Frequency Of Migrants Research Articles

Diversity and guild structure of the Petit Saut bird community

Bird communities were sampled at different scales at Saint-Eugène and on the Petit Saut dam reservoir between 1993 and 1 996. Global bird species richness of the Petit Saut area (ca . 35 sq. km investigated) was estimated to be over 400 species, of which 342 were observed. Relationships between species richness and surveyed area, sampling pressure, and habitat diversity, are described. A majority of species showed low contact frequencies and low abundance. Numerically dominant species on the mainland were also abundant on islands, but islands had a different pool of dominant species. Migrants accounted for a very small proportion of the community and were mainly aquatic birds, but the frequency of migrants in primary rainforest may be underestimated. Community organization is described in terms of guilds, feeding habits, habitats and social systems. Sixteen guilds can be identified. Insectivores are dominant in the bird community. Most species inhabit terra firme forest, but some guilds are preferentially associated with damp or liana forests. Thirty percent of the bird species live in single-species or mixed-species groups, in particular in canopy-, understory-, or ant-following flocks. The ecological organization of the Petit Saut bird community is similar to that found in other Guianan or Amazonian forest sites. Flooding of the reservoir lead to a very slow increase of aquatic bird diversity and abundance, particularly of Sungrebe (Heliomis fulica), while sightings of banks-dwelling species became scarce.

Relationships among climate, latitude and migration: Australian butterflies are not temperate-zone birds.

We examined the distribution of butterflies over the mostly arid and semi-arid continent of Australia and analyzed the proportion of migrant species and species diversity with respect to an array of climatic and geographic variables. On a continent-wide scale, latitude explained virtually no variance in either proportion of migrants (r 2=0.01) or species diversity (r 2=0.03) in Australian butterflies. These results are in marked contrast to those for temperate-zone birds from three continents where latitude explained between 82 and 98% of the variance in frequency of migrants and also accounted for much of the variance in bird species diversity. In eastern Australia where rainfall regimes are similar to those in temperate Europe and North and South America, latitude explains 78% of the variance in frequency of butterfly migrants. In both eastern and central Australia, latitude also accounts for relatively high proportions of the variance in species diversity. Rainfall patterns and especially soil moisture are negatively associated with migration frequency in Australian butterfly faunas, both alone and in combination with other climate variables. Where moisture levels are relatively high, as in eastern Australia, measures of temperature are associated with migration frequency, a result consistent with findings for temperate-zone birds, suggesting latitude is a surrogate for temperature. The ultimate causes of migration in temperate-zone birds and Australian butterflies are the uneven temporal, and in Australia also spatial, distribution of resources. Uneven distribution is brought about primarily by temperature in temperate regions and by erratic rainfall over much of arid Australia. As a key determinant of productivity, especially in the tropics and subtropics, aridity is likely to be an important determinant of the global distributions of migrants.

Biologia dos adultos de Methona themisto (Hübner, 1818) (Lepidoptera, Nymphalidae, Ithomiinae) em praças públicas de Uberlândia, Minas Gerais, Brasil

From April 1992 to September 1993 Methona themisto (Hübn.) adults were weekly marked and recaptured in two downtown plazas in Uberlândia, MG. Females accounted for a third of the 637 individuals marked. Nearly 10% of the butterflies were recaptured, with the maximum residence time of 62 days. The presence of the larval host plant (Brunfelsia uniflora (Pohl) D. Don, Solanaceae) in one of the plazas enhanced a higher abundance of the butterfly throughout the research and a higher proportion of new individuals, and frequency of migrants than the other plaza which lacked the host plant. Methona themisto adults were active through the sampling period with overlapping generations. Its abundance in plazas with polluted air and exotic flora, long adult life, and tolerance to humans in its vicinities place M. themisto among the ones indicated to have its life conditions improved in downtown areas of brazilian cities. Simple managing practices which benefit the lepitopteran are suggested.

Frequency of migrants and migratory activity are genetically correlated in a bird population: evolutionary implications.

Most migratory bird populations are composed of individuals that migrate and individuals that remain resident. While the role of ecological factors in maintaining this behavioral dimorphism has received much attention, the importance of genetic constraints on the evolution of avian migration has not yet been considered. Drawing on the recorded migratory activities of 775 blackcaps (Sylvia atricapilla) from a partially migratory population in southern France, we tested two alternative genetic models about the relationship between incidence and amount of migratory activity. The amount of migratory activity could be the continuous variable "underlying" the phenotypic expression of migratory urge, or, alternatively, the expression of both traits could be controlled by two separate genetic systems. The distributions of migratory activities in five different cohorts and the inheritance pattern derived from selective breeding experiments both indicate that incidence and amount of migratory activity are two aspects of one trait. Thus, all birds without measurable activity have activity levels at the low end of a continuous distribution, below the limit of expression or detection. The phenotypic dichotomy "migrant-nonmigrant" is caused by a threshold which may not be fixed but influenced both genetically and environmentally. This finding has profound implications for the evolution of migration: the transition from migratoriness to residency should not only be driven by selection favoring resident birds but also by selection for lower migratory activity. This potential for selection on two aspects, residency and migration distance, of the same trait may enable extremely rapid evolutionary changes to occur in migratory behavior.