Some aspects of frugivory by bird populations using coastal dune scrub in Lincolnshire

The fruit diets of Sylvia warblers, and some thrushes, were studied during 1989 at a coastal scrubland site, using faecal examination. Fruit feeding commenced as the earliest berries ripened in late July, but there were differences of timing between species. Amongst both warblers and thrushes, the larger species commenced earlier and ate more fruit. Almost all Blackcap and Garden Warbler faeces examined from early August contained some fruit remains, as did most Whitethroat faeces from mid-month. Mean fruit contents of faecal samples exceeded 80% infrequently, indicating that even the most frugivorous species always took other food, normally insects, to supplement their diet. Each species differed in their fruit diet: Blackcaps, and to a lesser extent Garden Warblers, ate Woody Nightshade berries in preference to Blackberries, as they are structurally adapted for feeding on the former plant. Only inexperienced juveniles of other species regularly chose the Nightshade berries. Elder berries, abundant from late August, became the preferred fruit for all Sylvia species. Using data from an earlier investigation, 1 it is shown that by feeding on Elder berries, migrating Blackcaps can obtain 75–90% of their daily energy requirements in 10% of daylight hours.     

Recognizability of different food types in faeces and in alimentary flushes of Sylvia warblers

The efficiency of the analysis of faeces and flushes of the digestive tract was examined by feeding captive Garden Warblers Sylvia borin and Blackcaps S. atricapilla with different food types (arthropods, fruits). Faecal analysis underestimated aphids; the analysis of flushes overestimated beetles and underestimated aphids and fruits. Correction factors to remove these biases are given. Flushing free-living birds showed no noticeable effect on recapture rates and body mass. Methods of calculating the number of prey items from different fragments are evaluated.     

Fruit size in wild olives: implications for avian seed dispersal

1. The response of frugivorous birds to an enlargement of fruit size, and the consequences for both birds and plants, are analysed for the interaction between avian seed dispersers and olives ( Olea europaea) .
2. The enlargement of fruit size promotes a shift in frugivorous birds' feeding behaviour, from swallowing fruits whole to pecking pieces of pulp. The relative frequency of olive consumption using each feeding behaviour was assessed by combining field data on frequency of appearance of olive pulp and seeds with data from laboratory trials.
3. Sardinian Warblers ( Sylvia melanocephala ) and European Robins ( Erithacus rubecula ) were mainly peckers both on cultivated and wild olives. Blackcaps ( Sylvia atricapilla ) consumed wild olives mainly by swallowing but consumed cultivated olives (larger than the wild ones) primarily by pecking. Song Thrushes ( Turdus philomelos ) were primarily swallowers of both types of fruits.
4. Laboratory trials with Song Thrushes, Blackcaps and European Robins showed that: (a) all were able to peck fruits; (b) fruit size determined a shift from swallowing to pecking, as pecking frequency increased with the enlargement of the fruit size; (c) all the species had an increased fruit handling failure rate when trying to swallow increasingly large fruits; and (d) from the birds' perspective, small shifts in fruit size may have important consequences on fruit profitability.
5. Pecking on olives turns the mutualistic fruit–frugivore interaction into a fruit-pulp predator interaction, thus arising a conflict between the plant and frugivorous birds.
6. This study shows that heavy dependence on fruit is not always simply related to seed dispersal. The same frugivorous bird species can act as a seed disperser or a pulp predator for the same plant species. The threshold between these roles is highly influenced by the ratio gape size/fruit size.     

Migratory and resident Blackcaps Sylvia atricapilla wintering in southern Spain show no resource partitioning

When different populations of the same bird species share non‐breeding habitats, competition for food may promote resource partitioning. We studied food choice by resident and migratory Blackcaps Sylvia atricapilla in sympatric wintering grounds in southern Spain. Resident Blackcaps have a larger bill, which may allow them to feed on a broader range of fruit sizes, and they may know the distribution of food better than do migrants. Based on fruit and bird counts, we transformed both fruit crop and bird abundance to a common energy currency. During two winters with low and high fruit production, available energy from fruit in mid‐January was estimated to be 80 and 1300 times, respectively, the daily requirements of Blackcaps. Furthermore, Blackcap numbers did not track between‐winter changes in fruit abundance during 10 consecutive years of monitoring, further suggesting that fruit food is not limiting. Analysis of food items from 760 samples of 717 individuals showed that migrants and residents fed primarily on fruits of Wild Olive Olea europaea sylvestris, the most energetic fruit resource. There was no evidence that the larger bills of resident Blackcaps provided any foraging benefit. Migratory Blackcaps fed on Wild Olives and invertebrates, two resources with high energetic and structural value, more frequently than did residents. This food choice could be more important for migratory Blackcaps because they have lower body mass to reduce wing load. Our results suggest that the wintering grounds of Blackcaps in Iberia provide abundant food that is used by sympatric migrants and residents without resource partitioning. Slight differences in food choice suggest that migrants might benefit from feeding on more nutritive food than residents to counteract the energetic constraints associated with a smaller body size.     

The role of fruit diet within a temperate breeding bird community in southern Spain

In southern temperate regions many fleshy-fruited plant species ripen their fruits during the breeding or early post-breeding season and these are consumed by resident birds. I studied the frugivore community of the summer-fruiting tree Frangula alnus ssp. baetica in southern Spain and the significance of fruit in the diet of adult and recently fledged birds. Twelve species consumed F. alnus fruits. Almost 97% of the foraging visits were carried out by the four most abundant species, Robin Erithacus rubecula, Blackcap Sylvia atricapilla, Blackbird Turdus merula and Blue Tit Parus caeruleus. During the ripening season Robins and Blackcaps concentrated at fruit-rich sites. Entire families of Blackcap and Blackbird regularly entered fruiting trees together. In contrast, adult Robins always foraged solitarily, and accounted for only 4% of the tree visits, while 96% were realized by recently fledged birds of the first brood. In all species, young birds foraged as efficiently as adults. F. alnus fruits are not part of the regular nestling diet, but they are a significant part of the diet for recently fledged Robins during their postnatal dispersal. Conversely, these are the most important and effective seed dispersers while adult Robins scarcely take part in the dispersal of F. alnus seeds.     

Habitat effects on resource tracking ability: do wintering Blackcaps Sylvia atricapilla track fruit availability?

If resource availability shapes population distribution, changes in resource abundance should cause parallel changes in population numbers. However, tracking ability may be disrupted by different environmental and behavioural factors that act at different spatial and temporal scales. Here we analyse the ability of wintering Blackcap Sylvia atricapilla populations to track spatio-temporal variation in fruit availability in southern Spain in two habitats (forests and shrublands) with different population structure. Former studies had shown that forests are equally used by both adult migrant and local Blackcaps, whereas shrublands are nearly monopolized by juvenile migrants. These differences might affect resource tracking: it should be disrupted in forests, as local birds remain over winter in their breeding territories, but not in shrublands where similarly competitive juvenile migrants can freely track the spatial distribution of fruits. We analysed the fruit-tracking ability of Blackcap populations among sites and years in both habitat types using a habitat-matching model, which predicts spatio-temporal changes in population abundance proportional to changes in resource availability. We counted Blackcaps and fruiting shrubs (dominated by Lentiscs Pistacia lentiscus and Wild Olives Olea europaea sylvestris ) during four winters in forest and shrubland patches. The abundance of fruits was always higher in shrublands than in forests. In shrublands, Blackcaps seemed to move freely across fruit-rich habitat patches, tracking changes in fruiting-shrub abundance among sites and years. However, such tracking was not observed in forests. This supports the view that fruit-tracking ability may be constrained by local factors, such as the social structure of populations occurring in different habitat types, which introduces spatio-temporal variation in the way fruit availability shapes the abundance distribution of these birds in their Mediterranean wintering grounds.     

Blackaps Sylvia atricapilla stopping over at the desert edge; physiological state and flight-range estimates

Migrating Blackcaps Sylvia atricapilla were mist netted at the desert edge in northern Israel and in Elat (southern Israel) during spring and autumn migrations between 1970 and 1991. Birds in spring in northern Israel were representative of birds that had completed the crossing of the Sahara, while those in Elat still had to cross the 150 km of the Negev Desert, which separates Elat and northern Israel. In autumn, birds captured in northern Israel were representative of those about to cross the Sahara Desert, while those in Elat had already started to cross the desert. The data allowed analysis of seasonal and location differences in the physiological state of Blackcaps before and after crossing the Sahara. Data analysed included body mass, visible fat score and calculated fat content. Autumn migrants were in better physiological condition than spring migrants at both locations, probably as a consequence of their migration route through fertile areas in autumn compared with the crossing of the Sahara in spring. Body mass was less variable after the Sahara crossing in spring than before the crossing in autumn. In spring, 71% and 67% of the birds were fat depleted (fat scores 0 and 1) at Elat and in northern Israel, respectively, while in autumn 34% and 42% were fat depleted. Blackcaps at Elat were 1.6 g lighter than those in northern Israel in autumn and 1.9 g lighter in spring. Potential flight ranges were estimated on the basis of meteorological conditions and flight altitude of passerines above the Negev in Israel (northern Sahara edge) during migration and on a simulation model that considered both energy and water as potential limiting factors for flight duration and distance. The simulation model predicted that half of the Blackcaps that stopped over in Elat and the majority of those that stopped over in northern Israel could not make a nonstop flight over the Sahara Desert in autumn without the assistance of at least an 8 m per s tailwind. Such a wind would still not be sufficient for 34% of the birds in Elat and 42% in northern Israel, and clearly they had insufficient fat reserves to cross the Sahara in a single flight. Although the fattest Blackcaps had accumulated sufficient fat to enable them to traverse the Sahara in a single flight, they probably faced dehydration by at least 12% of their initial body mass when they reached the southern Sahara edge. These birds should use intermittent migration with stopovers at sites with drinking and feeding potential. Their decision to stop over during the day in the desert at sites with shade but without food and water would be beneficial if the meteorological conditions during daytime migration imposed greater risks of dehydration than at night. Spring migrants could not reach their breeding areas in Europe without feeding, but those examined in Elat could cross the remainder of the desert in a single flight.     

A method for differentiating between sedentary and migratory Blackcaps Sylvia atricapilla in wintering areas of southern Iberia

We used the morphological differentiation of southern, sedentary Iberian Blackcaps Sylvia atricapilla to distinguish them from migratory Blackcaps during the winter. To do so, we studied three morphometric traits (wing length, wing shape and tail length) of three migratory Iberian populations and two sedentary ones during the breeding season. We obtained a discriminant function which allowed us to differentiate the two population types (>90% correct classifications). We discuss the utility of the method for differentiating between sedentary and migratory Blackcaps in their sympatric wintering areas, as well as the implications of our results for the study and conservation of sedentary, perhaps differentiated Blackcaps in southern Iberia.     

Flight-range estimates for small trans-Sahara migrants

Arguments in support of the non-stop and the intermittent strategies for crossing the Sahara have been based on data on the fat reserves of birds before the crossing and of birds grounded in the desert. In this paper, flight-range estimates were calculated and the necessary assumptions about air speed, energy input during flight, and energy equivalent of body reserves were evaluated. As examples, Willow Warblers Phylloscopus trochilus and Garden Warblers Sylvia borin were investigated during autumn migration from two study sites north of the Sahara and two study sites in the desert. In still air, the flight-range for both species at all study sites was too short to reach the Sahel zone without refuelling. It is concluded that birds depend on tailwinds for a successful crossing, independent of a non-stop or an intermittent migratory strategy, and that weather conditions in autumn allow them to rely on tailwinds.     

Sex and age-specific differences in wing pointedness and wing length in blackcaps Sylvia atricapilla migrating through the southern Baltic coast

The blackcap Sylvia atricapilla shows a complex migratory pattern and is a suitable species for the studies of morphological migratory syndrome, including adaptations of wing shape to different migratory performance. Obligate migrants of this species that breed in northern, central, and Eastern Europe differ by migration distance and some cover shorter distance to the wintering grounds in the southern part of Europe/North Africa or the British Isles, although others migrate to sub-Saharan Africa. Based on ˃40 years of ringing data on blackcaps captured during autumn migration in the Southern Baltic region, we studied age- and sex-related correlations in wing pointedness and wing length of obligate blackcap migrants to understand the differences in migratory behavior of this species. Even though the recoveries of blackcaps were scarce, we reported some evidence that individuals which differ in migration distance differed also in wing length. We found that wing pointedness significantly increased with an increasing wing length of migrating birds, and adults had longer and more pointed wings than juvenile birds. This indicates stronger antipredator adaptation in juvenile blackcaps than selection on flight efficiency, which is particularly important during migration. Moreover, we documented more pronounced differences in wing length between adult and juvenile males and females. Such differences in wing length may enhance a faster speed of adult male blackcaps along the spring migration route and may be adaptive when taking into account climatic effects, which favor earlier arrival from migration to the breeding grounds.     

The moult of Barred Warblers Sylvia nisoria in Kenya—evidence for a split wing-moult pattern initiated during the birds' first winter*

The moult of Barred Warblers Sylvia nisoria was studied during three winter seasons in southeastern Kenya at a southward passage site (Ngulia) and a wintering site (Mtito Andei). Most Barred Warblers migrating through Ngulia in November had yet to commence winter moult. These birds probably moulted subsequently in winter in northern Tanzania. In December, birds were found in heavy moult at Mtito Andei, and some of these birds were known to stay throughout the winter. By contrast, most birds reaching southeastern Kenya from late December onwards had already completed part or all of their winter moult, presumably at stopover sites in northern and eastern Kenya or in Ethiopia. Thus, winter moult in Barred Warblers takes place mainly in late November and December, either just before or soon after the final leg of autumn migration. In general, first-year birds renewed all tertials and tail feathers, about three to five secondaries per wing and commonly also the outer one to four large primaries per wing. Adults renewed all tertials and tail feathers, almost all secondaries and only occasionally an outer primary. The replacement of relatively fresh juvenile secondaries during the birds' first winter implies that the split moult pattern of this species (secondaries, tertials and tail moulted in winter; primaries and tertials in summer) is endogenously controlled.     

GENETIC BASE AND EVOLUTIONARY CHANGE OF MIGRATORY DIRECTIONS IN A EUROPEAN PASSERINE MIGRANT SYLVIA ATRICAPILLA

Helbig, A. J. 1994. Genetic basis and evolutionary change of migratory directions in a European Passerine migrant Sylvia atricapilla. Ostrich 65: 157–159.

Early displacement experiments and more recent experimental studies with hand-raised Blackcaps Sylvia atricapilla are reviewed in order to describe our current state of understanding of the genetic basis and recent evolutionary changes of migratory directions in birds. Hand-raised Blackcaps from east and west of the Central European migratory divide, when tested under identical conditions, exibited population-specific migratory directions in orientation cages. Cross-breeding of birds from these two populations demonstrated an intermediate mode of inheritance of this behavioural character.

New data on the mitochondrial genetic population structure of the Blackcap indicate that population differentiation at the behavioural level is not necessarily related to long periods of geographic isolation (e.g. during ice ages). Migratory adaptations may have evolved recently, in some cases rapidly, as is illustrated by the establishment of a new migration route of central European Blackcaps to winter quarters in the British Isles. This new route is shown (in a captive breeding experiment) to be based on a novel, genetically determined WNW migratory direction that must have spread from almost zero to 7–11% frequency in parts of central Europe within only three decades. The inheritance of this novel trait also follows a phenotypically intermediate mode and is not influenced by the origin of the female parent (i.e. non-genetic factors can be excluded). The evolutionary flexibility of migratory adaptations is discussed in relation to changes in the environment, both natural and accelerated by man. The need is stressed to study the population-genetic mechanisms of such adaptation with the help of molecular markers as well as large-scale ringing on the breeding grounds.     

Nectar consumption of warblers after long-distance flights during spring migration

The consumption of nectar by European passerines has been reported only occasionally. In this study we investigated the occurrence and significance of nectar consumption of small passerine birds on spring migration after crossing the Mediterranean Sea. On Ventotene Island in the Tyrrhenian Sea, four migrating species of Sylvia warblers [Garden Warbler S. borin , Subalpine Warbler S. cantillans , Whitethroat S. communis , Blackcap S. atricapilld ) regularly foraged on the two most common flowering species at that time of year, Brassica fruticulosa (Cruciferae) and giant fennel Ferula communis (Umbelliferae), while other species visited flowers only occasionally or not at all. Feeding behaviour, pollen traces on the head, and the examination of pollen and sugar remains in droppings indicated that nectar was the main target of the Sylvia warblers, rather than pollen or insects on the flowers. This was confirmed by food choice experiments indicating a clear preference by Garden Warblers and Whitethroats for nectar from artificial flowers over mealworms. Although conclusive experiments are not available, we hypothesize that nectar might be a diet easy to obtain and to absorb for birds after a long-distance flight in which they have incurred a depletion of energy stores and a reduction of the digestive tract.     

Age-related variation in wing shape of migratory and sedentary Blackcaps Sylvia atricapilla

In many passerines, juveniles have shorter and more rounded wings than adults. Given that (1) long and pointed wings improve endurance in migratory flights, (2) shorter and rounded wings improve manoeuvrability, and (3) juvenile birds are more vulnerable to predators than adults, it has been hypothesised that ontogenetic variation in wing shape results from a greater importance of predation avoidance relative to migration performance during the first year of life. If so, wing shape should not change with age in the absence of migration-related selection for longer and more pointed wings. We test this by studying the variation with respect to age in wing length and wing pointedness of migratory and sedentary Blackcaps wintering in southern Spain. Migratory Blackcaps had longer and more pointed wings than sedentary Blackcaps. Juveniles had shorter wings than adults in migratory populations, but not in sedentary populations. The variation with age in wing pointedness was less pronounced, and was found in migratory females only. These differences between the two traits could be related to a stronger selection for pointed wings than for longer wings with increasing distance of migration, and to an increased migratoriness of females in partially migratory Blackcap populations. We hypothesise that, in migratory Blackcaps, a shorter and more rounded wing in juveniles could be selected for if the decrease in predation rate compensated for the somewhat greater costs of the first migration attempt. On the other hand, there are no costs of migration in sedentary Blackcaps, which hence maintain a similar wing shape, giving high manoeuvrability, both as juveniles and as adults.     

Neue Aspekte zur zugzeitlichen Frugivorie der Gartengrasmücke (Sylvia borin)

Zusammenfassung Freilandbeobachtungen und frühere Laboruntersuchungen (Berthold 1976) führten bisher zu einer widersprüchlichen Bewertung der zugzeitlichen Frugivorie bei der Gartengrasmücke. Deshalb wurden das Wahlverhalten gegenüber verschiedenen Beeren und Früchten in deren natürlichem Gehänge in einer Voliere geprüft sowie die Auswirkung unterschiedlich frugivorer Ernährung auf Körpermasse und tägliche Nahrungsaufnahme unter kontrollierten Laborbedingungen untersucht. Zwischen dem Präferenzverhalten gegenüber bestimmten Beeren und Früchten und deren Profitabilität bzw. Nährstoffgehalt bestand kein eindeutiger Zusammenhang. Demnach ist die Wahl zwischen verschiedenen Vegetabilien nicht einfach abhängig von ihrer Größe oder dem Nährstoffgehalt. Erhebliche Unterschiede zwischen den getesteten Beeren und Früchten zeigten sich bei der täglichen Aufnahmerate und den Auswirkungen auf die Körpermasse. Während bestimmte Beeren nur als Zusatzfutter zu einem immer, wenn auch geringfügig notwendigen, animalischen Standardfutter verabreicht werden konnten, waren die Gartengrasmücken mit Schwarzem Holunder und Feige auch bei ausschließlicher Gabe nicht nur in der Lage, ihre Körpermasse zu halten, sondern konnten dabei sogar normale Fettdeposition vollziehen. Offensichtlich sind bestimmte Vegetabilien für die Gartengrasmücke wesentlich bedeutsamer als bisher angenommen. Die Ergebnisse stimmen somit mit Beobachtungen aus dem Freiland überein. Die beobachteten qualitativen Unterschiede zwischen den Früchten und deren Wirkung auf die Körpermasse der Versuchsvögel ließen sich nicht allein mit dem Nährstoffgehalt der Beeren erklären. Vermutlich spielen die spezifische Qualität der Nährstoffe (z. B. Fettsäurezusammensetzung) und möglicherweise auch sekundäre Pflanzenstoffe eine wichtige Rolle.

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The significance of seasonal frugivory in migratory Garden WarblersSylvia borin

Summary In contrast to field observation, exhibiting an important role of seasonal frugivory,Berthold (1976) concluded that fruits are only a supplementary food, and he rejected the hypothesis that songbirds shift to a fruit diet during migration in order to render or to accelerate fat deposition.We investigated the preference of Garden Warblers upon various berries and pulpy fruits in an outdoor aviary. Furthermore, the effects of frugivory on body mass and food intake is measured in caged specimens under controlled laboratory conditions.In almost all feeding trials a significant selection between the two alternative fruit diets are obvious. However, no clearcut relationship between profitability or nutritive content of the various fruits and food choice is evident.In the experiments under controlled laboratory conditions the effects of varying extends of frugivory on body mass and food intake of captive Garden Warblers reveal a considerable difference between various berries and fruits. In some berries, the birds were not able to compensate for a reduction of the standard animal food by increasing frugivory. With Black Elder berries or figs, however, they did not only compensate for to maintain body mass rather they were able to gain mass during migratory fat deposition, even kept on an exclusive fruit diet.These results clearly exhibit a significant role of fruit diets in migrating Garden Warblers, with particular regards to Black Elder berries and figs, respectively.The observed differences between the various fruit diets do not correspond to the crude nutritive content of the pulp. Presumably, the specific quality of nutrients (for instance, fatty acid composition) or even secondary plant compounds have to be considered.The experimental results agree with field observations, indicating the importance of frugivory for the Garden Warbler, and they contradict Berthold's suggestion that fruits are only a least important supplementary food to an always necessary animal food.

     

STOPOVER AND FAT DEPOSITION BY BLACKCAPS SYLVIA ATRICAPILLA FOLLOWING SPRING MIGRATION OVER THE SAHARA

Maitav, A. & Izhaki, I. 1994. Stopover and fat deposition by Blackcaps Sylvia atricapilla following spring migration over the Sahara. Ostrich 65:160-166.

Length of stopover and rate of weight change were studied in Blackcaps that stopover in Eilat (southern Israel) in spring after a trans-Saharan flight. The birds studied had already completed 2000 km of desert crossing and faced just 200 km more. The potential predicted flight range for these transients suggests that the fat depleted Blackcaps (fat class=0) could not reach the east Mediterranean scrublands, which offer more extensive foraging opportunities than the desert area around Eilat, without refueling. However, 46% of the Blackcaps had sufficient fat (fat class > 0) to reach central or northern Israel in a single flight in still air. Recaptured birds stayed in Eilat for 2,4±1,9 days. Although the body mass of an average recaptured bird increased by 0,27±1,44 g/day, which was an increase of 0,8%±7,0% of its body mass per day during stopover, these results were not significantly different from zero. Stopover length, mass change during stopover, and the rate of change in mass did not differ between males and females and between those who stopped over for only one day to those which stayed > 1 day. However, there was a tendency of these lean birds to stay for a shorter period in Eilat than relatively fat birds. There was also some tendency for lean birds to gain more mass during their stopover than fat birds. It seems that in spring, when less time is allotted for the whole migration program, the migrants resume their journey before fat reserves have been replenished.     

Population differentiation of migratory directions in birds: comparison between ringing results and orientation behaviour of hand-raised migrants

Summary Blackcaps (Sylvia atricapilla) from five areas in Central Europe were hand-raised. Their autumn migratory orientation was tested in funnel-shaped cages. Their directional choices were compared to recoveries of conspecifics ringed during the breeding season in the same areas, which are situated on a transect across a migratory divide between southeastward and southwestward migrating populations. Results from ringing and orientation tests were in good agreement with respect to mean direction and dispersion of flight directions. An exception is the area around Linz (NW Austria), right on the migratory divide, where ringing yielded a strong scatter, but hand-raised birds chose westerly directions. The recent establishment of a novel migration route toward the British Isles was reflected in both data sets: in southern Germany the percentage of northwestward migrants is 6.8% according to orientation tests and 11.8% according to ringing recoveries. Testing the orientation of young passerines in captivity can yield valuable information about population differentiation of migratory behaviour. It is more efficient than ringing in this respect, because it circumvents the low recovery rates and is free of biases affecting ringing data. In the blackcap, geographic differentiation of migratory directions occurs on a finer scale than previously recognized and can change significantly within 2–3 decades.     

Does the Jarman–Bell principle at intra-specific level explain sexual segregation in polygynous ungulates? Sex differences in forage digestibility in Soay sheep

Fat accumulation by blackcaps (Sylvia atricapilla) is a prerequisite for successful migratory flight in the autumn and has recently been determined to be constrained by availability of drinking water. Birds staging in a fruit-rich Pistacia atlantica plantation that had access to water increased their body mass and fat reserves both faster and to a greater extent than birds deprived of water. We conducted a series of laboratory experiments on birds captured during the autumn migration period in which we tested the hypotheses that drinking water increases food use by easing limitations on the birds’ dietary choices and, consequently, feeding and food processing rates, and that the availability of drinking water leads to improved digestion and, therefore, to higher apparent metabolizable energy. Blackcaps were trapped in autumn in the Northern Negev Desert, Israel and transferred to individual cages in the laboratory. Birds were provided with P. atlantica fruit and mealworms, and had either free access to water (controls) or were water-deprived. In experiment 1, in which mealworm availability was restricted, water-deprived birds had a fourfold lower fruit and energy intake rates and, consequently, gained less fat and total mass than control birds. Water availability did not affect food metabolizability. In experiment 2, in which mealworms were provided ad libitum, water availability influenced the birds’ diet: water-restricted birds ate more mealworms, while control birds consumed mainly P. atlantica fruit. Further, in experiment 2, fat and mass gain did not differ between the two treatment groups. We conclude that water availability may have important consequences for fat accumulation in migrating birds while they fatten at stopover sites, especially when water-rich food is scarce. Restricted water availability may also impede the blackcap’s dietary shift from insectivory to frugivory, a shift probably necessary for successful pre-migratory fattening.     

Migratory Orientation: Magnetic Compass Orientation of Garden Warblers (Sylvia borin) after a Simulated Crossing of the Magnetic Equator

Since the birds' magnetic compass works as an inclination compass using the axial course of the magnetic field lines and their inclination, transequatorial migrants have to reverse their reaction with respect to the magnetic field after crossing the magnetic equator. Garden Warblers, long distance migrants breeding in Europe and wintering in tropical and southern Africa, were tested during autumn in the local geomagnetic field on the northern hemisphere. The experimental group was exposed to a field with horizontal field lines, simulating equator crossing, at the beginning of October; afterwards the birds were tested in the local geomagnetic field again. While the controls showed southerly tendencies during the entire season, the experimentals reversed their directional tendencies after staying in the horizontal field and now preferred northerly directions. In a field of the southern hemisphere, this preference corresponds to a southern course which would have meant the continuation of their migration flight.     

Crosswise migration by Yellow Warblers,Nearctic‐Neotropical passerine migrants

Yellow Warblers (Setophaga petechia) are abundant breeding birds in North America, but their migratory and non‐breeding biology remain poorly understood. Studies where genetic and isotopic techniques were used identified parallel migration systems and longitudinal segregation among eastern‐ and western‐breeding populations of Yellow Warblers in North America, but these techniques have low spatial resolution. During the 2015 breeding season, we tagged male Yellow Warblers breeding in Maine (N = 10) and Wisconsin (N = 10) with light‐level geolocators to elucidate fine‐scale migratory connectivity within the eastern haplotype of this species and determine fall migration timing, routes, and wintering locations. We recovered seven of 20 geolocators (35%), including four in Maine and three in Wisconsin. The mean duration of fall migration was 49 d with departure from breeding areas in late August and early September and arrival in wintering areas in mid‐October. Most individuals crossed the Gulf of Mexico to Central America before completing the final eastward leg of their migration to northern South America. Yellow Warblers breeding in Maine wintered in north‐central Colombia, west of those breeding in Wisconsin that wintered in Venezuela and the border region between Brazil, Colombia, and Venezuela. Our results provide an example of crosswise migration, where the more easterly breeding population wintered farther west than the more westerly breeding population (and vice versa), a seldom‐documented phenomenon in birds. Our results confirm earlier work demonstrating that the eastern haplotype of northern Yellow Warblers winters in northern South America, and provide novel information about migratory strategies, timing, and wintering locations of birds from two different populations.