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Commentary

The 118(2):557-562, 2001

Habitat Selection Studies in Avian :A Critical Review

JASONJONES • Departmentof ,Queenk University, Kingston, Ontario K7L-3N6, Canada

The study of use and selectionin birds has sponsesthat may result in the disproportionateuse a long tradition (Grinnell 1917, Kendeigh 1945, of to influence survival and fitness of indi- Svardson 1949, Hild6n 1965; Block and Brennan viduals (Hutto 1985, Block and Brennan 1993). Hab- 1993). Early habitat-selectiontheory was character- itat selectioncarries a connotationof understanding ized by correlativemodels of habitat characteristics complex behavioral and environmental processes and (MacArthur and Pianka that habitatuse does not; habitat-use patterns are the 1966, Verner et al. 1986, Rosenzweig1991), which end result of habitat-selectionprocesses. -site se- subsequentlyevolved into modelsthat involvedden- lectionis a subsetof habitat selectionfocusing solely sity dependence:the "ideal-free distribution" and on nest sites. "ideal-despoticdistribution" models (Fretwell and Context.--Theability of researchersto place their Lucas1970, Fretwell 1972).More recently,habitat-se- findings in an appropriatebehavioral or - lection studies have shown that many factors, such ary contextvaries widely. Much of that variationlies as , can influence exactly how in researchers'ability to generatespecific questions "ideal" and "free" are while moving and to place the answers to those questionsinto througha landscapeand selectinghabitats (Karr and broader theoretical frameworks. There are two as- Freemark 1983, Pulliam and Danielson 1991, Petit pects of habitat selectionthat are crucial to under- and Petit 1996). standingthe adaptive significanceof disproportion- Habitat-selectionstudies have recently assumeda ate use of habitats; demonstration of choice and an new urgency,partially as a result of the importance assessmentof the fitness consequencesassociated of incorporatingboth habitat and demographicin- with the choice. formation into conservation planning (Caughley First, habitat selection is a decision-making pro- 1994). Nevertheless,ornithologists tend to be incon- cessand researchersneed to make an attempt to de- sistentin their conceptualframework and terminol- scribe how the observedpatterns reflect individual ogy with regard to (1) what constituteshabitat use choice.That attempt can be improved by the recog- versus selection,(2) the behavioral and evolutionary nition that individuals are faced with choices that contextof their findings,and (3) the order or scaleof differ not only in termsof habitatquality, but alsoin their study, from microhabitatto geographicrange terms of the costsand benefits of acquiring (Johnson1980, Orians and Wittenberger1991). The (Fretwell and Lucas 1970, Kennedy and Gray 1994). purposeof this review is to addressthose concerns Second, habitat preferencesare assumed to be through a surveyof recentliterature and highlight adaptivewithout demonstrationof increasedfitness areaswhere improvementsor advancescan be made in preferred habitats (Robertson1972, Pulliam and in avian habitat ecology. Danielson 1991, Martin 1998). There is no guarantee ThreeAreas of Concern.--Definitions.--The semantic that the presenceof individuals in a given habitat is and empirical distinctionsbetween the termshabitat positivelyrelated to habitatquality (VanHorne 1983, use and habitat selectionare often unclear (Hall et al. Pulliam 1988, Caughley1994). In the absenceof be- 1997). Habitat refers to a distinctive set of physical havioral or -history information, there is no way environmentalfactors that a speciesuses for its sur- to know if detected differenceshave any bearing on vival and (Block and Brennan 1993). choicesof individuals (Martin 1992, 1998). Habitat userefers to the way in which an individual In addition, many nonhabitat-relatedphenomena or speciesuses habitats to meet its life history needs influence habitat selection in birds (Cody 1981, (Block and Brennan1993). The study of habitat-use 1985), including nest (Sonerud1985, Mar- patternsdescribes the actual distributionof individ- uals acrosshabitat types (Hutto 1985). Habitat selec- tin 1993), (Svardson 1949, Martin 1993, tion refers to a hierarchicalprocess of behavioral re- Petit and Petit 1996), intraspecific attraction (Dan- chin et al. 1998, Forsman et al. 1998, POysaet al. 1998), and limitation (Martin 1993, McCollin E-mail: [email protected]. ca 1998). There needs to be explicit recognitionof how

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thosephenomena affect both the choicesmade by in- availability,although that assumptionis seldomtest- dividuals and the fitness consequencesof those ed in the . choices. Second,both the spatialand temporalscales of the Methodology.--Theempirical and statisticalmeth- study influencethe perceptionof habitatavailability ods by which habitat selection is inferred differ (Wiens 1973, Kotliar and Wiens 1990, Orians and greatly in their precisionand applicability(Alldred- Wittenberger1991) and, hence,our senseof habitat ge and Ratti 1986, 1992).There are two main ways in selection (Clark and Shutler 1999). Johnson (1980) which habitat selectionis tested for breeding birds defined four orders of habitat selection that acknowl- with territorial systems:comparing used habitats edgeits hierarchicalnature and provide a usefulem- with unused habitat and comparing used habitats pirical framework for habitat studies. Johnson's with available habitats. Used habitat is habitat cur- frameworkranges from the macroscaledescriptions rently occupiedby the focal individual or species; of the geographicalor physical range of a species unusedhabitat is not currently occupied.Available (first-order selection)to microscaledescriptions of habitat refers to all habitat types in a prescribedarea the actual attainment of food items or selection of and includeshabitats currently in use. nest sites from those available (fourth-order selec- The used versusunused comparison is considered tion). If the scaleof study and analysisis not tailored the less informative of the two methods (Johnson to the speciesand questionof interest,key influences 1980).Information on the quality of used versusun- on habitat selectionmay be missed by the research used spaceis only informativeabout habitat selec- (Orians and Wittenberger1991). For example,when tion if the unused habitat is actually availableto the examininghabitat usewithin territories,the individ- birds of interest. Further, absencefrom a particular ual has likely already made a crucial selectionby habitatdoes not meanthat the habitatis beingavoid- choosinga territory. Researchersshould be explicit ed (Wiens1989, Haila et al. 1996). density about the constraintsthat prior decisionsmade by and demographicsmay havea major effecton which the place on its current options (Johnson habitats are used or unused (Rotenberry and Wiens 1980). 1980, Wiens 1986, Wiens et al. 1987, Haila et al. 1996). Third, habitat availability often is not assessedin In addition, there are statisticalissues concerning a manner relevant to the individual or speciesin the comparisonof used and unusedareas. Of partic- question(Aebischer et al. 1993, Gates and Evans ular importanceis the concernraised over statistical 1998). Available habitat is usually assessedwithin a methods that fail to consider that an individual's use study area, the boundariesof which are often arbi- of a particular habitat affectsits use of other habitats trarily drawn. A more accurateassessment of habitat (Thomasand Taylor 1990,Aebischer et al. 1993).The availability is one that is informed by the natural- lack of independencenegatively affects the power of and life-history characteristicsof the focal species. many of the statistical techniquesused to analyze For example,when consideringselection of habitat such proportional use (Alldredge and Ratti 1986, componentsfrom within territories or home ranges 1992; James and McCulloch 1990, Aebischer et al. (third-order selection;sensu Johnson 1980), the mea- 1993). Analytical techniquesshould test for depar- surement of habitat availability should be con- tures from random use. If nonrandom use is detect- strained by the boundariesof the territory or home ed, techniquesshould then assesswhich habitats are range (Evansand Gates1997, Gates and Evans1998). used more or lessthan expectedby chance(Aebisch- Methods.--I surveyed the last 14 years (1986 to er et al. 1993). Johnson's(1980) unit-sum constraint 1999) of four North American ornithologicaljour- is an exampleof sucha technique. nals-The Auk, TheCondor, Journal of Field , Used-versus-availabletests involve comparisons and The Wilson Bulletin. I considered the content of of habitatscurrently used by individuals to habitats thosefour journals to be representativeof the state availableto be used.Those comparisons are prefer- of the avian research to date. I chose 1986 as the start- able to used-versus-unusedcomparisons because ing point becauseit was the first full publicationyear they allow researchersto make inferences about following the publicationof HabitatSelection in Birds, choice.However, the used-versus-availablecompar- (Cody 1985). I searchedtitles, abstracts,and key isonsare also problematicin that the measurement words for the terms habitat use, habitat characteris- of habitat availability is very difficult. First, avail- tics, habitat associations, habitat selection, and nest- ability refers to both the accessibilityand procura- site selection.Papers were groupedinto three exclu- bility of ,not just their abundance(Wiens sive categories:habitat-use papers, habitat-selection 1984, Hall et al. 1997, Martin 1998). The definition of papers,and nest-siteselection papers. Within each availabilitybased solely on the proportionalarea of category,papers were classifiedaccording to the habitat types makesa rarely tested assumptionthat characteristics of both the research within each man- all parts of the study area are equally available(Ken- uscript and how the researchwas presented.I asked nedy and Gray 1994, Arthur et al. 1996, Spenceret severalquestions of eachpaper: (1) Were the authors al. 1996).In addition, many researchersassume that consistentand accuratein their usageof habitatter- a random sampling of habitats estimates habitat minology throughoutthe paper?(2) If the paper ex- April 2001] Commentary 559

amined habitat or nest-site selection, did the authors rendered many of the results difficult to interpret contrast used habitats with unused habitats or did and, possibly,inaccurate. There was no differencein they addresshabitat availability?(3) If the authors the propensityfor error acrossthe three periods addressedhabitat availability,did they define avail- (X2 = 2.13, df = 140, P = 0.34). Most of the reviewed ability arbitrarily (e.g. within preset study area papers that examined habitat selectiondescribed a boundaries),or did they considerthe ecologyof the used-versus-availablecomparison (76%) rather than study systemwhen designingtheir habitatsampling a used-versus-unusedcomparison (24%), although method (e.g. within territory boundaries when as- the latter is widely used in management sessingnest-site selection)?(4) If the authors ad- (White and Garrott 1990). Very few of the reviewed dressed habitat or nest-site selection,did they at- papers that employedused-versus -unused compar- tempt to place their findings in a behavioral or isonsaddressed issues of accessibilityand availabil- fitness context? ity. For example,Frederick and Gutidrrez (1992) test- I assessedwhether or not the frequencyof errors ed habitat selection in White-tailed Ptarmigan changedover time by groupingpapers into the fol- (Lagopusleucurus) by restrictingthe locationof un- lowing time intervals:1986 to 1989,1990 to 1994,and used sitesto siteswithin regionsof concentrateduse, 1995to 1999.I comparedthe percentageof total pa- therebyguaranteeing habitat accessibility. Unless ac- pers committing semantic,methodological, and con- cessibilitycan be addressed,a better approachis to textual, errors across each of the three time intervals examine areas where birds are found and look at using chi-squaretests. Below, I deal with semantic probabilityor frequencyof usageacross used areas concernsfirst becausethe papers that suffered from (e.g. Andersonand Tacha1999). Finally, few papers such concerns were not included in the assessment used statisticalmethodology designed to accountfor of methodologicaland contextualissues. the nonindependenceof proportionaluse of habitats Resultsand Discussion.--In total, 185 papers were (e.g. Ryan and Renkin 1987). included in this review: 73 habitat use, 51 habitat se- The majority of papersthat actuallyexamined hab- lection, and 61 nest-site selection. Forty-two were itat selectionemployed a form of the used-versus- publishedin the period 1986-1989,56 during 1990- available habitat comparison.Although that is en- 1994, and 87 during 1995-1999. couraging, many researchersfailed to explicitly Semantics.•Overall,24% (44/185) of the surveyed recognize that not all habitats are equally available paperssuffered from semanticinconsistencies. Prob- for use and did not structure their habitat sampling lems rangedfrom usinghabitat use and habitatselec- methodologyaccordingly. Less than half of the pa- tion as synonyms(e.g. Darveau et al. 1992) to solely pers that employeda used-versus-availablecompar- describingnest-site characteristics without reference ison to test habitat selectiondefined availability in a as evidence of nest-site selection (e.g. Schaffner manner relevantto the speciesor individual in ques- 1991). This study is not the first to raiseconcern over tion. For example, when examining nest-site selec- the lack of semantic standardization in the habitat tion, many authors compared nest-site characteris- field (Romesburg1981, Morrison et al. 1992,Hall et tics with habitat characteristics at random sites that al. 1997).That nearly one-quarterof the papers sur- were selectedwithout referenceto territory bound- veyed in this review misinterpreted or misused the aries (e.g. Pampushand Anthony 1993, Linder and terms habitat use, habitat selection and nest-site se- Anderson 1998). The assessmentof habitat availabil- lection indicatesthat the problem remainspervasive ity likely included habitat not availableto the focal in avian ecology.Further, the situationdoes not ap- individual and, consequently,erroneous differences pear to be improving;there was no differencein the between nest-sites and available habitat could have propensityfor error acrossthe three time periods(X 2 been described.Constraining the assessmentof hab- = 3.05, df = 184, P = 0.22). Why the problemremains itat availability to within territory boundaries will is perhaps related to the commonnessof the termi- provide a more accuratepicture of nest-siteor for- nology;authors may assumethat everybodyknows aging site selection(e.g. Ramsayet al. 1999). what habitat selectionis and, therefore, pay less at- The issueof availabilitycan be compoundedwhen tention to providing operational definitions when dealing with specieshaving unique habitat require- presenting their research.Although the avian habitat ments. Smith et al. (1999) documented nest-site se- literature appears to have fewer semantic inconsis- lection by Great Horned (Bubovirginianus), a tenciesthan other fields (Hall et al. 1997), researchers species that requires nest structure built by other need to strive to insure that essential conceptsare species.However, in their selectionof random sites clearlydefined if habitatecology is to continueto de- to document availability, they did not record the velop and maintain its position as one of the central presenceor absenceof potential nest sites, such as fields in avian research.The lack of semanticclarity old corvid .If there are no potential nest sites, carriesover into the ability of researchersto develop the habitat is technicallynot available.On the other a meaningfulcontext surrounding their results. hand, Sieg and Becker (1990) provided a truer as- Metholodogy.--Ofthe 141 paperswithout semantic sessmentof availability for Merlins (Falcocolumbar- concerns,46% (n = 65) made empirical decisionsthat ius),which alsorequire nestsbuilt by heterospecifics, 560 Commentary [Auk,Vol. 118 by centeringtheir nonnesthabitat plots on unused oughtto be informedby the natural- and life-history Black-billedMagpie (Picapica) nests. characteristicsof the focal species. Context.--Thirty-eightpercent (46/121) of the pa- Acknowledgments.--Alist of referencesfor papers pers that examined habitat or nest-siteselection did included in this review is available on requestfrom not provide a behavioralor fitnesscontext for their the author. This manuscript has benefited greatly findings.There was no differencein the propensity from commentsby J. Barg, J. Dickinson,R. Holmes, for error acrossthe three time periods (X2 = 0.32, df J. Marks, L. Ratcliffe, R. Robertson,and two anony- = 120, P = 0.85). Given so many known nonenviron- mous reviewers, and from conversations with C. mental influenceson habitat selection,it is perhaps Eckert,C. Francis,J. Pither, and S. Ramsay.Financial troubling that over one-third of habitat selectionpa- support while preparing this review was provided pers neglectedto acknowledgethe potential effects by a scholarshipfrom the Natural Sciencesand En- of nonenvironmentalfactors on patterns they de- gineeringResearch Council of Canada. scribe.Obviously, no singleresearch project can cov- er all potential influences,but the existenceof mul- LITERATURE CITED tiple constraintson individual behavior needsto be explicitlynoted. For example,Hooge et al. (1999)fo- AEBISCHER,N.J., P. A. ROBERTSON,AND R. E. KEN- cusedtheir effortsin documentingnest-site selection WARD.1993. Compositional analysis of habitat by (Melanerpes formicavorus) on use from animal radio-tracking data. Ecology the potential influenceof microclimateon habitat se- 74:1313-1325. lection,but expandedtheir discussionto includethe ALLDREDGE,J. R., ANDJ. T. RATTI.1986. Comparison role of nest predation. In contrast, Wilson et al. of somestatistical techniques for analysisof re- (1998) offer only a cursoryexplanation of observed source selection.Journal of Wildlife Manage- patternsof habitatselection by peatlandbirds, which ment 50:157-165. limits the ability of the reader to appreciatethe his- ALLDREDGE,J. R., AND J. T. RATTI. 1992. Further com- torical dynamicsof the system. parison of some statisticaltechniques for anal- The results of this review indicate that few habitat ysis of selection.Journal of Wildlife and nest-siteselection papers have addressedwhy Management 56:1-9. the selectionof certain habitatswas adaptive for the ANDERSON,J. T., AND T. C. TACHA. 1999. Habitat use speciesin question.One notableexception was Bad- by Masked Ducks along the Gulf Coastof Texas. yaev et al.'s (1996) examinationof habitat selectionin Wilson Bulletin 111:119-121. female Wild Turkeys (Meleagrisgallopavo); not only ARTHUR, S. M., B. F. J. MANLY, L. L. MCDONALD, AND did the authors examine the reproductive conse- G. W. GARNER.1996. Assessinghabitat selection quencesof habitatselection by individuals,they doc- when availability changes.Ecology 77:215-227. umented the processby which femalesbehaviorally BADYAEV,A. V., T. E. MARTIN, AND W. J. ETGES.1996. sampledhabitat availability. Habitat sampling and habitat selectionby fe- Summary.--Theresults of my time-period analy- male Wild Turkeys:Ecological correlates and re- sis, coupledwith the frequencyof error detectedin productive consequences.Auk 113:636-646. this review,indicate that my concernsregarding the BLOCK,W. M., AND L. A. BRENNAN. 1993. The habitat generalstate of avian habitat selectionresearch were concept in ornithology: Theory and applica- valid. One concernwas that ornithologiststend not tions. Current Ornithology 11:35-91. to consistentlyevaluate the behavioral and fitness CAUGHLEY, G. 1994. Directions in conservation biol- contextof their findings.That can be amelioratedby ogy. Journalof Animal Ecology63:215-244. recognizingthat (1) habitat selectionrefers to a pro- CLARK, R. G., AND D. SHUTLER. 1999. Avian habitat cessand not a pattern, (2) that there are many ex- selection:Pattern from processin nest-siteuse trinsic factorsthat influencehabitat selection,and (3) by ducks?Ecology 80:272-287. that a complete test of habitat selectioninvolves an CODY, M. L. 1981. Habitat selection in birds: The assessment of whether or not the documented habi- roles of structure, competitorsand tat preferencesare adaptive. A secondconcern was . BioScience31:107-113. that ornithologistsdo not consistentlyuse and per- CODY, M. L., ED. 1985. Habitat Selection in Birds. Ac- ceive habitat-relatedterminology. That lack of con- ademic Press, New York. sistencycan be remedied by providing operational DANCHIN, E., T. BOULINIER, AND M. MASSOT. 1998. definitionsto limit misunderstanding.A third con- Conspecificreproductive successand breeding cernwas that methodologiescommonly employed to habitat selection:Implications for the study of document habitat selection do not account for the hi- coloniality. Ecology79:2415-2428. erarchicalnature of habitatselection and do not gen- DARVEAU,M., J.-L. DESGRANGES,AND G. GAUTHIER. erate accuraterepresentations of habitatavailability. 1992.Habitat useby three insectivorousbirds in Comparisonsof used habitat with availablehabitat declining maple .Condor 94:72-82. are more appropriatethan comparisonsof usedand EVANS, D. R., AND J. E. GATES. 1997. Cowbird selec- unused habitat. Definitions of habitat availability tion of breeding areas:The role of habitat and April2001] Commentary 561

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