Nearctic Migrants Research Articles

Cause and effect in population declines of migratory birds

-Population declines have been reported for nearly one-third of all Nearctic migrant birds that winter in the Neotropics. Speculative explanations for these reports have been presented emphasizing the importance of events occurring during one portion of the annual cycle versus another. These explanations are difficult to test directly. However, certain predictions can be made regarding the characteristics of populations controlled by breedingground factors as opposed to those associated with migration or wintering-ground phenomena. These characteristics can be measured for and tested experimentally on a species-byspecies basis. We present 14 such predictions, and review data currently available to assess their relevance to observed declines. Based on these data, populations of many species of Nearctic migrants appear to be controlled by wintering-ground events. Received 30 September 1993, accepted 5 January 1994. DECLINES IN AT LEAST 109 species of Nearctic migrants have been reported by one or more survey methods (Droege and Sauer 1989, Morton and Greenberg 1989, Terborgh 1989, Robbins et al. 1989a, Askins et al. 1990, DeGraaf and Rappole in press). These figures are cause for concern despite conflicting regional population trends (Sauer and Droege 1992), and despite arguments that at least some purported evidence of decline may be the result of procedural errors (e.g. survey design and analysis flaws; Hutto 1988, James et al. 1990, 1992, Rappole et al. 1993a). If the declines are real, what are the

Temporal Dynamics of Mangrove Bird Communities in Venezuela with Special Reference to Migrant Warblers

ABsmAcr.-We studied bird phenology and abundance patterns relative to rainfall, ground inundation, and food abundance in three mangrove sites of northeastern Venezuela over two complete annual cycles. Bird abundance, breeding and molting activities were assessed through mist-netting sessions scheduled twice monthly. At the same frequency, food abundance was evaluated through the percentage of trees bearing flowers, and by sweep-netting arthropods. The birds' breeding season occurred from the mid-dry to the mid-wet seasons and was linked to the abundance of some arthropod taxa including caterpillars. The higher abundance of arthropods and the presence of breeding activities during the dry season suggest that environmental factors are less limiting in mangroves than in adjacent arid habitats. The abundance of most birds, grouped according to their diet and frequency of mangrove use, varied significantly over time. Many species from other habitats exploited the mangroves in the late-wet and dry seasons to take advantage of a higher food abundance and to join mixedspecies flocks. Abundance of Nearctic migrants was correlated with the abundance of several arthropod taxa, and diet analyses suggest that these birds feed opportunistically on seasonal abundant arthropods during their stay in mangroves. Received 3 March 1993, accepted 17 November 1993. ALTHOUGH MANGROVES are widely distributed along the littoral in tropical and subtropical

Defense of Homopteran Honeydew by Birds in the Mexican Highlands and Other Warm Temperate Forests

Nearctic migrants aggressively compete for the honeydew of scale insects (Margarodidae, Xylococcini) throughout the winter in oak forests of the highlands of Chiapas, Mexico. Yellow-rumped warblers (Dendroica coronata) defend territories around 3-7 infested oak trees. Aggression is directed primarily against adult male Townsend's warblers (Dendroica townsendi), which in turn defend the territory against conspecifics and other migrant warblers when the dominant yellow-rumped warbler is off territory. The white-eared hummingbird (Hylocharis leucotis) is the only resident species to commonly feed on honeydew. Birds have only rarely been reported to feed on honeydew. The reports are restricted to moist, equable temperate forests in Australia, New Zealand, the Brazilian highlands, the Andes, and the mountains of Mesoamerica

Perspectives on Palaearctic and Nearctic bird migration; comparisons and overview of life‐history and ecology of migrant passerines

In this paper we compare ecological attributes of tropical migrant passerines from the Nearctic and western Palaearctic, focusing particularly on habitat association patterns during both breeding and wintering seasons. Three regions were compared: Europe, western and eastern North America. Breeding bird census data from 32 studies (each including at least four stages of forest succession) were used to assess the association patterns of breeding habitats among tropical migrants. For each species we calculated an index of habitat diversity and habitat preference.Tropical migrants preferred earlier successional stages than other birds in Europe. The opposite was true in eastern North America. In eastern North America, tropical migrants tended to be associated with a smaller range of serai stages than other passerine species. In their winter quarters, Palaearctic migrants live primarily in open habitats, such as savannas, whereas eastern Nearctic migrants make more frequent use of evergreen forests. Migrants from western North America show the greatest match between breeding and wintering habitats.We relate the results to the taxonomy and probable history of contemporary avifaunas and vegetation formations of the Old and New World. Taxonomically, tropical migrants from different parts of the Holarctic are less closely related to each other than residents and short‐distance migrants. Tropical and temperate avifaunas are more closely related to each other in the New World than in the Old World. Conservation implications of the between‐continent differences are briefly discussed.

Evolutionary Precursors of Long-Distance Migration: Resource Availability and Movement Patterns in Neotropical Landbirds

Seasonal movements are common in Neotropical forest birds. Species that engage in the largest movements, altitudinal and intratropical migrants, are predominantly frugivores or nectarivores of canopy/edge or dry habitats. Migration by such species may be necessitated by the high spatial and temporal variation in their resource base. We hypothesize that seasonal movements within the tropics predisposed these birds to migration out of the tropics. Indeed, most Nearctic passerine migrants are drawn from Neotropical taxa that exhibit altitudinal or intratropical migration. Conversely, taxa comprising the most sedentary group of Neotropical residents (understory insectivores) are poorly represented among Nearctic migrants. Further evidence of an evolutionary link between seasonal movements within the tropics and long-distance migration is provided by similarities in habitat use and diet between overwintering Nearctic migrants and closely related tropical residents. Both groups tend to be frugivorous/nectarivor...

Effects of Hurricane Gilbert on Birds in a Dry Tropical Forest in the Yucatan Peninsula

On 14 September 1988 Hurricane Gilbert, the strongest tropical storm ever recorded in the Western Hemisphere, passed through an area of tropical forest in Quintana Roo, Mexico, where I had been studying winter bird populations since 1984. The hurricane defoliated the forest, and created a nearly impenetrable tangle of fallen branches and vines at ground level. Point counts and mist-netting conducted 5-6 mo after the storm revealed an influx of fieldand shrub-associated bird species. Most previously common insectivores and feeding generalists were present in FebruaryMarch 1989, but many had declined significantly in abundance. Resident frugivores and nectarivores were especially scarce after the hurricane. Wildfires that swept through northeastern Quintana Roo in the summer of 1989 had a greater effect on birds than did the storm itself. By late winter 1990 the bird community within unburned forest was clearly converging toward its prehurricane composition. In contrast, the bird community in burned forest resembled that associated with recently abandoned agricultural fields and pastures. As a group, overwintering nearctic migrants appeared to be more resilient than year-round residents to the effects of both the hurricane and the associated fires.

Avian Pollination and Nectar Use in Combretum fruticosum (Loefl.)

In a tropical dry forest of southwestern Mexico, the flowers of the liana Combretum fruticosum (Loefl.) were visited by a diverse assemblage of birds (31 spp.), dominated by passerines but including hummingbirds. Nearctic migrants constituted a high percentage of the individuals (42%i) and species (40%) visiting the plant. The flowers produced abundant, hexose-dominated dilute nectar, and varied among plants in both the total amount of nectar produced and in the temporal pattern of secretion. The morphology of inflorescences and flowers, nectar chemistry and the species of birds identified as visitors of C. fruticosum indicate that this plant is pollinated by passerine birds rather than hummingbirds.

Distribution of Overwintering Nearctic Migrants in the Yucatan Peninsula, I: General Patterns of Occurrence

Grâce, principalement, a des comptages ponctuels bases sur une technique combinant l'index partiel d'abondance (IPA) et l'echantillonnage frequentiel progressif (EFP) et a des captures-recaptures, la distribution d'oiseaux migrateurs a hivernage neartique et d'oiseaux residents de la Peninsule de Yucatan est etalonnee en relation avec les types de milieu, qui sont principalement la foret tropicale et les zones d'agricultures et de friches. Pour les migrateurs, le type de milieu choisi pour la reproduction est compare a celui observe lors de l'hivernage

THE BIRDS OF THE SANTA ELENA PENINSULA, S.W. ECUADOR

SummaryA systematic account is given of the birds of the Santa Elena peninsula in S.W. Ecuador, based on three years observations (1954‐7) and a collection of 425 skins. The status of individual species is decribed and the different elements of the avifauna discussed, viz. the Equatorial arid fauna, Nearctic migrants, sea‐birds, and other wide‐ranging forms and waterfowl. Seasonal movements of species of the Equatorial arid fauna are indicated in greater detail than has been previously known. The effect of a recently constructed reservoir on the bird life is mentioned. More detailed knowledge of the occurrence of species from the cold Humboldt current is provided. Several additions to the Ecuadorian list are given, and some birds recorded which were hitherto unknown or very rarely recorded from the west coast of S. America. Various matters of taxonomy affecting races of birds along the whole west coast of S. America are discussed.