January1988] Commentaries 205

The published records of Kirtland's Warblers in Natl. Aeronauticsand SpaceAdmin., NASA SP- springand autumnmigration indicate that they may 262. not pauseenroute until at or near their destinations. LOWER¾,G.H.,JR.1960. Louisianabirds.Baton Rouge, If this is true of one warbler, it may alsobe true of Louisiana State Univ. Press. other small land birds. MAYFIELD, H. F. 1960. The Kirtland's Warbler. I thank CameronB. Kepler, Paul W. SykesJr., and BloomfieldHills, Michigan, CranbrookInst. Sci. Elliot J. Tramerfor readinga draft of this manuscript. NISBET,I. C.t. 1970. Autumn migrationof Blackpoll Warblers:evidence for long flight provided by regionalsurvey. Bird-Banding 41: 207-240. LITERATURE CITED STONE,A.E. 1986. Migrationand winteringrecords of Kirtland's Warbler: an annotated bibliog- COCHRAN,W. W. 1972. Long-distancetracking of raphy.Athens, Georgia, U.S. Fish and Wildl. Serv. birds. Pp. 39-59 in Animal orientation and nav- (unpublished). igation(Sidney R. Caller, KlausSchmidt-Koenig, GeorgeJ. Jacobs,and RichardE. Belleville,Eds.). Received2 July 1987, accepted7 July 1987.

Bird Migration Terminology

SCOTT B. TERRILL 1'2AND KENNETH ]3. ABLE •

The patternsof migratory behavior in birds spana poll Warbler (Dendroicastriata). Data from someex- continuum from obligate long-distanceannual mi- tensivelystudied European species indicate that an- grations to irregular eruptive movements. Current nual migratorydisposition expressed in cagedbirds studiesreveal ever more variability in avian migra- is often based on endogenousprocesses (i.e. occurs tion systems.Effective communicationrequires that in the absence of external stimuli; see Gwinner 1986 establishedterminology be used preciselyand that for a recent review). It is, however, becoming increas- somenew terms be employed to reflect our increased inglydear that the migrationof at leastsome annual knowledge of migration patterns. We proposestan- migrantsmay not be entirelythe resultof an endog- dard usagefor severalcommon migration terms.We enousmotivation. Rather, late stagesof autumn mi- have been as conservativeas possible in retaining grationcan occur only in responseto externalstimuli establishedterminology, and where possiblewe sug- (e.g.failure in foodsupply, unusually high density gest simply adding modifiers that more accurately of conspecifics,extreme weather conditions) and may describethe phenomena. not occurannually. For example,field evidencein- Much of the variability within migration patterns dicatesthat Yellow-rumped Warblers (Dendroicaco- probably is based on underlying differencesin the ronata;Terrill and Ohmart 1984),American Tree Spar- mechanismsthat control migratory behavior. We rows (Spizellaarborea; Niles et al. 1969), Harris' mention some of these possible differences, but for Sparrows(Zonotrichia querula; Rohwer 1978), and some only a fraction of speciesdo we have any information speciesof Palearcticnocturnal migrants (e.g. Lack 1983, on the mechanisms.Therefore, the terms we propose Haila et al. 1986)exhibit a delayedor facultativephase are intended to be descriptiveof the observedphe- of migratorybehavior after the initialportion, or ob- nomenaand carry no implication regarding the spe- ligatephase, of annualmigration. In addition,there cific mechanisms involved. is experimentalevidence for facultativemigration in The term annualmigrant should be applied to pop- Dark-eyedJuncos (Junco hyemalis; Terrill 1987, 1988) ulations (speciesor geographicallydefined breeding and Garden Warblers (Sylvia borin; Gwinner pers. populationsof a species)in which all individuals mi- comm.).Thus, in termsof regulatorymechanisms, it grate from their breeding siteson an annual basis. is misleadingto assumethat the entire annual mi- This descriptiveterm is preferable to others, e.g. ob- grationis underendogenous control simply because ligate, true, or regular migrant, that are often used a speciesis an annual migrant.Indeed, an endoge- synonymously.A well-known exampleis the Black- nouscomponent need not be involved in annual mi- gration,although we knowof no exampleswhere an annualmigration is stimulatedexclusively by exog- • Department of BiologicalSciences, State Univer- enous factors. sity of New York, Albany, New York 12222 USA. We thereforepropose the termsobligate phase and 2Present address: Vogelwarte Radolfzell, Schloss facultativephase as modifiers to describethe behavior MtSggingen,D-7761 MtSggingen, Federal Republic of of individualannual migrants. The obligatephase is Germany. the initial portion of migrationfrom the breeding 206 Commentaries [Auk,Vol. 105 area, performed annually regardlessof environmen- BIEBACH,H. 1983. Genetic determination of partial tal conditions.In many speciesthe fundamentalstim- migration in the EuropeanRobin (Erithacusru- ulus for the obligate phase of migration may be en- becula). Auk 100: 601-606. dogenous. In some individuals this phase may BRUUN,B. 1970. Birds of Europe. New York, Mc- constitutethe entire migration. The facultativephase Graw-Hill. is a period following the obligate phase in at least GAUTHREAUX,S. A., JR. 1982. The ecologyand evo- some individuals of annual migrant populations in lution of avian migration systems.Pp. 93-168 in which further migration is induced by deteriorating Avian biology, vol. 6 (D. S. Farner, J. R. King, environmental conditions (see Terrill 1987, 1988 for and K. C. Parkes,Eds.). New York, Academic Press. a detailed discussion; see also Lack 1983). GWINNER,E. 1986. Circannual rhythms in the con- Partial migrantpopulations include some individ- trol of avian migrations.Pp. 191-228in Advances uals that do and some that do not migrate from the in the studyof behavior,vol. 16 (J.S. Rosenblatt, samebreeding area (Lack 1944,Shtiz and Meise 1968, C. Beer, M.-C. Busnel, and P. J. B. Slater, Eds.). Gauthreaux1982). The term has also been applied to New York, Academic Press. speciesthat are migratory in part of their breeding HAILA, Y., J. TIAINEN, & K. VEPS,•L,i•INEN. 1986. De- range and resident elsewhere (e.g. Bruun 1970, Hay- layed autumn migrationas an adaptivestrategy man et al. 1986). To avoid confusion, we recommend of birdsin northernEurope: evidence from Fin- that the term be used only to reflect the behavior of land. Ornis Fennica 63: 1-9. individuals within a population, not as a descriptor HAYMAN,P., J. MARCHANT,& T. PRATER.1986. Shore- of speciesconsisting of migratory and resident pop- birds. Boston,Houghton Mifflin. ulations. KETTERSON,E.D., & V. NOLAN JR. 1983. The evolu- Partialmigration is sometimesused synonymously tion of differential bird migration. Pp. 357-402 with "facultativemigration" (e.g. Gauthreaux1982). in Current ornithology, vol. 1 (R. F. Johnston, Whereaspartial migration can certainly be faculta- Ed.). New York, Plenum. tive, as in the Blue Tit (Parus caeruleus;Smith and LACK,D. 1944. The problem of partial migration. Nilsson 1987), it can also be under more rigid, en- Brit. Birds 37: 122-130. dogenouscontrol (Berthold 1985). We thereforepro- LACK, P. 1983. The movements of Palaearctic land- posethe term obligatepartial migration to refer to the bird migrantsin TsavoEast National Park,Kenya. behavior of those individuals of a partial migrant J. Anim. Ecol. 52: 513-524. populationthat migrate each year regardlessof an- NILES, D. M., S. A. ROHWER,& R. D. ROBBINS. 1969. nual environmental variation or fluctuationsin pop- An observation of midwinter nocturnal tower ulation density. Presumably,the behavior reflectsa mortalityof TreeSparrows. Bird-Banding 40: 322- geneticpolymorphism, and the impetusto migrateis 323. primarily endogenous(e.g. Germanpopulations of ROHWER,S. 1978. Reply to Shields on avian winter the EuropeanRobin, Erithacus rubecula; Biebach 1983). plumage variability. Evolution 32: 670-673. We suggestthat the term facultativepartial migration SHt•Z,E., & W. MEISE. 1968. Zum Begriff des Teil- be used for individuals that may or may not migrate ziehers.Vogelwarte 24: 213-217. in any given year. Whether an individual migrates SMITH,H. G., & J.-,•.NILSSON. 1987. Intraspecific appearsto depend largely, if not entirely, on envi- variation in migratory pattern of a partial mi- ronmentalconditions (e.g. Blue Tit; Smith and Nils- grant,the Blue Tit (Paruscaeruleus): an evaluation son 1987). of different hypotheses.Auk 104: 109-115. The term differentialmigration refers to the situation TERRILL,S.B. 1987. Socialdominance and migratory in which migration in some distinguishableclasses restlessnessin the Dark-eyed Junco (Juncohye- of individuals(ages, sexes, races) differs with respect malis). Behav. Ecol. Sociobiol. 21: 1-11. to timing, distance,or both (seeGauthreaux 1982). ß 1988. The relative importanceof ecological Recently, Ketterson and Nolan (1983) restricted use factorsin bird migration.Proc. 19th Int. Ornithol. of the term to populationsin which all individuals Congr. in press. migrate (annual migrants),and that definition has ---, & g.D. OHMART. 1984. Facultative extension been adoptedby Berthold(1985). Of course,partial of fall migration by Yellow-rumped Warblers migrants can also exhibit differential timing or dis- (Dendroicacoronata). Auk 101: 427-438. tance,and we proposethat the term be used in its original, broader senseas a modifier of any of the Received11 September1987, accepted7 October1987. migration categoriesoutlined above.

LITERATURE CITED

BERTHOLD,P. 1985. The control of partial migration: a review. Ring 10: 253-265.