Overseas seed dispersal by migratory birds

Long-distance dispersal (LDD) promotes the colonization of isolated and remote habitats, and thus it has been proposed as a mechanism for explaining the distributions of many species. Birds are key LDD vectors for many sessile organisms such as plants, yet LDD beyond local and regional scales has never been directly observed nor quantified. By sampling birds caught while in migratory flight by GPS-tracked wild falcons, we show that migratory birds transport seeds over hundreds of kilometres and mediate dispersal from mainland to oceanic islands. Up to 1.2% of birds that reached a small island of the Canary Archipelago (Alegranza) during their migration from Europe to Sub-Saharan Africa carried seeds in their guts. The billions of birds making seasonal migrations each year may then transport millions of seeds. None of the plant species transported by the birds occurs in Alegranza and most do not occur on nearby Canary Islands, providing a direct example of the importance of environmental filters in hampering successful colonization by immigrant species. The constant propagule pressure generated by these LDD events might, nevertheless, explain the colonization of some islands. Hence, migratory birds can mediate rapid range expansion or shifts of many plant taxa and determine their distribution.     

Is the range size of migratory birds constrained by their migratory program?

Aim Intuitively, species in which the individuals migrate long distances between summer and winter quarters should be more likely to disperse and colonize new breeding areas than resident species. However, it has repeatedly been noted that many bird species with large ranges are residents. This paradox was tested on land birds breeding in the boreal forest of the Palaearctic, the largest uninterrupted stretch of habitat on earth. Methods The longitudinal distribution of two land bird communities on each side of the Eurasian continent, in Scandinavia and eastern Siberia, were used to test whether migratory birds indeed have a lower colonization success than resident birds. Results The migratory species are significantly less likely than resident species to have a range including both regions. The pattern held true even after controlling for latitudinal effects and local abundance, and was also observed at the level of genus and family. Main conclusions The relatively low colonization success of migratory species into new breeding areas may be because these new areas require novel migratory programs (migratory distance, direction and timing) in order for the birds to reach suitable wintering grounds.     

Passive internal dispersal of insect larvae by migratory birds

It has long been assumed that the resistant eggs of many zooplankton are able to survive passage through the gut of migratory waterbirds, thus facilitating their dispersal between isolated aquatic habitats. We present the first evidence that such passive internal transport within birds may be relevant for insect populations. In three out of six faecal samples from black-tailed Godwits on autumn migration in southwest Spain, we found larvae of the chironomid Chironomus salinarius which had survived gut passage. Although adult chironomids can fly, they are likely to disperse greater distances when transported as larvae via birds. In insects with discrete generations, such passive transport also enables colonization of new habitats at times when flight by adults is not an option.     

Optimal moult strategies in migratory birds

Avian migration, which involves billions of birds flying vast distances, is known to influence all aspects of avian life. Here we investigate how birds fit moult into an annual cycle determined by the need to migrate. Large variation exists in moulting patterns in relation to migration: for instance, moult can occur after breeding in the summer or after arrival in the wintering quarters. Here we use an optimal annual routine model to investigate why this variation exists. The modelled bird's decisions depend on the time of year, its energy reserves, breeding status, experience, flight feather quality and location. Our results suggest that the temporal and spatial variations in food are an important influence on a migratory bird's annual cycle. Summer moult occurs when food has a high peak on the breeding site in the summer, but it is less seasonal elsewhere. Winter moult occurs if there is a short period of high food availability in summer and a strong winter peak at different locations (i.e. the food is very seasonal but in opposite phase on these areas). This finding might explain why only long-distance migrants have a winter moult.     

Radionuclide transfer to reptiles

Reptiles are an important, and often protected, component of many ecosystems but have rarely been fully considered within ecological risk assessments (ERA) due to a paucity of data on contaminant uptake and effects. This paper presents a meta-analysis of literature-derived environmental media (soil and water) to whole-body concentration ratios (CRs) for predicting the transfer of 35 elements (Am, As, B, Ba, Ca, Cd, Ce, Cm, Co, Cr, Cs, Cu, Fe, Hg, K, La, Mg, Mn, Mo, Na, Ni, Pb, Po, Pu, Ra, Rb, Sb, Se, Sr, Th, U, V, Y, Zn, Zr) to reptiles in freshwater ecosystems and 15 elements (Am, C, Cs, Cu, K, Mn, Ni, Pb, Po, Pu, Sr, Tc, Th, U, Zn) to reptiles in terrestrial ecosystems. These reptile CRs are compared with CRs for other vertebrate groups. Tissue distribution data are also presented along with data on the fractional mass of bone, kidney, liver and muscle in reptiles. Although the data were originally collected for use in radiation dose assessments, many of the CR data presented in this paper will also be useful for chemical ERA and for the assessments of dietary transfer in humans for whom reptiles constitute an important component of the diet, such as in Australian aboriginal communities.     

Orientation of migratory birds under ultraviolet light

In view of the finding that cryptochrome 1a, the putative receptor molecule for the avian magnetic compass, is restricted to the ultraviolet single cones in European Robins, we studied the orientation behaviour of robins and Australian Silvereyes under monochromatic ultraviolet (UV) light. At low intensity UV light of 0.3 mW/m², birds showed normal migratory orientation by their inclination compass, with the directional information originating in radical pair processes in the eye. At 2.8 mW/m², robins showed an axial preference in the east–west axis, whereas silvereyes preferred an easterly direction. At 5.7 mW/m², robins changed direction to a north–south axis. When UV light was combined with yellow light, robins showed easterly ‘fixed direction’ responses, which changed to disorientation when their upper beak was locally anaesthetised with xylocaine, indicating that they were controlled by the magnetite-based receptors in the beak. Orientation under UV light thus appears to be similar to that observed under blue, turquoise and green light, albeit the UV responses occur at lower light levels, probably because of the greater light sensitivity of the UV cones. The orientation under UV light and green light suggests that at least at the level of the retina, magnetoreception and vision are largely independent of each other.     

The scheduling of molt in migratory birds

Summary We model the yearly cycle of small migratory birds to explain the variation in scheduling of complete molt, in particular why some birds molt immediately after breeding on the breeding grounds (summer molt) whereas others migrate to their wintering grounds before molt is initiated (winter molt). We employ the method of dynamic programming, because of its suitability for modelling life history traits. Feather quality and latitude entered the model as state variables and were assumed to affect survival rate and reproductive success. Migration and molt were assumed to be associated with increased mortality risks. By changing the parameters in the model we were able to generate most existing molt patterns, including summer and winter molt, biannual (summer and winter) molt, and molt migration. Our model suggests that the scheduling of molt is basically a result of a trade-off between having a high feather quality during breeding versus during the non-breeding period. A high impact of feather quality on survival rate in combination with low costs of molt resulted in biannual molt. Winter molt became more likely as the survival rateper se increased. A low seasonal amplitude in survival rate is a prerequisite for the occurrence of molt migration. Molt duration, migration costs and reproductive successper se were found to have no impact on the timing of molt. We also investigated the effect of benefits from prior occupancy at breeding and winter grounds.     

Seasonal climatic niches diverge in migratory birds

Migratory birds occupy different geographical areas during breeding and non-breeding periods, and thus different factors may determine their range limits depending on each season. One such factor is the spatial climatic component of the niche, which is widely used to model species distributions, yet the temporal component is often neglected and is generally assumed to be constant. We tested the hypothesis that the climatic niche is conserved between breeding and non-breeding areas in 355 bird species migrating through Eurasian–African flyways. For this, we performed niche overlap analyses and compared niche differences between sister or phylogenetically closely related species, as well as linking the differences to migratory distances. For more than 80% of the species, there was no or very little overlap between their breeding and non-breeding climatic niches. For most closely related species, the degree of overlap of their breeding climatic niches was larger than the overlap observed within each species, but not for their wintering climatic niches, suggesting a phylogenetic conservation of breeding climatic niches. Finally, there was a clear negative relationship between migratory distances and climatic niche overlap within each species. Our results confirmed that the climatic niche of most Eurasian–African migratory species differs between both breeding and non-breeding ranges, suggesting distinctive seasonal climatic requirements. Given these results and the geographically uneven effects of climate change, the impact of global change is likely to have different effects in each seasonal range. Hence, both breeding and non-breeding climatic data need to be considered when using species distribution models.     

Species richness of migratory birds is influenced by global climate change

Aim  Global climate change is increasingly influencing ecosystems. Long-term effects on the species richness and composition of ecological communities have been predicted using modelling approaches but, so far, hardly demonstrated in the field. Here, we test whether changes in the composition of bird communities have been influenced by recent climate change.
Location  Europe.
Methods  We focus on the proportion of migratory and resident bird species because these groups are expected to respond differently to climatic change. We used the spatial relationship between climatic factors and bird communities in Europe to predict changes in 21 European bird communities under recent climate change.
Results  Observed changes corresponded significantly to predicted changes and could not be explained by the effects of spatial autocorrelation. Alternative factors such as changes in land use were tested in a first approximation as well but no effects were found.
Main conclusions  This study demonstrates that global climate change has already influenced the species richness and composition of European bird communities.     

The potential for dispersal of microalgal resting cysts by migratory birds

Most microalgal species are geographically widespread, but little is known about how they are dispersed. One potential mechanism for long‐distance dispersal is through birds, which may transport cells internally (endozoochory) and deposit them during, or in‐between, their migratory stopovers. We hypothesize that dinoflagellates, in particular resting stages, can tolerate bird digestion; that bird temperature, acidity, and retention time negatively affect dinoflagellate viability; and that recovered cysts can germinate after passage through the birds’ gut, contributing to species‐specific dispersal of the dinoflagellates across scales. Tolerance of two dinoflagellate species (Peridiniopsis borgei, a warm‐water species and Apocalathium malmogiense, a cold‐water species) to Mallard gut passage was investigated using in vitro experiments simulating the gizzard and caeca conditions. The effect of in vitro digestion and retention time on cell integrity, cell viability, and germination capacity of the dinoflagellate species was examined targeting both their vegetative and resting stages. Resting stages (cysts) of both species were able to survive simulated bird gut passage, even if their survival rate and germination were negatively affected by exposure to acidic condition and bird internal temperature. Cysts of A. malmogiense were more sensitive than P. borgei to treatments and to the presence of digestive enzymes. Vegetative cells did not survive conditions of bird internal temperature and formed pellicle cysts when exposed to gizzard‐like acid conditions. We show that dinoflagellate resting cysts serve as dispersal propagules through migratory birds. Assuming a retention time of viable cysts of 2–12 h to duck stomach conditions, cysts could be dispersed 150–800 km and beyond.     

Patterns of phenological changes in migratory birds

The phenology of avian migration appears to be changing in response to climate change. Seemingly contradictory differences in the timing of these annual cycles have been reported in published studies. We show that differences between studies in the choice of songbird species, as well as in the measurements of migration phenology, can explain most of the reported differences. Furthermore, while earlier spring arrival is evident across these studies, trends in timing of departure show large variation between species and according to individual timing of migration (early-arriving vs. late-departing individuals). Much of the variation in departure between species could be explained by each species’ migratory status. We present a detailed analysis of migrants recorded at a Danish migration site, and reveal that although shifts in migration timing can be demonstrated for almost all species, these shifts are either most pronounced in the early arriving/late departing individuals or the changes are similar. Thus most individuals do not seem to change their breeding-area residence time (BART). As BART is likely to reflect ecologically important factors, e.g. number of clutches, we expect that only small effects have been exerted on the breeding ecology of the studied species in the time period investigated. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Genetic control of migratory behaviour in birds

Although it has long been suspected that biannual migration in birds has a direct genetic basis, only in the last decade have details of the inheritance of behavioural traits such as migratory activity and directional preferences been demonstrated. A model has now been developed to estimate how inexperienced first-time migrants manage to reach their unknown winter quarters on the basis of inherited spatio-temporal programs. Furthermore, in obligate partial migrants the decision to migrate or not has been shown to have a strong genetic base. Migratoriness and sedentariness in partial migrants have been shown to have a high potential for rapid evolution. A recent set of results has suggested that novel migratory habits can evolve in less than 25 years. A possible consequence is that environmental changes, including 'greenhouse' effects, might considerably alter avian migration systems by acting on genetic variation for migratory tendencies.     

Macroecology of habitat choice in long-distance migratory birds

Patterns of habitat choice in ecological communities are not only influenced by present-day selective forces but also by historical processes, such as the biogeographical history of the lineages they are composed of. Nevertheless, it has been very difficult to test historical factors. The possible tropical origin of long-distance migratory birds provides an opportunity for such a test. If habitat choice of long-distance migrants is inherited from their tropical ancestors then Nearctic long-distance migrants might have acquired their habitat choice from Neotropical forest species and European long-distance migrants from African savannah species. Here we use a macroecological approach to show that this hypothesis can be confirmed. Long-distance migrants in the Nearctic are found in forested habitat types, while those in Europe are found in open ones. In comparison, the habitat choice of residents and short-distance migrants (in genera without long-distance migration) does not differ between the Nearctic and Europe. These results demonstrate that habitat choice in temperate bird communities can be explained by the tropical history of long-distance migrants. Thus, habitat choice seems to be shaped not only by local mechanisms, but also by processes acting on much larger spatial and temporal scales.     

Protandry and sexual dimorphism in trans-Saharan migratory birds

Earlier arrival to reproductive sites of males relative to females(protandry) is widespread among migratory organisms. Diversemechanisms have been proposed that may select for protandry,including competition for limiting resources (e.g., territories)or mates. In species with large variation in male reproductivesuccess, such as polygamous species and those with intense spermcompetition, early arriving males may accrue a fitness advantagebecause they acquire more mates or have larger chances of paternity.Comparative studies of birds have shown that sexual size dimorphism(SSD) is positively associated with the level of polygyny, whereasintense sperm competition is associated with sexual dichromatism(SD). Positive correlations between protandry and SSD or SDcan therefore be expected to exist across avian species. Becauselarge males are predicted to be better able to cope with adverseecological conditions early in the breeding season, selectionfor protandry, in turn, may have a correlated response on SSDamong migratory species breeding in boreal latitudes. Althoughprevious studies of birds have analyzed the association betweenSSD and protandry, none has analyzed SD in relation to protandry.Here we analyze the association between protandry during springmigration, SSD, and SD in 21 trans-Saharan monogamous migratorybird species. The difference in median migration dates betweenfemales and males, reflecting protandry, was positively associatedwith SD but not with SSD. Because dichromatism is positivelyrelated to sperm competition across species, present resultsare consistent with predictions derived from sexual selectionhypotheses for the evolution of protandry mediated by spermcompetition.     

Examining the total arrival distribution of migratory birds

This paper reports on the total distribution of spring migration timing of willow warbler, chiffchaff and pied flycatcher at locations in the UK, Germany, Russia and Finland. This is the first time that high‐quality data based on known‐effort monitoring has been examined on a continental scale. First arrival dates, commonly reported in the literature, were positively correlated with mean arrival dates although they would not make good predictors of the latter. At all locations, at least one aspect of the arrival distribution of each species had got significantly earlier in recent years. The trend towards earliness was associated with warmer local temperatures and more positive winter North Atlantic Oscillation index. In years that were early, the arrival distribution became more elongated and skewed. Researchers should now investigate the consequences of earlier arrival on current and future bird populations.     

Orientation cues used by migratory birds: A review of cue-conflict experiments

During the late 1960s and early 1970s the accumulating evidence of magnetic orientation forced the conclusion that the orientation of migratory birds and homing pigeons is based upon multiple stimuli. 'Cue-conflict experiments' have provided a powerful means of asking how these directional cues relate one to another. The weight of evidence suggests that in short-term orientation decision making, magnetic cues take precedence over stars, and visual information at sunset overrides both these stimuli. Recent experiments point to polarized skylight patterns as the relevant cue in dusk orientation. Although cue-conflict experiments have now been performed on a diversity of species, generalizations are weakened because of differences in experimental design, in the cues examined and in our ability to manipulate those cues. There remains a need for carefully designed comparative studies.     

Modelling the potential spread of Hyalomma marginatum ticks in Europe by migratory birds

This study modelled the probability of introduction of Hyalomma marginatum into Europe by predicting the potential migratory routes of 28 bird species and the probability to carry immatures of the tick. Flyways were modelled as a spatio-temporal feature, at weekly intervals, using satellite-derived data of temperature and vegetal phenology, together with cost surfaces derived from speed and direction of the wind (years 2002–2018). The expected period of activity of tick immatures defined the probability of ticks being carried by birds along the modelled flyways. The probability of moulting of the engorged nymphs was modelled as a linear relationship of the daily sum of temperatures after tick introduction by birds. Positive probabilities of tick introduction extend the known northern range of permanent populations to central and western France, and large portions of central Europe. The flight of birds into an area and thence the risk of introduction of H. marginatum is very heterogeneous, with sites receiving “waves” of different bird species at diverse times of the year. Therefore, there is not a clear period of time for introduction, as it depends on the modelled behaviour of the bird species. The probability of introduction into Baltic and Nordic countries is small. We hypothesise that conditions of a warmer climate might support permanent populations of H. marginatum if a high number of immatures is introduced. Active surveys in risky territories, where the tick is not yet established, are advisable for rapid intervention.     

Climatic patterns in the establishment of wintering areas by North American migratory birds

Long‐distance migration in birds is relatively well studied in nature; however, one aspect of this phenomenon that remains poorly understood is the pattern of distribution presented by species during arrival to and establishment of wintering areas. Some studies suggest that the selection of areas in winter is somehow determined by climate, given its influence on both the distribution of bird species and their resources. We analyzed whether different migrant passerine species of North America present climatic preferences during arrival to and departure from their wintering areas. We used ecological niche modeling to generate monthly potential climatic distributions for 13 migratory bird species during the winter season by combining the locations recorded per month with four environmental layers. We calculated monthly coefficients of climate variation and then compared two GLM (generalized linear models), evaluated with the AIC (Akaike information criterion), to describe how these coefficients varied over the course of the season, as a measure of the patterns of establishment in the wintering areas. For 11 species, the sites show nonlinear patterns of variation in climatic preferences, with low coefficients of variation at the beginning and end of the season and higher values found in the intermediate months. The remaining two species analyzed showed a different climatic pattern of selective establishment of wintering areas, probably due to taxonomic discrepancy, which would affect their modeled winter distribution. Patterns of establishment of wintering areas in the species showed a climatic preference at the macroscale, suggesting that individuals of several species actively select wintering areas that meet specific climatic conditions. This probably gives them an advantage over the winter and during the return to breeding areas. As these areas become full of migrants, alternative suboptimal sites are occupied. Nonrandom winter area selection may also have consequences for the conservation of migratory bird species, particularly under a scenario of climate change.