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Migratory Strategies (migratory + strategy)

Distribution by Scientific Domains

Life Sciences	80%

Selected Abstracts

Migration strategies of sylviid warblers: chance patterns or community dynamics?

JOURNAL OF AVIAN BIOLOGY, Issue 1 2000
Peter Howlett
The effects of community dynamics in birds on the optimisation of their migratory strategies is a neglected area. For three years, we captured migrating warblers on autumn passage at a coastal site in western Britain. We used canonical correspondence analysis (CCA) to assess spatio-temporal patterns of occurrence, and principal components analysis (PCA) to assess morphological variation. We calculated Euclidean distance in ordination and morphological space to assess separation between species pairs, and used Monte-Carlo simulations to assess the probability of pattern occurring by chance. Ordination revealed five species-groups separated by habitat type and time of passage. Reed Warbler Acrocephalus scirpaceus and Sedge Warbler A. schoenobaenus (Group 1) occurred in wet habitats and peaked simultaneously. In drier habitats with scrub, a first wave of Blackcap Sylvia atricapilla (Group 2) significantly preceded Grasshopper Warbler Locustella naevia, Willow Warbler Phylloscopus trochilus, Whitethroat Sylvia communis and Lesser Whitethroat Sylvia curruca (Group 3), which in all but one case (Lesser Whitethroat) significantly preceded Garden Warbler Sylvia borin (Group 4); peak numbers of Chiffchaffs Phylloscopus collybita and a second wave of Blackcaps (Group 5) occurred later still. Age effects were found only in Acrocephalus, with adults peaking before juveniles. For seven out of eight pairings within genera, separation in time of passage increased significantly in species that were morphologically similar. The only exception was Blackcap and Lesser Whitethroat which differed substantially in both passage time and morphology. Monte-Carlo simulations showed that chance was unlikely to be responsible for ordination patterns, nor for inter-specific variation in passage time and its relationship with species morphology. These data provide annually consistent evidence that migrating sylviid warblers are separated ecologically by habitat use, time of passage and morphology: we cannot refute the hypothesis that community dynamics have influenced niche use and autumn migratory strategy. We call for further tests of the ,migrant interaction' hypothesis in other geographical locations and taxa, particularly where migrants are allopatric and interact ecologically only on migration. [source]

Patterns of density, diversity, and the distribution of migratory strategies in the Russian boreal forest avifauna

JOURNAL OF BIOGEOGRAPHY, Issue 11 2008
Russell Greenberg
Abstract Aim, Comparisons of the biotas in the Palaearctic and Nearctic have focused on limited portions of the two regions. The purpose of this study was to assess the geographic pattern in the abundance, species richness, and importance of different migration patterns of the boreal forest avifauna of Eurasia from Europe to East Asia as well as their relationship to climate and forest productivity. We further examine data from two widely separated sites in the New World to see how these conform to the patterns found in the Eurasian system. Location, Boreal forest sites in Russia and Canada. Methods, Point counts were conducted in two to four boreal forest habitats at each of 14 sites in the Russian boreal forest from near to the Finnish border to the Far East, as well as at two sites in boreal Canada. We examined the abundance and species richness of all birds, and specific migratory classes, against four gradients (climate, primary productivity, latitude, and longitude). We tested for spatial autocorrelation in both dependent and independent variables using Moran's I to develop spatial correlograms. For each migratory class we used maximum likelihood to fit models, first assuming uncorrelated residuals and then assuming spatially autocorrelated residuals. For models assuming unstructured residuals we again generated correlograms on model residuals to determine whether model fitting removed spatial autocorrelation. Models were compared using Akaike's information criterion, adjusted for small sample size. Results, Overall abundance was highest at the eastern and western extremes of the survey region and lowest at the continent centre, whereas the abundance of tropical and short-distance migrants displayed an east,west gradient, with tropical migrants increasing in abundance in the east (and south), and short-distance migrants in the west. Although overall species richness showed no geographic pattern, richness within migratory classes showed patterns weaker than, but similar to, their abundance patterns described above. Overall abundance was correlated with climate variables that relate to continentality. The abundances of birds within different migration strategies were correlated with a second climatic gradient , increasing precipitation from west to east. Models using descriptors of location generally had greater explanatory value for the abundance and species-richness response variables than did those based on climate data and the normalized difference vegetation index (NDVI). Main conclusions, The distribution patterns for migrant types were related to both climatic and locational variables, and thus the patterns could be explained by either climatic regime or the accessibility of winter habitats, both historically and currently. Non-boreal wintering habitat is more accessible from both the western and eastern ends than from the centre of the boreal forest belt, but the tropics are most accessible from the eastern end of the Palaearctic boreal zone, in terms of distance and the absence of geographical barriers. Based on comparisons with Canadian sites, we recommend that future comparative studies between Palaearctic and Nearctic faunas be focused more on Siberia and the Russian Far East, as well as on central and western Canada. [source]

Does avian species richness in natural patch mosaics follow the forest fragmentation paradigm?

ANIMAL CONSERVATION, Issue 1 2007
D. C. Pavlacky Jr.
Abstract As one approaches the north-eastern limit of pinyon (Pinus spp.) juniper (Juniperus spp.) vegetation on the Colorado Plateau, USA, woodland patches become increasingly disjunct, grading into sagebrush (Artemisia spp.)-dominated landscapes. Patterns of avian species richness in naturally heterogeneous forests may or may not respond to patch discontinuity in the same manner as bird assemblages in fragmented agricultural systems. We used observational data from naturally patchy woodlands and predictions derived from studies of human-modified agricultural forests to estimate the effects of patch area, shape, isolation and distance to contiguous woodland on avian species richness. We predicted that patterns of species richness in naturally patchy juniper woodlands would differ from those observed in fragmented agricultural systems. Our objectives were to (1) estimate the effect of naturally occurring patch structure on avian species richness with respect to habitat affinity and migratory strategy and (2) assess the concordance of the effects to predictions from agricultural forest systems. We used the analogy between populations and communities to estimate species richness, where species are treated as individuals in the application of traditional capture,recapture theory. Information-theoretic model selection showed that overall species richness was explained primarily by the species area relationship. There was some support for a model with greater complexity than the equilibrium theory of island biogeography where the isolation of large patches resulted in greater species richness. Species richness of woodland-dwelling birds was best explained by the equilibrium hypothesis with partial landscape complementation by open-country species in isolated patches. Species richness within specific migratory strategies showed concomitant increases and no shifts in species composition along the patch area gradient. Our results indicate that many patterns of species richness considered to be fragmentation effects may be general consequences of patch discontinuity and are ubiquitous in naturally heterogeneous systems. There was no evidence for the effects of patch shape and distance to contiguous woodland in juniper woodland, suggesting edge effects and dependence upon regional species pools are characteristics of fragmented agricultural systems. Natural patch mosaics may provide benchmarks for evaluating fragmentation effects and managing forests by mimicking natural landscape patterns. [source]