The problem of estimating wind drift in migrating birds

Whether migrating birds compensate for wind drift or not is a fundamental question in bird migration research. The procedures to demonstrate and quantitatively estimate wind drift or compensation are fraught with difficulties and pitfalls. In this paper, we evaluate four methods that have been used in several studies over the past decades. We evaluate the methods by analysing a model migratory movement with a realistic scatter in flight directions, for the ideal cases of full drift and complete compensation. Results obtained with the different methods are then compared with the "true behaviour" of the model movement, illustrating that spurious patterns of drift and compensation arise in some cases. We also illustrate and evaluate the different methods of estimating drift for a real case, based on tracking radar measurements of bird migration in relation to winds. Calculating the linear regression of mean geographic track (resulting flight direction) and heading directions (directions of the birds' body axis) of a migratory movement under different wind conditions in relation to the angle alpha (the angle between mean track and heading) always provides robust and reliable results. Comparing mean flight directions between occasions with winds from the left and right of the mean flight direction of the whole migratory movement also always provides expected and correct measures of drift. In contrast, regressions of individual flight directions in relation to alpha (the angle between track and heading for the specific individuals or flocks) are liable to produce biased and spurious results, overestimating compensation/overcompensation if following winds dominate in the analysis and overestimating drift/overdrift if opposed winds are dominating. Comparing mean directions for cases with winds from the left and right in relation to individual flight directions also gives biased and spurious results unless there is full variation in wind directions or an equal distribution of crosswinds from left and right. The results of the methodological evaluation and the analysis of the real case indicate that some earlier analyses of wind drift may have to be re-evaluated.     

Bird orientation: compensation for wind drift in migrating raptors is age dependent

Despite the potentially strong effect of wind on bird orientation, our understanding of how wind drift affects migrating birds is still very limited. Using data from satellite-based radio telemetry, we analysed the effect of changing winds on the variation of the track direction of individual birds. We studied adults and juveniles of two raptor species, osprey Pandion haliaetus and honey buzzard Pernis apivorus, on autumn migration between North Europe and Africa, and demonstrate an important difference between the age categories of both species in the extent of wind drift. For juveniles, side- and following-wind components affected the rates of movement perpendicular to and along the mean direction, respectively, to a similar degree, suggesting full wind drift. By contrast, for adults the rate of crosswind displacement was significantly smaller than the effect of wind on forward movement, showing much reduced wind drift (29%). This indicates that adults have acquired a more sophisticated orientation system, permitting detection of and compensation for wind drift, than juveniles. These drift effects are likely to reduce the ability of juveniles to locate species-specific wintering areas in case of rapid climatic wind change.     

Wind selectivity and partial compensation for wind drift among nocturnally migrating passerines

A migrating bird's response to wind can impact its timing, energy expenditure, and path taken. The extent to which nocturnal migrants select departure nights based on wind (wind selectivity) and compensate for wind drift remains unclear. In this paper, we determine the effect of wind selectivity and partial drift compensation on the probability of successfully arriving at a destination area and on overall migration speed. To do so, we developed an individual-based model (IBM) to simulate full drift and partial compensation migration of juvenile Willow Warblers (Phylloscopus trochilus) along the southwesterly (SW) European migration corridor to the Iberian coast. Various degrees of wind selectivity were tested according to how large a drift angle and transport cost (mechanical energy per unit distance) individuals were willing to tolerate on departure after dusk. In order to assess model results, we used radar measurements of nocturnal migration to estimate the wind selectivity and proportional drift among passerines flying in SW directions. Migration speeds in the IBM were highest for partial compensation populations tolerating at least 25% extra transport cost compared to windless conditions, which allowed more frequent departure opportunities. Drift tolerance affected migration speeds only weakly, whereas arrival probabilities were highest with drift tolerances below 20°. The radar measurements were indicative of low drift tolerance, 25% extra transport cost tolerance and partial compensation. We conclude that along migration corridors with generally nonsupportive winds, juvenile passerines should not strictly select supportive winds but partially compensate for drift to increase their chances for timely and accurate arrival.     

A place to land: spatiotemporal drivers of stopover habitat use by migrating birds

Migrating birds require en route habitats to rest and refuel. Yet, habitat use has never been integrated with passage to understand the factors that determine where and when birds stopover during spring and autumn migration. Here, we introduce the stopover‐to‐passage ratio (SPR), the percentage of passage migrants that stop in an area, and use 8 years of data from 12 weather surveillance radars to estimate over 50% SPR during spring and autumn through the Gulf of Mexico and Atlantic coasts of the south‐eastern US, the most prominent corridor for North America’s migratory birds. During stopovers, birds concentrated close to the coast during spring and inland in forested landscapes during autumn, suggesting seasonal differences in habitat function and highlighting the vital role of stopover habitats in sustaining migratory communities. Beyond advancing understanding of migration ecology, SPR will facilitate conservation through identification of sites that are disproportionally selected for stopover by migrating birds.     

Traveling or stopping of migrating birds in relation to wind: an illustration for the osprey

Although it is often assumed that birds strongly prefer tailwindsfor their migratory flights, we predict that a strategy of nowind selectivity (traveling independently of winds) may be morefavorable than wind selectivity (traveling on tailwind occasionsbut stopping to rest under headwind occasions) for birds withlow energy costs of travel relative to rest and for birds thatcannot use stopover time for efficient fuel deposition. We testthis prediction by analyzing the daily traveling or stoppingas recorded by satellite tracking of five ospreys Pandion haliaetus,a species often using energy-saving thermal soaring, duringtheir migration between northern Europe and Africa. Besideswind, precipitation is another weather factor included in theanalyses because thermal soaring migrants are expected to stopand rest in rainy weather. In logistic regression analyses,taking into account the effects of latitude, behavior on previousday, season, date, and individual for discriminating betweentraveling and stopping days, we found a lack of influence ofwinds, suggesting that the ospreys travel or stop without regardto wind. This lack of wind selectivity under light and moderatewinds is in agreement with our prediction. We expect a low degreeof wind selectivity and thus regular flights under headwindsalso among other types of birds that cannot use stopping timefor efficient foraging and fuel deposition. We also found anunexpected lack of influence of precipitation, possibly becauseof relatively few instances with rainfall in combination withpoor geographic precision for estimates of this weather variable.     

Orientation and navigation of migrating birds

The question of how migrating birds find their way to winter quarters and back has fascinated humans since the beginning of scientific research into avian biology. Migrating birds have been shown to possess compass systems that allow them to select and maintain certain compass directions. Three such systems are known, solar, stellar and magnetic. Their details are not quite clear and need further research. Hierarchy and interaction of compass systems of migrating birds are poorly studied; different species may vary in this respect. During migration, birds learn to use maps that make true navigation possible, i.e. to detect their position relatively to the goal of movement. The physical nature of navigational maps is an object of intensive research; currently the most promising concepts are the geomagnetic and possibly olfactory maps. A significant contribution to the study of formation of navigational maps was made by Soviet/Russian researchers, whose work was published in Zoologicheskii Zhurnal (Sokolov et al., 1984). Migrating birds have no innate map, and first-autumn individuals reach their species-specific wintering areas by using compass sense and counting time that should be spent moving in certain genetically fixed directions. However, in recent years more and more data surface that suggest that juveniles (maybe not of all species) do have some mechanism of controlling their position on the migratory route that allows them to compensate for errors of the spatio-temporal programme of migration.     

Routes of migrating soaring birds

Soaring migrants travelling through Israel use three principal routes which are used in the opposite directions during the spring and autumn: (1) the Western Route lies mainly along the western edge of the central mountain range, (2) the Eastern Route lies mainly along the Jordan Valley, crossing the mountain range during part of the day, continuing southward along the Dead Sea towards the Sinai, and joining the Western Route in autumn and (3) the Southern-Elat Mountains Route. The geomorphological structure of Israel, with a central mountain range dividing the country roughly into three landscape units, plays a central role in route selection. In the autumn, the Western Route migration axis is deflected at the beginning of the day from east to west for 10–25 km, depending on weather conditions and the flock's roosting locations. Between 10.00 h and 11.00 h, the daily breeze blowing from the Mediterranean Sea influences the migration axis, which is slowly deflected back to the east. A parallel deflection of the migration axis occurs in the Eastern Route in the autumn. The route moves southwest over the eastern slopes of the central mountain range during the morning hours and over the slope, which absorbs direct radiation from the sun, creating good soaring conditions. Towards late afternoon, when the breeze from the sea starts, the axis is deflected to the east, to the Jordan Valley. In the Elat Mountains, the wind flow plays a similar role, but because the topography of the southern Arava Valley causes a change in wind direction, the axis moves during the day in a north-south direction. In addition to the axis movement on a daily scale, a seasonal deflection of the migration axis from east to west also exists. During autumn migration, early migrants (e.g. White Storks Ciconia ciconia) tend to travel on an eastern route, while late migrants (e.g. White Pelican Pelecanus onocrotalus) travel along the Mediterranean coast. This fluctuation was probably because of sub-optimal soaring conditions along the coastal plain during August. In September, temperature differences between the sea and land decrease and the influence of the marine inversion gradually declines, until its influence disappears completely in October. A comparison of the numbers of soaring birds seen over Israel in the autumn and spring shows significant seasonal differences in the use of the various routes. For example, only one species, the Steppe Eagle Aquila nipalensis, flies over the Elat Mountains in the autumn, compared to more than 30 species in the spring. In the autumn, White Storks pass over only along the Jordan Valley axis, whereas in the spring, about half the migrating storks also pass over the western edge of the central mountain range. Honey Buzzards Pernis apivorus fly along the Western Route in large numbers in the autumn, while concentrating almost totally over the Elat Mountains in the spring. These differences are related to the global migration routes between the breeding and the wintering grounds in relation to the Red Sea, which birds avoid crossing, thus causing them to follow different routes in autumn, and spring.     

Artificial light at night confounds broad‐scale habitat use by migrating birds

With many of the world's migratory bird populations in alarming decline, broad‐scale assessments of responses to migratory hazards may prove crucial to successful conservation efforts. Most birds migrate at night through increasingly light‐polluted skies. Bright light sources can attract airborne migrants and lead to collisions with structures, but might also influence selection of migratory stopover habitat and thereby acquisition of food resources. We demonstrate, using multi‐year weather radar measurements of nocturnal migrants across the northeastern U.S., that autumnal migrant stopover density increased at regional scales with proximity to the brightest areas, but decreased within a few kilometers of brightly‐lit sources. This finding implies broad‐scale attraction to artificial light while airborne, impeding selection for extensive forest habitat. Given that high‐quality stopover habitat is critical to successful migration, and hindrances during migration can decrease fitness, artificial lights present a potentially heightened conservation concern for migratory bird populations.     

Optimal stopover decisions of migrating birds under variable stopover quality: model predictions and the field data

Dataset on departure fuel loads, stopover length and fuel deposition rate of the European robins Erithacus rubecula during their migration in the Baltic area is presented. We test these empirical data against the predictions of an optimal migration model assuming that robins minimize time spent on migration, and that fuel deposition rate varies stochastically. The latter assumption sets this model apart from the alternative ones and makes it more realistic. In particular, it is applicable in frequently observed situations when fuel deposition rate is negative. Our model assumes stochastic variation of the fuel deposition rate at sites along the migratory rout and thus is applicable when negative values of fuel deposition rate are recorded. The model predicts the relationship between fuel deposition rate and departure fuel load rather well. The agreement between the observed and the predicted values of optimal stopover duration is much poorer. Predictions of optimal migration theory are known to be dependent on the form of flight equation chosen. Our model fits the data best when the costs of transport are low. This supports the idea that transport costs of fuel stores may be low, especially when fuel stores are modest.     

The audibility of frog choruses to migrating birds

Breeding choruses of Hyla crucifer and H. versicolor are loud enough to be audible to migrating birds up to at least 1 km from their source, both vertically and horizontally, provided that no large obstacles intervene. During May in south-eastern New York State sound pressure levels (A weighting) at altitudes of 200 to 965 m and slant ranges from the frogs of 225 to 1020 m varied from 28 to 52 dB SPL.     

Long-distance migrating species of birds travel in larger groups

How individuals migrate over long distances is an enduring mystery of animal migration. Strong selection pressure for travelling in groups has been suggested in long-distance migrating species. Travelling in groups can reduce the energetic demands of long migration, increase navigational accuracy and favour group foraging at migratory halts. Nevertheless, this hypothesis has received scant attention. I examined evolutionary transitions in migration distance in all North American breeding species of birds. I documented 72 evolutionary shifts in migration distance in the pool of 409 species. In contrasting clades, long-distance migration, as opposed to short-distance migration, was associated with a larger travelling group size. No other transitions occurred alongside in other traits such as group size in the non-breeding season or body mass. The results suggest that larger group sizes have been beneficial in the evolution of long-distance migration in a large clade of birds.     

Effect of endurance flight on haematocrit in migrating birds

The effects of an endurance flight on the haematocrit, the percentage of packed red blood cells per blood volume, were examined within the framework of six possible factors explaining possible changes in the haematocrit. Two approaches were adopted: (1) the haematocrit was studied in four species of passerine birds which landed on an Italian island after having crossed the Mediterranean Sea on their spring migration in a non-stop flight; (2) the haematocrit was evaluated in six individual red knots after a flight of 1, 2, 4 and 10 h in a wind tunnel and the data thus obtained compared with data on resting birds with or without food. In the four passerine species, the haematocrit decreased from 51% in fat birds to 48% in lean birds. In lean birds, the haematocrit dropped from 48% in birds with well-developed breast muscles to 36% in birds with emaciated breast muscles. In the red knots, the haematocrit was dependent on body mass in flying and resting birds. The haematocrit decreased from about 51% pre-flight to about 49% within 1 h of flight and remained at this level for up to 10 h of flight. Taking the results from the passerines and the red knots together, it seems that the haematocrit drops by a few percentage points within 1 h after the onset of flight, decreases very slowly with decreasing body mass and decreases more steeply in very lean birds having entered stage III of fasting. This indicates that dehydration is not an underlying factor in decreased haematocrit because if this were the case we would expect an increase with endurance flight. We found no effect of the presence of blood parasites on haematocrit. With the onset of flight, haemodilution may be adaptive, because it reduces blood viscosity and, thereby, energy expenditure by the heart, or it may be a sign of water conservation as an insurance against the risk of dehydration during long non-stop flights. During endurance flight, a reduction in the haematocrit may be adaptive, in that oxygen delivery capacity is adjusted to the decreased oxygen needs as body mass decreases. A decreasing haematocrit would also allow birds to reduce heart beat frequency and/or heart size, because blood viscosity decreases disproportionally with decreasing haematocrit. However, when energy stores are about to come to an end and birds increase protein breakdown, the haematocrit decreases even further, and birds probably become anaemic due to a reduced erythropoiesis.     

Habitat use by seed-eating birds: a scale-dependent approach

The seedbank in arable farmland represents an important foraging resource for birds, particularly in grassland landscapes where alternative foraging opportunities may be scarce. We used a stepwise approach to examine the importance of seed food resources for farmland birds in winter. First, results from a large-scale experiment in which seed resources were manipulated to test the notion that birds aggregate at food resources, subject to a minimum threshold level. Secondly, a multiscale approach was used to characterize habitat use at a landscape scale and how this may inform agri-environment implementation. Overall seed resources declined sharply over the winter and were relatively low in most arable fields. Large-scale declines in the arable seedbank mean that much habitat may not be of sufficient quality to support foraging bird populations through a winter. At a landscape scale, extensive analyses of breeding season abundance show that bird abundance is most influenced by arable (i.e. seed-rich) habitat in grassland landscapes. The scale at which birds respond to habitat differs between species, and is related to the extent of between-season movements. Implementation of agri-environment schemes will need to consider both the quality of habitat and the context in which it is provided if recent declines in farmland bird populations are to be reversed.     

Mass changes in migrating birds: the evidence for fat and protein storage re-examined

The fact that one cannot kill a bird twice makes it very difficult to determine the relative contributions of fat and non-fat components to increases in body mass before migratory flights in individual birds. Knowing the relative contributions of these components is of obvious energetic interest since fat yields about eight times as much energy as fat-free muscle tissue. Several recent studies have failed to demonstrate convincingly, due to flaws in their analyses, that fat-free mass in addition to fat is accumulated before long-distance flights. We point out that regressions of fat or the non-fat component on total body mass cannot yield reliable estimates of the composition of individual mass changes in view of inter-individual variation in structural size, reserve levels or timing of storage. We suggest that studies over time of synchronous populations or marked individuals will give better answers. A re-analysis of published data indicates the widespread existence of fat-free tissue deposition during migration, whereas in some species fat alone explained the increase in total body mass. Larger species tend to incorporate a relatively higher proportion of non-fat components when increasing in mass. However, the comparative data set is not yet of sufficient quality to allow general statements on why, and to what extent in different individuals and species, non-fat tissue in addition to fat is deposited before take-off on migratory flights.     

Ticks (Acari: Ixodidae) parasitizing migrating and local breeding birds in Finland

Ticks are globally renowned vectors for numerous zoonoses, and birds have been identified as important hosts for several species of hard ticks (Acari: Ixodidae) and tick-borne pathogens. Many European bird species overwinter in Africa and Western Asia, consequently migrating back to breeding grounds in Europe in the spring. During these spring migrations, birds may transport exotic tick species (and associated pathogens) to areas outside their typical distribution ranges. In Finland, very few studies have been conducted regarding ticks parasitizing migrating or local birds, and existing data are outdated, likely not reflecting the current situation. Consequently, in 2018, we asked volunteer bird ringers to collect ticks from migrating and local birds, to update current knowledge on ticks found parasitizing birds in Finland. In total 430 ticks were collected from 193 birds belonging to 32 species, caught for ringing between 2018 and 2020. Furthermore, four Ixodes uriae were collected from two roosting islets of sea birds in 2016 and 2020. Ticks collected on birds consisted of: Ixodes ricinus (n?=?421), Ixodes arboricola (4), Ixodes lividus (2) and Hyalomma marginatum (3). Ixodes ricinus loads (nymphs and larvae) were highest on thrushes (Passeriformes: Turdidae) and European robins (Erithacus rubecula). The only clearly imported exotic tick species was H. marginatum. This study forms the second report of both I. uriae and I. arboricola from Finland, and possibly the northernmost observation of I. arboricola from Europe. The importation of exotic tick species by migrating birds seems a rare occurrence, as over 97% of all ticks collected from birds arriving in Finland during their spring migrations were I. ricinus, a species native to and abundant in Finland.

     

Lyme disease and migrating birds in the Saint Croix River Valley

During a study of migrating land birds in 1987, we examined over 9,200 individual birds representing 99 species from the Saint Croix River Valley, a Lyme disease-endemic area of east central Minnesota and northwestern Wisconsin. We found that 250 deer tick (Ixodes dammini) larvae and nymphs infested 58 birds from 15 migrant species; 56 ticks (22.4%) were positive for the Lyme disease spirochete Borrelia burgdorferi. Five ground-foraging migrant bird species favoring mesic habitats, veery (Catharus fuscescens), ovenbird (Seiurus aurocapillus), northern waterthrush (S. novaboracensis), common yellowthroat (Geothlypis trichas), and swamp sparrow (Melospiza georgiana), accounted for nearly three-quarters of parasitized individuals. Nearly half of the spirochete-positive ticks were removed from migrating birds taken in a riparian floodplain forest. Recaptured migrants with infected ticks indicate that they transmit B. burgdorferi to hexapod larvae. We suggest that birds may be both an important local reservoir in the upper Mississippi Valley and long-distance dispersal agents for B. burgdorferi-infected ticks to other regions of the continent.     

Flower power: tree flowering phenology as a settlement cue for migrating birds

1. Neotropical migrant birds show a clear preference for stopover habitats with ample food supplies; yet, the proximate cues underlying these decisions remain unclear. 2. For insectivorous migrants, cues associated with vegetative phenology (e.g. flowering, leaf flush, and leaf loss) may reliably predict the availability of herbivorous arthropods. Here we examined whether migrants use the phenology of five tree species to choose stopover locations, and whether phenology accurately predicts food availability. 3. Using a combination of experimental and observational evidence, we show migrant populations closely track tree phenology, particularly the flowering phenology of honey mesquite (Prosopis glandulosa), and preferentially forage in trees with more flowers. Furthermore, the flowering phenology of honey mesquite reliably predicts overall arthropod abundance as well as the arthropods preferred by migrants for food. 4. Together, these results suggest that honey mesquite flowering phenology is an important cue used by migrants to assess food availability quickly and reliably, while in transit during spring migration.