The Condor9 1:515-544 0 The CooperOrnithological Society 1989 DISTRIBUTION OF OVERWINTERING NEARCTIC MIGRANTS IN THE YUCATAN PENINSULA, I: GENERAL PATTERNS OF OCCURRENCE ’ JAMES F. LYNCH SmithsonianEnvironmental Research Center, P.O. Box 28, Edgewater,MD 21037 Abstract. Point countsand mist-net surveyswere employed to studythe winter distri- bution of nearcticmigratory landbirds in Mexico’s YucatanPeninsula. Overwintering mi- grantscomprised 42 regularlyoccurring species, and accountedfor 30-58% (mean = 41%) of the individual birds encounteredin surveysof a wide rangeof natural and disturbed habitats.Some migratory species were encountered most frequently in pastures,agricultural fields,or brushysecond growth, but the main habitatfor many migrantswas tropical forest. Migratoryand residentspecies showed similar degreesof specializationwith respectto the successionalmaturity of their habitats,but residentswere more likely to specializeon particulartypes of matureforest. After mist-netcapture data werestandardized by rarefac- tion, therewere no statisticallysignificant differences in the ratio of migrantsto permanent residentsin habitatsthat ranged from pasture,through brushy old fields,to maturesemiever- greenforest. For any given number of captures,species richness of both migrantsand residentswas substantiallyhigher in mist-net samplesfrom pasturesand brushyold fields than in thosefrom maturesemievergreen forest. With few exceptions,nearctic migratory species that breed in mature temperate-zone forestoccurred both in forestand in brushysecond growth during winter, althoughsome specieswere substantiallyless frequentin the latter habitat. In contrast,overwintering migrantsthat breedin edgeor field habitatstended to avoid tropicalforest in the Yucatan. Key words: Parulinae; migratory birds;distribution; habitat use; Yucatan Peninsula. INTRODUCTION reflect species-widereductions that are causally In recent years, biologists and conservationists related to the destruction of overwintering hab- have expressedincreasing concern over the sta- itat in the neotropical region (Briggsand Criswell tus of nearctic migratory birds, speciesthat breed 1979, Morse 1980, Terborgh 1980, Rappole et in arctic or temperate North America, but spend al. 1983, Wilcove and Terborgh 1984, Powell the nonbreeding season south of the Tropic of and Rappole 1986). Certainly, there can be no Cancer (e.g., Keast and Morton 1980, Rappole doubt as to the rapid pace of deforestation in et al. 1983, Wilcove and Whitcomb 1983, Powell Middle America and the Caribbean islands and Rappole 1986). Since the 1950s breeding (Myers 1980), areas where the majority of mi- populations of some nearctic migrants that breed gratory speciesand individuals overwinter (Keast in forest have undergone alarming local declines, and Morton 1980). particularly in regions where remaining wood- While a link between tropical deforestationand lands are highly fragmented (e.g., Lynch and declining populations of migratory landbirds in Whitcomb 1978, Morse 1980). Studiesin eastern North America is both plausible and a cause for North America have shown that the area, iso- great concern, there is surprisingly little direct lation, physiognomy, and floristic composition evidence for such a connection. Indeed, the ex- of remnant forest patches all influence the oc- istence of a pattern of global decline in migrant currence and density of breeding birds, partic- populations, as opposed to purely local pertur- ularly nearctic migrants (e.g., Robbins 1980, bations, has yet to be unequivocally demonstrat- Whitcomb et al. 198 1, Lynch and Whigham 1984, ed. The only published continent-wide data set Askins et al. 1987). For these long-distance mi- on population trends, the Breeding Bird Survey grants, it has also been suggestedthat population (BBS),indicates that breedingpopulations of most declines in local portions of the breeding grounds forest-associatedmigratory specieswere stable, or actually increased, between 1966 and 1979, the only period for which data have been pub- I Received12 July 1988. Final acceptance16 Feb- lished (Robbins et al. 1986). This latter finding, ruary 1989. which has struck some biologists as counter-in- [5151 516 JAMES F. LYNCH tuitive, seems to harken back to an earlier, and Maya Mountains of Belize, essentially the entire largely discredited, view (e.g., Slud 1960, Willis peninsula is a low-lying shelf of Tertiary to 1966, Leek 1972) that moderate levels of tropical Quaternary marine limestone. The present-day forest disturbance might actually benefit the con- Yucatan gradually emerged from the sea floor, siderable number of nearctic migrant speciesthat beginning at the southern end during the Oli- utilize edge habitats and successionalvegetation gocene-Miocene epoch, and continuing north- on their tropical wintering grounds. ward into Pleistocene and Recent times (Flores Analysis of more recent BBS data indicates 1952, Hatt et al. 1953, West 1964). that many ofthe increasesobserved between 1966 The northern half of the peninsula lacks rivers and 1979 have levelled off, or even have been and streams, and surface water is restricted to reversed (Robbins et al., in press),but most mi- scatteredshallow seasonalpools and a few lakes. grant populations still appear to be as high or Rainfall, which is stronglyconcentrated in a May- higher than they were in the late 1960s according November wet season,increases markedly from to BBS data. In large part, current uncertainties north to south, and, in the northern half of the as to the specific threats posed to nearctic mi- region, from west to east (Garcia 1965). At Pro- grants by tropical deforestation reflect our ig- greso,on the northwesterncoast of Yucatan state, norance of the winter ecology of even the com- the annual rainfall is 450 mm, and the native moner migrant species(Keast and Morton 1980, vegetation is low thorn scrub. Merida, only 30 Rappole et al. 1983). km inland, receives about twice as much precip- Within the northern neotropical region, the itation, and the native vegetation there is low 240,000-km* Yucatan Peninsula stands out as deciduous forest, termed “selva baja caducifol- an important overwintering center for nearctic ia” (Miranda 1958). Between Merida and the migratory landbirds. Previous studies (e.g., Caribbean coast,some 300 km to the east,annual Paynter 1955, Tramer 1974, Waide 1980, Waide precipitation increasesto about 1,200 mm, and et al. 1980) had shown that overwintering mi- low deciduousforest gives way to moderately tall grants are both diverse and abundant in the semideciduous forest (“selva mediana subcadu- northern Yucatan Peninsula, but a comprehen- cifolia”), then to moderately tall semievergreen sive, quantitative survey of migrant numbers and forest (“selva mediana subperennifolia”) in habitat associationshas not previously been at- northern and central Quintana Roo. According tempted. In 1982 I undertook such a survey in to the Holdridge systemof life zone classification order to addressthe following questions:(1) What (Holdridge et al. 197 l), this west-east gradient are the relative densities of individual migratory corresponds to a transition from thorn wood- species,and of migrants as a group, in the most land, through arid tropical forest, to dry tropical widespread categories of natural and disturbed forest (Thien et al. 1982). vegetation? (2) Are migrants as a group more In southernmost Quintana Roo and Cam- tolerant of human-related habitat disturbance peche, rainfall increases to 1,500 mm, streams than are resident species?(3) What will be the and rivers are common, and taller, floristically likely impact of future land-use changeson mi- richer semievergreen forest (“selva alta subpe- gratory landbirds in the Yucatan? rennifolia”) appears. This forest, which is inter- mediate between “tropical dry forest” and “trop- STUDY AREA AND METHODS ical moist forest” according to the Holdridge Fieldwork was concentratedin the Mexican states system, is inhabited by a number of resident bird of Quintana Roo and Yucatan, but migrants were speciesthat do not range farther northward into also surveyed in the states of Campeche and the drier portions of the peninsula (Paynter 1955). Chiapas, and in neighboring Belize and Guate- The wettest climate in the Yucatan Peninsula mala (Fig. 1). The vegetation, climate, and ge- occurs along the Caribbean-facing slope of the ology of the Yucatan Peninsula have been well Maya Mountains in southern Belize. Here, an- described from a zoogeographicalviewpoint by nual rainfall approaches 4,000 mm (Russell Lee (1980), and only a brief account is presented 1964), and the climax vegetation is “tropical wet here. forest” according to the Holdridge classification. A number of distinctive natural vegetation CLIMATE AND NATIVE VEGETATION types (e.g., mangrove swamp, sawgrass-palmetto The Yucatan Peninsula is fairly uniform in ge- savanna, dune scrub) occur along the Yucatan ology and topography. With the exception of the coast(see Lynch et al. 1985: fig. 2). For migratory OVERWINTERING MIGRANTS 517 I I 9o” 1 89O 88O 87’ YUCATAN 0 !OO CAMPECHE 190 IS’ GUATEMALA BELIZE i 7O 5,o 100 kilometers 170, I 900 +89O I I 88O 87O F IGURE 1. Map of the Yucatan Peninsula showing sites where point counts were made 1982-1987. Large ? fmbols represent > 12 counts;small symbols represent4-l 2 counts.Several localities in northern Chiapas (just SCmthwest of the area mapped) are not indicated. 518 JAMES F. LYNCH FOREST WOTt!,KEW, BWWA A <=>A YEWA INBU RBGRyT A PABU,. _ / ACAHUAL