2 Chickadee Use of Artificial Snags 195

terms from the final models because they werenever significant. We report the inverse link-logitto express the least squares means in terms ofthe original scale with approximated standarderrors. We also repeated these analyses using the12 sites that contained only filled boxes and filledsnags (i.e., no unfilled boxes in the replicate andremoving the variable representing the numberof cavities). To further examine the importanceof filling artificial cavities with wood shavings,we also repeated the analysis of nest initiationusing the eight sites that contained pairs of filledboxes and empty boxes.

RESULTS

Black-capped Chickadees excavated morefilled snags (12–14, 60–70%) than filled boxes(8–10; 40–50%) during each year of our study(Logistic Mixed Model [LMM], df = 97, P =0.01). Chickadees also initiated more nests infilled snags (5–6; 25–30%) than in filled boxes(3; 15%) each year (LMM, df = 97, P = 0.03;Fig. 1). We found no evidence that the numberof nesting sites per replicate influenced nest siteselection by chickadees (LMM, df = 96, P =0.27 for nest start and P = 0.27 for excavation).Because our small sample size may have limitedour ability to detect an effect of the numberof nesting sites per replicate, we repeated theanalyses using the 12 sites with only two nestingsites per replicate (filled boxes and filled snags)and found similar results. Chickadees excavatedmore filled snags (7–8; 58–67%) than filledboxes (4; 33%) during each year of our study(LMM, df=57, P =0.01). Chickadees initiatednests in marginally more filled snags (4–8; 17–25%) than filled boxes (4–5; 8–17%) each year(LMM, df = 5, P = 0.059). Chickadees alsoinitiated significantly more nests in filled boxes(LMM, df = 30, P = 0.03) than in empty boxes(Fig. 1).

We assumed that all cavities were excavated byBlack-capped Chickadees and classified HouseWren nests in a filled snag or box as a usurpedcavity. House Wrens initiated the same numberof nests in filled snags (3–7; 15–35%), filledboxes (4–5; 20–25%), and unfilled boxes (4–6;19–29%) each year.

Mice were less likely to occupy artificial snagsthan either type of wooden box (� 2

2 = 16.4,P = 0.0003). During our three-year study, micenever occupied a filled snag, but occupied filled

Fig. 1. Inverse link-logit of the least squared means(± SE) from mixed logistic models illustrating thatthe probability of nest initiation by Black-cappedChickadees (BCCH) from 2005 to 2007 was (top)higher in artificial snags filled with wood shavingsthan wooden boxes filled with wood shavings (N =60; 20 cavity pairs over 3 yr ) and (bottom) higher inwooden boxes filled with wood shavings than woodenboxes with no shavings (N = 24; eight cavity pairsover 3 yr).

boxes six times and unfilled boxes 15 times.Clutch size did not differ with cavity type (� 2

8 =12.2, P = 0.14). Mean clutch sizes in filledboxes, filled snags, and unfilled boxes were 6.4 ±0.9 (1 SD; N = 8; range = 5–7), 6.9 ± 1.2(N = 11; range = 5–9), and 6 ± 0 (N = 2),respectively.

DISCUSSION

Although standing dead trees are common inSapsucker Woods and natural nesting cavitiesare presumably abundant, the artificial snagsand boxes filled with wood shavings were usedby nesting Black-capped Chickadees. We foundthat Black-capped Chickadees were more likelyto nest in snags filled with wood shavings thanfilled or unfilled boxes. Although wooden boxesfilled with wood shavings attracted more chick-adees than unfilled wooden boxes, artificial snags

196 C. Cooper and D. Bonter J. Field Ornithol.

filled with wood shavings were used most, wereno more likely than boxes to be usurped byHouse Wrens, and were less likely to be occupiedby mice than boxes. The absence of mice inartificial snags was likely due to accessibility, withPVC tubes likely restricting access to climbingrodents.

Because we installed the three cavity types inclose-proximity pairs (N = 20) and trios (N =8), each pair or trio of artificial cavities was likelylocated in the territory of one pair of chickadeesand gave them a choice of nesting sites. Ifmore widely spaced (e.g., one artificial snag perterritory), occupancy rates could potentially behigher.

Our experimental design minimized the pos-sible influence of habitat and entrance-hole ori-entation on cavity selection, but we did notcontrol for the height of the entrance holesin comparisons of filled snags and filled boxes.The greater height (0.9 m) of the snag cavitiesmay partially explain the apparent preferenceby chickadees for filled snags over filled boxes(Grubb and Bronson 1995).

Artificial nest sites that can be accessed easilyare a valuable aid to researchers, particularlythose wishing to band, measure, or manipulateyoung in the nest or conduct clutch manipula-tions. Natural nest cavities, frequently in deadlimbs or trees and in soft wood, can be difficultfor investigators to access. Nevertheless, there arepossible disadvantages to using artificial snags.Snags were more expensive to build than stan-dard nest boxes and required additional main-tenance. The lids of the snags required routinelubrication to prevent the PVC parts from stick-ing together. In addition, we frequently observeddroplets of water on the inside lid of the snagsand elevated moisture levels in PVC snags werealso noted by Grubb and Bronson (1995). Thepossible effect of this moisture on chickadees andtheir eggs and young is unknown, but ventilationholes drilled near the top of the nesting chambermay help reduce moisture levels.

Additional benefits of artificial snags are thatthey would be more resistant to decay in a long-term study (hence reducing long-term costs),and may be less susceptible to climbing predatorsthan wooden nest boxes. Further, the additionalconstruction costs may be offset by the higheroccupancy rates (and, hence, the need for fewerartificial cavities). Finally, nests in artificial snags

may be less prone to disturbance by people thannests in wooden boxes that may be easier to open.

In sum, we found that chickadees nested inboth artificial snags and boxes if cavities werefilled with wood shavings. However, artificialsnags were used more often and may be the betteroption for investigators studying the breedingbiology of chickadees.

ACKNOWLEDGMENTS

We thank J. Curran for constructing the artificial snagsand D. Lisk for building the wooden nest boxes. Weexpress special gratitude to T. Phillips and M. Whitmanfor overseeing the numerous volunteers who helped in-stall and monitor the nest cavities. We extend thanksto those volunteers, including S. Barker, B. Bermudez,M. Ciborowski, C. Crane, D. Danowski, L. Galford, K.Gifford, M. Guthrie, A. Johnson, A. Hobbs, M. Kuklis, C.McCarthy, L. Minde, W. Morris, E. Rowland, J. Schaus,K. Sanders, S. Spear, L. Stenzler, and K. Stratton.

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