Summary 0[ Dispersal is of critical ecological and evolutionary importance for several issues of population biology\ particularly population synchrony\ colonization and range expansion\ metapopulation and sourceÐsink dynamics\ and population genetic struc! ture\ but it has not previously been possible to compare dispersal patterns across a wide range of species or to study movement outside the con_nes of local study areas[ 1[ Using resampling methods\ we veri_ed that statistically unbiased estimates of average dispersal distance and of intraspeci_c variance in dispersal distance could be extracted from the bird ringing data of the British Trust for Ornithology[ 2[ Using data on 64 terrestrial bird species\ we tested whether natal and breeding dispersal were in~uenced by a species| habitat requirements\ diet\ geographical range\ abundance\ morphology\ social system\ life history or migratory status[ We used allometric techniques to ascertain whether these relationships were independent of body size\ and used the method of phylogenetically independent contrasts to ascertain whether they were independent of phylogeny[ 3[ Both natal and breeding dispersal distances were lower among abundant species and among species with large geographical ranges[ Dispersal distances and life!history variables were correlated independent of phylogeny\ but these relationships did not persist after controlling for body size[ All morphometrical variables "wing length\ tarsus length and bill length# were not signi_cantly correlated with dispersal distances after correcting for body size or phylogenetic relatedness[ 4[ Migrant species disperse further than resident ones\ this relation was independent of body size but not of phylogeny[ A signi_cant positive relation was observed between breeding dispersal distance and long!term population decline among migrants\ but not among residents[ 5[ The species living in wet habitats disperse further than those living in dry habitats\ which could be explained by the greater patchiness of wet habitats in space and:or time[ This relationship was observed only for breeding dispersal\ suggesting that this habitat variable does not impose the same constraint on natal dispersal[ Key!words] body size\ comparative analyses\ dispersal distances\ phylogeny\ ring recoveries[ Journal of Animal Ecology "0887# 56\ 407Ð425

No abstract

We used extensive atlas and census data to assess trends in the distribution and population levels of birds on lowland farmland in Britain between the late 1960s and early 1990s. Many species of farmland birds have become less widespread or have declined in numbers, or both, but few have become more widespread or have increased. Of the 28 species classified as farmland birds the distributions of 24 contracted between 1970 and 1990. Of the 18 farmland species for which it was possible to assess population change, 15 were less abundant in 1990 than in 1970. Seven of the species were estimated to have undergone population decreases of at least 50%. Farmland species showing the largest population declines tended also to show substantial range contractions. Farmland species underwent an appreciably larger contraction of distribution than species associated with any other habitat. Furthermore, farmland species tended to decrease in abundance, whereas woodland species tended to increase. Population declines among farmland birds became evident in the mid-to late 1970s, a period when several fundamental changes were taking place in British agricultural practices. These included a great reduction in the spring sowing of cereals, a simplification of crop rotations, increased use of chemical pesticides and inorganic fertilizers, and more-intensive grassland management. We suggest that the declines of farmland bird species have been caused or aggravated by this pervasive intensification of agriculture. Existing research on declining farmland birds, however, indicates that there is no single mechanism underlying the population changes. We identify priorities for research, focusing mainly on relationships between bird populations and agricultural practices, but we also recognize a need for a better understanding of the role of predation.Declinaciones poblacionales y contracciones en el firea de distribuci6n de aves que habitan tierras bajas agricolas en Gran Bretafia Population Declines and Range Contractions of British Farmland BirdsFuller et al.fundamentales en las pr~cticas agrfcolas britdtntcas. Estos cambios incluyeron una gran reducci6n en la siembra primaveral de cereales, una simpliflcaci6n en la rotact6n de las cosechas, un aumento en el uso de pestictdas quimicos y ferttlizantes inorg~ntcos y un manejo mds intensivo de las pasturas. Sugerimos que la declinaci6n de las especies de aves de tlerras agrtcolas, ha sido causada o aumentada por esta tntenstflcact6n de la agrtcultura. Sin embargo, las investigaciones existentes sobre las aves de tierras agrfcolas en decli-naci6n indtcan que no existe un mecantsmo anico responsable estos cambtos poblacionales. Identiflcamos prioridades para la investtgaci6n, enfatizando prtnctpalmente las relactones entre poblactones de aves y pr~cticas agrfcolas, pero tambi6n reconocemos que es necesario una mejor comprensi6n del papel que juega de la predaci6n.

Summary 0[ Widespread declines in the populations of many British farmland birds have occurred since the early 0869s[ We must understand the causes of these declines to make recommendations about conservation and agricultural management\ and this can be approached by investigating the relationships\ across species\ between abundance and agricultural change[ We describe novel\ quantitative approaches to the interpretation of abundance indices from which reliable inferences about conservation status can be made[ 1[ We calculated farmland Common Birds Census indices for 31 species\ smoothed the series to reveal underlying trends and estimated con_dence intervals for the changes in abundance[ 2[ Between 0857 and 0884\ the abundance of 01 species declined signi_cantly and that of 03 species increased[ 3[ Specialization was the only signi_cant determinant of changes in abundance "of 09 tests against species characteristics#] 02 farmland specialists declined\ on average\ by 29)\ whilst 18 more generalist species underwent an average increase of 12)\ con_rming that farmland birds should engender conservation concern[ 4[ Smoothed abundance curves\ transformed to emphasize trend direction and timing\ were then compared quantitatively to identify whether groups of species had shared common trends[ 5[ Species tended not to be strongly grouped\ but small groups of species with common trends were identi_ed[ Similarities in ecology among grouped species clarify the possible environmental causes of their population trends\ indicating future research priorities[ 6[ The groups identi_ed included] one group consisting of three thrush species Turdus and the skylark Alauda arvensis L[ which all declined from the mid!0869s after being stable previously^one group comprising three trans!Saharan migrant warblers "Sylvidae#\ whose abundance fell in the early 0869s and later increased^and a diverse group of six smoothly increasing species[ 7[ Turning points were identi_ed as where each species| population trend turned signi_cantly\ revealing critical periods during which populations are likely to have been a}ected by environmental change[ 8[ Three collections of downward turning points were found\ including one in the mid!0869s when many farmland bird declines began[ Four other periods each included many upturns[ The groups of turning points should facilitate the identi_cation of environmental changes which have had widespread e}ects[ Management prescriptions can then be designed to reverse or to mirror such key changes and thereby focus conservation e}ort e}ectively[ Key!words] agriculture\ bird populations\ conservation\ environmental change\ turn! ing points[

No abstract