Passerine Bird Species Research Articles

Fuel deposition of three passerine bird species along the migration route.

The rate at which migrant birds replenish their energy stores at intermittent stopovers largely determines overall migration speed, the manner in which migration proceeds and success of migration. In this study, data on the fuel deposition rate (FDR) of three long-distance migrants from 17 ringing sites along their autumn migration route were used to examine: (1) effects of endogenous factors on FDR, and (2) how relationships between exogenous factors and FDR affect the organisation of migration. We developed a model to estimate FDR from retrapped birds which takes into account time of day and various other factors which might influence FDR. The two endogenous factors, moult and current energy stores, generally reduced FDR. This may result in lower departure energy loads and more stopovers than expected from optimal migration theory. Differences between species with respect to seasonal, year-to-year and geographical patterns of FDR could be related to differences in availability and predictability of food resources, and help to explain differences in the organisation of migration. A low FDR in northern and central Europe could be related to low, but predictable, food resources and an early departure during moult of the reed warbler (Acrocephalus scirpaceus); FDRs varying between years were related to large spatial and year-to-year variation in the density of the main prey of the sedge warbler (Acrocephalus schoenobaenus); and a high FDR in the garden warbler (Sylvia borin) was related to abundant food resources, due to a switch from a purely invertebrate diet to a mixed diet including fruits which are abundant over large areas of Europe and north Africa. This study demonstrated that the organisation of migration is the outcome of a complex interplay of the seasonal timing of moult, food availability and predictability and a seasonal switch in diet, and can be modified by individual birds in response to a limited amount of time in which to migrate.

Developmental changes in 7-ethoxyresorufin- O-deethylase (EROD) and δ-aminolevulinic acid dehydratase (ALA-D) activities in three passerines

In this study, we report changes in 7-ethoxyresorufin- O-deethylase (EROD) and δ-aminolevulinic acid dehydratase (ALA-D) enzyme activities during development of three wild passerine bird species: pied flycather ( Ficedula hypoleuca), great tit ( Parus major) and blue tit ( P. caeruleus). Activities were determined from late embryos, newly hatched, 1-week-old and 15-day-old nestlings and adult birds. EROD activity from hepatic microsomes and ALA-D activity from liver tissue and erythrocytes were measured. In all species investigated EROD and ALA-D activities were increased during the first posthatching week and decreased thereafter. Also, differences between activities in late-term nestlings and adults were seen. Based on these results, we suggest that the developmental status of an animal must always be taken into account when evaluating the results obtained in the biomarker studies.

Semi‐quantitative assessment of wing feather mite (Acarina) infestations on passerine birds from Portugal. Evaluation of the criteria for accurate quantification of mite burdens

AbstractWing feather mite burdens on seven species of passerine birds (Carduelis carduelis– goldfinch;C. chloris– greenfinch;Serinus serinus– serin;Sylvia atricapilla– blackcap;Sylvia melanocephala– Sardinian warbler;Turdus merula– blackbird;Passer domesticus– house sparrow) from Portugal were assessed by the subjective semi‐quantitative scoring system of Behnkeet al. (1995) in order to evaluate more fully the accuracy and reliability of the technique. Our analysis indicated that in all species, scores allocated to flight feathers showed a significant positive relationship with mite counts as assessed through microscopical examination of the same feathers. However, there were differences between species of birds. Of the species examined, goldfinches and greenfinches showed the weakest relationships between assigned mite scores and actual mite numbers indicating that the technique was less accurate when applied to these species compared with the remaining five. No evidence was found that anything more was to be gained from scoring both wings, rather than just one. Feather mites (Proctophyllodesspp.,Trouessartia incisa) were also detected on tail feathers, but the assessment of these feathers presented additional problems and it was concluded that in the interests of minimizing handling time of birds, tail scores had little more to offer. We conclude that scoring all the flight feathers (including all primary, secondary, and tertiary feathers) on one entire wing, but alternating between left and right wings of birds within a species, represents an acceptable compromise between sufficiently detailed examination and minimization of bird handling time in the field.

Semi-quantitative assessment of wing feather mite (Acarina) infestations on passerine birds from Portugal. Evaluation of the criteria for accurate quantification of mite burdens

Wing feather mite burdens on seven species of passerine birds (Carduelis carduelis– goldfinch; C. chloris– greenfinch; Serinus serinus– serin; Sylvia atricapilla– blackcap; Sylvia melanocephala– Sardinian warbler; Turdus merula– blackbird; Passer domesticus– house sparrow) from Portugal were assessed by the subjective semi-quantitative scoring system of Behnke et al. (1995) in order to evaluate more fully the accuracy and reliability of the technique. Our analysis indicated that in all species, scores allocated to flight feathers showed a significant positive relationship with mite counts as assessed through microscopical examination of the same feathers. However, there were differences between species of birds. Of the species examined, goldfinches and greenfinches showed the weakest relationships between assigned mite scores and actual mite numbers indicating that the technique was less accurate when applied to these species compared with the remaining five. No evidence was found that anything more was to be gained from scoring both wings, rather than just one. Feather mites (Proctophyllodes spp., Trouessartia incisa) were also detected on tail feathers, but the assessment of these feathers presented additional problems and it was concluded that in the interests of minimizing handling time of birds, tail scores had little more to offer. We conclude that scoring all the flight feathers (including all primary, secondary, and tertiary feathers) on one entire wing, but alternating between left and right wings of birds within a species, represents an acceptable compromise between sufficiently detailed examination and minimization of bird handling time in the field.

Do local abundances of British birds change with proximity to range edge?

Summary Aim Species generally vary in the density they attain at different sites, prompting the question of whether this variation is systematic across their range. We investigate this question using data on the abundance and distribution of thirty‐two species of passerine birds across Britain derived from censuses organized by the British Trust for Ornithology. Methods Analysis is complicated by the issue of quantifying the distance of any particular census location from the edge of the range of a species when the study area encompasses only part of its entire distribution. No measure of this quantity is a priori the correct one, and so we use a variety of different measures which make differing assumptions about how abundances might be structured across species ranges. Results None of the measures used reveal any consistent relationships between the density attained by species at census sites and the spatial positions of those sites. Only thirteen species show significant relationships with any of the measures, and no more than seven species with any single measure. Main conclusion In summary, there is no convincing evidence that passerine bird densities are usually lower towards range edges in Britain. We discuss possible reasons for these findings.

Egg sizes of nine passerine bird species in a subalpine birch forest, Swedish Lapland

Egg measurements together with calculated volumes are presented for nine passerine species (Prunella modularis, Luscinia svecica, Phoenicurus phoenicurus, Turdus pilaris, Turdus philomelos, Turdus iliacus, Phylloscopus trochilus, Fringilla montifringilla, and Carduelis flammea), breeding in rich subalpine birch forests on a south-faced mountain slope in southern Swedish Lapland. There were no clear regional size trends in comparisons with egg collections from other parts of Europe. There was a significant increase in egg size in the latter part of the laying sequence. The ultimate egg is from 2.1% (F. montifringilla) to 6.5% (Ph. trochilus) larger than the mean of the clutch. According to several years’ measurements of T. pilaris and T. iliacus eggs there was no significant size variation between seasons. This constancy together with the relatively large egg sizes might be a consequence of the productive breeding habitat of the study area.

MATERNAL EFFECTS AND THE PERVASIVE IMPACT OF NESTLING HISTORY ON EGG SIZE IN A PASSERINE BIRD.

Environmental conditions experienced during ontogeny may have important and permanent consequences for life-history traits of individuals. The aim of this work was to determine whether conditions experienced during early life, as expressed by prefledgling condition, have any bearing on the volume of eggs subsequently laid by females in a small passerine bird species, the pied flycatcher (Ficedula hypoleuca). I first show a genetic influence on egg volume by dam-offspring regression. The existence of maternal effects is shown by comparing resemblances between average egg volumes of offspring to those of their paternal and maternal grandmothers. Path analysis shows that when controlling for the influence of maternal egg volume, the average volume of eggs produced in a lifetime is positively related to the body condition of a female measured while she is still at the nest. Thus, body condition at fledgling affects egg volume beyond the ability of maternal egg volume to predict offspring egg volume. Although there may be a persistent maternal effect of egg volume on itself or linked to a property correlated with egg volume, the maternal performance trait involved remains unknown, except for maternal egg volume itself. Given the effects that egg volume may have on reproductive success, which are partly determined prior to leaving the nest, maternal effects may have important consequences for lifetime fitness of offspring.

Effects of Vasoactive Intestinal Peptide on Prolactin Secretion in Three Species of Passerine Birds

Previous work on domesticated species has indicated that vasoactive intestinal peptide (VIP) is an important prolactin-releasing factor in these birds, but no comparative work in passerine birds has been reported. This study showed that iv injections of VIP (50–100 μg/kg body mass) result in a dramatic, but transitory, rise in plasma prolactin in Mexican jays (Aphelocoma ultramarina). Significant increases in prolactin were also observed following VIP injection in blue jays (Cyanocitta cristata) and zebra finches (Poephilla guttata). At the dosage we used, maximum levels of prolactin attained were slightly lower (Mexican jays) or very similar (blue jay and zebra finch) to the maximum prolactin levels observed in other, breeding birds of the same species. In zebra finches that initially had low prolactin, VIP injection resulted in a greater than 10-fold increase in prolactin within 10 min, but those individuals that already had elevated prolactin showed no further increase in response to VIP. Slow-release pellets of VIP implanted subcutaneously in Mexican jays and releasing 10 or 15 μg VIP/day (two or three pellets) produced a significant increase in plasma prolactin (78 and 92% rise, respectively) compared to birds with placebo pellets or with with one pellet releasing only 5 μg/day.

Participation of birds (Aves) in the emergence of Lyme disease in southern Maine.

The contribution of migratory and resident birds to the introduction of Lyme disease will vary with the degree to which various species expose themselves to, and are infested by, juvenile vector ticks, and their ability to support and transmit the infectious agent. To examine the relative contribution of various passerine species during the emergence of this disease, we compared the abundance and infection rates of the blacklegged tick, Ixodes scapularis Say, removed from mist-netted birds with those from live-trapped mice at a coastal study site in southern Maine, collected during an 8-yr period in which the range of this tick and the incidence of Lyme disease increased in the state. Weekly bird-banding sessions using six 12-m Japanese mist nets were carried out from May through August 1989-1996. In 1989, 1991 and 1993, mice were live-trapped in a Sherman trap grid (7 by 7 m) during five 3-night sessions, June through August; in 1994-1996, 2 such grids were similarly trapped. Annual adult tick abundance was estimated by flagging vegetation. We removed 2,633 juvenile deer ticks from 1,713 of 1,972 birds examined. Twenty-five of 64 bird species were infested. The percentages of birds infested and the rate of infection among removed larvae and nymphs increased over the years, but species varied markedly in their ability to infect ticks. No infected larvae were removed from catbirds or towhees. The larval to nymphal ratio was higher in mice than in birds. Infection rates among bird-derived larvae were less than among mice-derived larvae, but increased with time. Because of the different ways in which individual species of passerine birds contribute to the availability of vector ticks and respond to the agent of Lyme disease in emerging areas, further research into host competency and borreliacidal mechanisms is needed.

Red squirrels,Tamiasciurus hudsonicus, produce predator-class specific alarm calls

Red squirrels, can produce alarm calls when they detect a potential predator. Observations of natural interactions between red squirrels and large birds, and predator-presentation experiments in the field, showed that red squirrels produce acoustically different alarm calls in response to aerial danger (live birds and a model hawk flown towards them) versus danger approaching from the ground (dogs and humans). The alarm call produced in response to aerial danger is acoustically convergent on the ‘seet’ alarm call produced by many species of passerine birds in response to raptors. The squirrels' ‘seet’ alarm is a short, low-amplitude, high-frequency call. These characteristics make the call difficult to localize, and is in a frequency range that is poorly perceived by raptors. Red squirrels produce much louder, wide-bandwidth bark calls in response to terrestrial danger. This is the first demonstration of predator-class specific alarm calls of red squirrels.

Bird species assemblages as indicators of biological integrity in Great Basin rangeland

The study evaluates the potential for bird species assemblages to serve as indicators of biological integrity of rangelands in the Great Basin in much the same way that fish and invertebrate assemblages have been used as indicators in aquatic environments. Our approach was to identify metrics of the bird community using relatively simple sampling methods that reflect the degree of rangeland degradation and are consistent over a variety of vegetation types and geographic areas. We conducted the study in three range types (i.e., potential natural plant community types) in each of two widely separated areas of the Great Basin: south-eastern Idaho (sagebrush steppe range types) and west-central Utah (salt-desert shrub range types). Sites were selected in each range type to represent three levels of grazing impact, and in Idaho included sites modified for crested wheatgrass production. Birds were sampled by point counts on 9 100-m radius plots at 250-m spacing on each of 20 sites in each area during the breeding season. In sagebrush-steppe, 964 individuals in 8 species of passerine birds were used in analyses. Five metrics were significantly related to impact class, both when analyzed within range type and when analyzed with all range types combined. Species richness, relative abundance of shrub obligate species, and relative abundance of Brewer's sparrow were generally lower for the higher impact classes, whereas the reverse was true for dominance by a single species and for relative abundance of horned larks. In contrast, total number of individuals did not differ significantly as a function of impact class. In salt-desert shrub, a total of 843 birds in 4 species were included in analyses, 98% of which were horned larks. None of the metrics identified above was significantly related to impact class. Two metrics for breeding birds in sagebrush steppe (species richness and dominance) showed little overlap between values for the extremes of impact class, and thus they have potential as indicators of biological integrity. However, the sensitivity of these metrics appears to be greatest at the high impact end of the spectrum, which suggests they may have limited utility in distinguishing between sites having light and moderate impact.

Blood parasites of passerine birds from central Spain.

Sixteen species of passerine birds captured during a 2.5 yr period in Central Spain were examined for hematozoa. Haemoproteus spp., Leucocytozoon spp., Trypanosoma spp., Plasmodium spp., and microfilariae were observed. The most prevalent species were in the genus Leucocytozoon. The majority of the records are new for Spain and some represent new host records. More than one-half of the birds examined were infected with at least one parasite species. These records are similar to those reported from other areas in northern Europe and the Iberian Peninsula.

THE EVOLUTION OF SPERM SIZE IN BIRDS.

Sperm size varies enormously among species, but the reasons for this variation remain obscure. Since it has been suggested that swimming velocity increases with sperm length, earlier studies proposed longer (and therefore faster) sperm are advantageous under conditions of intense sperm competition. Nonetheless, previous work has been equivocal, perhaps because the intensity of sperm competition was measured indirectly. DNA profiling now provides a more direct measure of the number of offspring sired by extrapair males, and thus a more direct method of assessing the potential for sperm competition. Using a sample of 21 species of passerine birds for which DNA profiling data were available, we found a positive relation between sperm length and the degree of extrapair paternity. A path analysis, however, revealed that this relationship arises only indirectly through the positive relationship between the rate of extrapair paternity and length of sperm storage tubules (SSTs) in the female. As sperm length is correlated positively with SST length, an increase in the intensity of sperm competition leads to an increase in sperm length only through its effect on SST length. Why females vary SST length with the intensity of sperm competition is not clear, but one possibility is that it increases female control over how sperm are used in fertilization. Males, in turn, may respond on an evolutionary time scale to changes in SST size by increasing sperm length to prevent displacement from rival sperm. Previous theoretical analyses predicting that sperm size should decrease as sperm competition becomes more intense were not supported by our findings. We suggest that future models of sperm-size evolution consider not only the role of sperm competition, but also how female control and manipulation of ejaculates after insemination selects for different sperm morphologies.

A Test of the Skutch Hypothesis: Does Activity at Nests Increase Nest Predation Risk?

Small clutches in tropical species of passerine birds may be a response to nest predation. The Skutch hypothesis predicts that nest predation increases with activity at nests, and activity at nests is assumed to increase with clutch size. This suggests that predation should be greatest when activity is greatest, all else being equal. If true, then nest predation should be greatest when adult birds are feeding nestlings rather than incubating eggs. This hypothesis was examined with a neotropical understory passerine bird, the Western Slaty Antshrike Thamnophilus atrinucha. Nest predation was the only important cause of nest failure. Mayfield daily survival rates were compared during the incubation and nestling stages. Activity at nests was observed and compared during those two stages. Anecdotally, we also report results of an attempt to identify predators and whether they fit the Skutch hypothesis. Activity, measured as adult arrival and departure frequency at nests, is greatest during the nestling stage. Nest predation rates, however, are equal during both stages of nesting. Activity is not associated with predation, and Skutch's hypothesis is not supported. Snakes, the predator considered by Skutch to be most important, may not be. Of 12 species of snakes found on or near the study area, one species ate eggs, and several ate young birds. Yet predation on nestlings is not greater than on eggs. On the other hand, small marsupial mammals are common and, as suggested by captures and discoveries in nests, may be the important predators. They are nocturnal so diurnal bird activity should not influence predation. Small arboreal marsupials and rodents are common throughout the neotropics. The research presented here, and the preliminary data describing potential nest predators, do not support Skutch's idea that predators are attracted to activity at nests.

Change in Avian Abundance Across the Geographic Range in Response to Environmental Change

The abundance changes of three North American passerine bird species after the harsh winters of the late 1970s were analyzed to determine relationships between proportional abundance change and prewinter abundance, position within the geographic range, and winter severity. Analyses were made between both the prewinter period and a period of declining abundance immediately after the winters and between this decline period and a later period of recovery in which abundance increased. The proportional declines in abundance showed no relationship to prewinter abundances. Proportional abundance change was greatest at sites closer to the range edge and, for two species, at sites with a more severe winter. Proportional abundance increases of these two species between the decline and recovery periods similarly were greatest at sites near the range edges and with previously more severe winters. Abundance increases were greater at sites with the lowest predecline abundance. Geographic range structures showed changes parallel to the abundance changes, including overall contraction and expansion. In addition, there was evidence of fine-scale range dynamics, including extinction and colonization events toward the range margins and a roughening of the range boundary with abundance decrease followed by a smoothing of the boundary with abundance increase. Periods of major environmental change, such as these severe winters, may provide insight into how populations of organisms will respond to global climate change. In particular, this study suggests that population declines will be accompanied by a contraction toward core areas of the range that formerly had the highest abundance. However, more investigation is clearly needed into the precise nature of broad-scale population dynamics following environmental perturbations that cause abundance change.

CHANGE IN AVIAN ABUNDANCE ACROSS THE GEOGRAPHIC RANGE IN RESPONSE TO ENVIRONMENTAL CHANGE

The abundance changes of three North American passerine bird species after the harsh winters of the late 1970s were analyzed to determine relationships between proportional abundance change and prewinter abundance, position within the geographic range, and winter severity. Analyses were made between both the prewinter period and a period of declining abundance immediately after the winters and between this decline period and a later period of recovery in which abundance increased. The proportional declines in abundance showed no relationship to prewinter abundances. Proportional abundance change was greatest at sites closer to the range edge and, for two species, at sites with a more severe winter. Proportional abundance increases of these two species between the decline and recovery periods similarly were greatest at sites near the range edges and with previously more severe winters. Abundance increases were greater at sites with the lowest predecline abundance. Geographic range structures showed changes parallel to the abundance changes, including overall contraction and expansion. In addition, there was evidence of fine-scale range dynamics, including extinction and colonization events toward the range margins and a roughening of the range boundary with abundance decrease followed by a smoothing of the boundary with abundance increase. Periods of major environmental change, such as these severe winters, may provide insight into how populations of organisms will respond to global climate change. In particular, this study suggests that population declines will be accompanied by a contraction toward core areas of the range that formerly had the highest abundance. However, more investigation is clearly needed into the precise nature of broad- scale population dynamics following environmental perturbations that cause abundance change.

An Alternative Hypothesis for Heavier Parasite Loads of Brightly Colored Birds: Exposure at the Nest

Hamilton and Zuk (1982) proposed that bright plumage in birds evolved as an advertisement of parasite resistance in response to heavy parasite loads, and they predicted that sexual dichromatism should be strongly correlated with parasite loads across species. To test their hypothesis, we sampled 935 individuals of 19 species of passerine birds in Louisiana for the presence of blood Nest height, a variable generally not considered in most recent literature on the Hamilton-Zuk hypothesis, was as good or better a predictor of para- site prevalence as was sexual dichromatism or plumage brightness. Because certain ornitho- philic vectors are most common in the canopy, the relationship between nest height and para- site prevalence may follow from the natural history of parasitism. Ecological conditions may influence blood-parasite loads in the species studied, suggesting that genetically based resis- tance is less important. If parasite vectors are more common in the canopy, then more colorful bird species will be more heavily parasitized, on average, than less colorful species, because bird species that live high in the trees tend to be more colorful than that live closer to the ground. Received 18 July 1994, accepted 14 August 1995. HAMILTON AND ZUK (1982) predicted that bird species with strong plumage dichromatism also will have the highest prevalence of parasitic in- fection. They indicated that species with heavy parasite burdens have undergone strong selec- tion for ways to advertise relative healthiness, such as by evolution of bright plumage. They proposed that males of such species would ex- perience strong sexual selection, generating strong sexual dimorphism in plumage. Hamil- ton and Zuk's specific prediction was that those (species) with most evident sexual selec- tion are most subject to attack by debilitating parasites. Their review of data on blood para- sites in 109 species of North American passer- ines revealed that bright plumage was posi- tively associated with prevalence of blood para- sites, a finding consistent with predictions of their hypothesis. They concluded that although alternative explanations for their results could be offered, their results suggested that genetic variation for disease resistance contributes to sexual selection and evolution of bright plum- age.

The Effects of Post-Fire Management on Bird Community Succession

In September 1989 a wild fire burned over 300 ha of pine (Pinus halepensis) forest on Mt. Carmel, Israel. In winter 1990, logging took place on several burned plots. Since then, nonbreeding bird communities were censused in unburned plots, in burned plots and in burned logged plots. Canonical correspondence analysis was used to investigate the relationship between bird composition, post-fire managements and time since fire. Thirty-three species of passerine bird were recorded during fall, winter and spring (1991-1994). The most significant differences in avian community structure appeared between the unburned forest and the burned logged forest, and between the first period (1-2 years) post-fire and the following period (3-5 years post-fire). In general, the number of bird species in the ecosystem was increased as the result of imposing different post-fire managements. Therefore, creating a mosaic of different type of habitats contributed in maintaining high bird diversity.

Prey selection by sparrowhawks,Accipiter nisus: relative predation risk for breeding passerine birds in relation to their size, ecology and behaviour

Predation is a fundamental ecological process, but there are few studies of predation risk for adult, breeding birds. In this study, we quantified Relative Predation Risk (RPR) for 46 species of passerine birds preyed on by sparrowhawks during the breeding season in south-western Sweden. The sparrowhawk is a major predator of smaller birds in Eurasian forests and woodland. Near nesting hawk pairs, prey abundance was estimated by line transects. From prey remains at hawk nests, we estimated the frequency of prey species in the diet of hawks. For each hawk pair, RPR values for different prey species were calculated. The mean RPR for each prey species, based on hawk pairs as sample units, indicated degree of overrepresentation (positive RPR) or underrepresentation (negative RPR) of the prey in the diet. Prey species with a high positive RPR nested mainly near human habitations (villages or towns), at forest edges and in farmland. However, there were marked seasonal changes in RPR. Forest birds, such as the goldcrest and wren, had the highest RPR in April. Later, the hawks seemed to hunt mainly in the habitats mentioned above, where prey were more abundant or easier to catch, resulting in increasing RPR for the house sparrow, yellowhammer, greenfinch and other species. RPR values were analysed in relation to prey body size (mass) and relative density, as well as prey foraging height and nest height (categorized from the literature). In addition, the mean perch height and mean exposure of prey species were quantified in the field. Most predation was due to the smaller male hawks, providing food for their mates and young, RPR increased with prey body size up to a mass of about 40 g, then declined with increasing body size (larger prey being more difficult to catch), RPR decreased with increasing relative density of prey species. Nest height was not correlated with RPR, but foraging height seems to be an important factor: RPR decreased with increasing foraging height. A similar result was found for mean perch height, when larger prey (over 40 g) and singing individuals were excluded from the field data. RPR increased with mean exposure of prey species when larger prey and singing birds were excluded; a combination of perch height and exposure improved the correlation with RPR. Singing birds were generally perched high in the vegetation and may not suffer high predation risk, contrary to common belief. The results of this study are discussed in relation to temporal and geographical variation in RPR and antipredator adaptations in passerine birds.

Bimodal orientation and the occurrence of temporary reverse bird migration during autumn in south Scandinavia

Extensive ringing data from a coastal site (Falsterbo Bird Observatory) in southwesternmost Sweden were used to investigate the occurrence of reverse autumn migration among 20 passerine bird species of widely different migration categories. The data demonstrate that reverse migration is a widespread and regular phenomenon among nocturnal as well as diurnal migrants and among irruptive migrants, temperate zone migrants, and long-distance migrants destined for tropical winter quarters. The reoriented movements were directed approximately opposite to the normal migration direction, i.e. between NNW and ENE from the coast and towards inland. Median distances of reverse movements varied between 9 and 65 km. Some individuals of irruptive and partial migrants settled to winter in the reverse direction. Bird species with relatively small fat reserves at capture were more likely to perform reverse migratory movements than species with larger fat deposits. In two species birds performing forward migration were significantly heavier within 10 days after capture than individuals performing reverse movements. The reoriented movements probably are of adaptive significance for birds confronted with the sea and pre-disposed to refuelling during migration. A bimodal orientation mechanism will bring the birds from an area with high competition for food and high predation risk to more suitable resting and feeding grounds before resuming migration in the forward direction and crossing the barrier.