Autumn migratory fuelling: a response to simulated magnetic displacements in juvenile wheatears, Oenanthe oenanthe

Recent experiments exposing migratory birds to altered magnetic fields simulating geographical displacements have shown that the geomagnetic field acts as an external cue affecting migratory fuelling behaviour. This is the first study investigating fuel deposition in relation to geomagnetic cues in long-distance migrants using the western passage of the Mediterranean region. Juvenile wheatears (Oenanthe oenanthe) were exposed to a magnetically simulated autumn migration from southern Sweden to West Africa. Birds displaced parallel to the west of their natural migration route, simulating an unnatural flight over the Atlantic Ocean, increased their fuel deposition compared to birds experiencing a simulated migration along the natural route. These birds, on the other hand, showed relatively low fuel loads in agreement with earlier data on wheatears trapped during stopover. The experimental displacement to the west, corresponding to novel sites in the Atlantic Ocean, led to a simulated longer distance to the wintering area, probably explaining the observed larger fuel loads. Our data verify previous results suggesting that migratory birds use geomagnetic cues for fuelling decisions and, for the first time, show that birds, on their first migration, can use geomagnetic cues to compensate for a displacement outside their normal migratory route, by adjusting fuel deposition.     

The nature of the migration route shapes physiological traits and aerodynamic properties in a migratory songbird

Migration distance is supposed to represent an important selection pressure shaping physiological and morphological properties. Previous work has focussed on this effect, while the importance of ecological barriers in this context has been rarely considered. We studied two subspecies of a migratory songbird, the northern wheatear (Oenanthe oenanthe oenanthe and O. o. leucorhoa L.), on an island in the North Sea. The former subspecies reaches their Scandinavian breeding areas after a short sea crossing, whereas leucorhoa northern wheatears cross the North Atlantic towards Iceland, Greenland or Canada. Physiological traits (fuel deposition rate) and wings’ aerodynamic properties (wing pointedness independent of body size), both affecting migration speed, were hypothesized to be more pronounced in leucorhoa than in oenanthe northern wheatears. Within subspecies, the physiological and aerodynamic properties were hypothesized to explain arrival date at the stopover site with “fast migrants” arriving early. Physiological and aerodynamic properties in leucorhoa northern wheatears lead to a faster and less costly migration, favouring a sea crossing, but in trade-off lower flight manoeuvrability than in oenanthe birds. Wings’ aerodynamic properties affected the seasonal occurrence of leucorhoa females, whereas the physiological traits significantly influenced arrival date in oenanthe individuals. The less risky migration route in oenanthe birds with few short sea crossings may have favoured higher flight manoeuvrability for foraging (less pointed wings), in trade-off an energetically more costly flight. Hence, not the migration distance itself, but the presence/absence of a sea barrier presents an important selection pressure in migratory land birds favouring low flight costs.     

Differential timing of spring migration in northern wheatears Oenanthe oenanthe: hurried males or weak females?

In a field experiment on the island of Helgoland (southeast North Sea), we investigated whether migration strategy or competition between the sexes cause the differential timing of spring migration of male and female northern wheatears (Oenanthe oenanthe) (males migrating earlier). The study included two subspecies, heading towards Greenland/Iceland and Scandinavia, respectively, and is based on colour-ringing and remote weighing of individuals. Despite food offered ad libitum, most Scandinavian birds left the island on the day of arrival or stayed only 1–3 days, whereas more than half of Greenlandic/Icelandic birds stayed for up to 12 days and refuelled rapidly. In the latter subspecies, males showed a positive correlation of departure fuel load and fuel deposition rate, resembling time-minimizers in optimal migration theory. In contrast, females departed irrespective of fuel deposition rate, with an approximately constant level of fuel stores. This level was lower than in males, but sufficient to enable by-passing of stopover sites en route, allowing us to regard females as time-minimizers also. Since females are not able to reach Greenland without additional refuelling elsewhere and males appeared to have a larger potential for by-passing stopover sites, time-selection seems to be more pronounced in males and may be the reason for earlier migration of males. Intraspecific aggressive interactions between colour-ringed birds were predominantly won by the initiator, by males and by larger birds, whereas fuel load and subspecies did not affect the outcome. Although compared to females, males were more often dominant at the feeding stations or held territories, refuelling patterns could not be explained by dominance. Subordinate or non-territorial birds did not refuel at a lower rate or depart with lower fuel loads than dominant or territorial birds. In non-territorial birds, the restricted access to feeding stations was made up with larger doses of food taken per visit, leading to the same energy intake as that of dominant and territorial birds. Therefore, competition during stopover could be eliminated as the reason for differential timing of migration of male and female wheatears, but this result may be species-specific.Communicated by W. Wiltschko     

Obligatory barrier crossing and adaptive fuel management in migratory birds: the case of the Atlantic crossing in Northern Wheatears (<Emphasis Type="Italic">Oenanthe oenanthe</Emphasis>)

Behaviour on migration was often suggested to be selected for time-minimising strategies. Current optimality models predict that optimal fuel loads at departure from stopover sites should increase with increasing fuel deposition rates. We modified such models for the special case of the east Atlantic crossing of the Northern Wheatear (Oenanthe oenanthe). From optimality theory, we predict that optimal time-minimising behaviour in front of such a barrier should result in a positive correlation between fuel deposition rates and departure fuel loads only above a certain threshold, which is the minimum fuel load (f _min) required for the barrier crossing. Using a robust range equation, we calculated the minimum fuel loads for different barrier crossings and predict that time-minimising wheatears should deposit a minimum of 24% fuel in relation to lean body mass (m ₀ ) for the sea crossing between Iceland and Scotland. Fuel loads of departing birds in autumn in Iceland reached this value only marginally but showed positive correlation between fuel deposition rate (FDR) and departure fuel load (DFL). Birds at Fair Isle (Scotland) in spring, which were heading towards Iceland or Greenland, were significantly heavier and even showed signs of overloading with fuel loads up to 50% of lean body mass. Departure decisions of Icelandic birds correlated significantly with favourable wind situations when assuming a migration direction towards Spain; however, the low departure fuel loads contradict a direct non-stop flight.     

Night sky orientation of migratory pied flycatchers raised in different magnetic fields

Summary To test whether the initial night sky orientation response of migratory pied flycatchers (Ficedula hypoleuca) is calibrated from the ambient magnetic field experienced by birds during their first summer, three groups of pied flycatchers were hand-reared and then held under different magnetic field conditions during the course of the summer. All groups were held outdoors and given full exposure to the day and night sky. One group was exposed to the local earth's magnetic field. A second group was exposed to a magnetic field of local earth strength, local earth inclination shifted 105° counter-clockwise relative to the local earth's field. The last group was exposed to a vertical, and thus nondirectional magnetic field.In autumn, the birds were tested for their orientation under the night sky in the absence of a directional magnetic field. When tested, all three groups were oriented with mean directions varying from south to southeast. No statistical differences emerged in any between group comparisons. The data indicate that earth's magnetism does not serve as a calibrating reference in the development of a pied flycatcher's initial orientation response to the night sky.     

Fuel loss and flexible fuel deposition rates in a long-distance migrant

Most migrating birds alternate flight bouts with stopovers, during which they rest and replenish the fuel used during flight (refueling). The rate of refueling (fuel deposition rate, FDR) affects stopover duration, and hence is an important determinant of the overall time required for migration. Although environmental and endogenous factors affect FDR, the urge to refuel depends on the anticipated distance to be travelled and possibly also on the amount of fuel used during the flight preceding stopover. Combining a field study with a fasting–refueling experiment on long-term captive songbirds, we tested whether the extent of fuel loss prior to refueling indeed affects FDR. In the field study, we took a comparative approach and determined FDR in two subspecies of northern wheatear (Oenanthe oenanthe) that differ greatly in the distance flown, and thus the extent of fuel used to reach our study (stopover) site. As both winter in western sub-Sahelian Africa, they face the same remaining migration distance. We found that FDR was higher in the subspecies that uses more fuel to get to our study site. Solidifying this result, in the experiment on captive northern wheatears, we found that the extent of fuel loss as a consequence of fasting explained most of the variation in subsequent FDR. The observation that experimental birds losing little fuel did not maximize their FDR suggests there are costs to rapid refueling. Our study shows that FDR is shaped not just by current environmental and endogenous conditions but also by fuel loss prior to refueling.     

Geomagnetic field affects spring migratory direction in a long distance migrant

Night-migrating song birds travel to and from their wintering and breeding areas often separated thousands of kilometers apart and are clearly capable of finding intended goal areas from a distant location. Displacement experiments provide a useful way to highlight orientation and navigational skills in migrants. To investigate which cues birds actually use to compensate for displacement and the exact mechanism of each cue, experiments with manipulation of single cues are required. We conducted a simulated displacement of lesser whitethroats Sylvia curruca on spring migration. Birds were displaced not geographically but in geomagnetic space only, north and south of their breeding area to test whether they incorporate information from the geomagnetic field to find their breeding area. Lesser whitethroats held in southeast Sweden but experiencing a simulated displacement north of their breeding area (Norway) failed to show a consistent direction of orientation, whereas birds displaced south of their breeding area (Czech Republic) exhibited consistent northerly orientation, close to the expected seasonally appropriate direction, after displacement toward the trapping location. The absence of a clear compensatory direction in birds displaced north might be due to unfamiliar magnetic information or lack of sufficient information such as a magnetic gradient when moving around. By isolating one orientation cue, the geomagnetic field, we have been able to show that lesser whitethroats might incorporate geomagnetic field information to determine latitude during spring migration.     

The role of daytime cues in the development of magnetic orientation in a night-migrating bird

To assess the role of celestial rotation during daytime in the development of the magnetic compass course, pied flycatchers (Ficedula hypoleuca Pallas, Muscicapidae) were handraised in Latvia under various celestial and magnetic conditions. Tests were performed during autumn migration in the local geomagnetic field (50 000 nT, 73° inclination) in the absence of celestial cues. A group of birds that had never seen the sky showed a bimodal preference for the migratory southwest-northeast axis, whereas a second group that had been exposed to the natural sky from sunrise to sunset in the local geomagnetic field showed a unimodal preference for the seasonally appropriate southwesterly direction. A third group that had also been exposed to the daytime sky, but in the absence of magnetic compass information, also oriented bimodally along a southwest-northeast axis. These findings demonstrate that observing celestial rotation during daytime enables birds to choose the right end of the migratory axis for autumn migration at the Latvian test location. This transformation of axial behavior into appropriate migratory orientation, however, requires the birds to have simultaneous access to information on both celestial rotation and the geomagnetic field. Received: 19 September 1997 / Accepted after revision: 22 November 1997     

Orientation of juvenile barn swallows (<Emphasis Type="Italic">Hirundo rustica</Emphasis>) tested in Emlen funnels during autumn migration

Although hirundines have been used extensively in homing experiments, to date no investigation of their migratory orientation has been carried out, despite the well-known migratory habits of many species of this family. This paper reports on a study of the orientation of the barn swallow (Hirundo rustica), a typical diurnal trans-Saharan migrant. Modified Emlen funnels were used to verify the suitability of this species for cage experiments and investigate the role of visual and magnetic cues during the birds first migratory journey. Juvenile swallows were mist-netted at a roost site in central Italy and then tested in a site 19 km apart. Orientation experiments were performed under four experimental conditions: natural clear sky and simulated overcast, in both local and shifted magnetic fields (magnetic North=geographical West). Under clear sky, the swallows tended to orient phototactically toward the best-lit part of the funnel and failed to respond to the magnetic field shift. Under overcast conditions, they oriented northward and modified their directional choices as expected in response to the shifted magnetic North. On the whole, our data indicate that swallows can use magnetic information for compass orientation. Possible explanations for the northward orientation of birds tested under overcast conditions are discussed.Communicated by W. Wiltschko     

Magnetic compass orientation in the yellow-faced honeyeater,Lichenostomus chrysops,a day migrating bird from Australia

Summary In Australia, the southern populations of the yellow-faced honeyeater, Lichenostomus chrysops (Meliphagidae), perform annual migrations, with routes following the eastern coastline. In order to assess the role of magnetic cues in the migratory orientation of this diurnal migrant, its directional behaviour was recorded in recording cages under natural and experimentally manipulated magnetic-field conditions. During autumn the birds tested indoors in the local geomagnetic field showed a directional change from north initially to northwest later in the season (Fig. 1 a, b), which corresponds well with the general pattern of movement of this species in the field. Deflecting magnetic north to ESE resulted in a clockwise shift of the mean direction by 77° and 71°, respectively (Fig. 1 c, d), while no significant directional tendencies were observed in a magnetic field with a compensated horizontal component (Fig. 1 e, f; see Table 1). In outdoor tests in spring, the birds preferred southerly directions when tested in the local geo-magnetic field. In a magnetic field with a reversed vertical component (i.e. with an inclination pointing down instead of upwards) the birds reversed their directional tendencies and oriented northward (Fig. 2, Table 2). These results clearly show: (1) that yellow-faced honeyeaters can use the magnetic field for direction finding, and (2) that their magnetic compass functions as an inclination compass, as has been shown for several holarctic migrants.Correspondence to: W. Wiltschko     

The magnetic compass mechanism of birds and its possible association with the shifting course directions of migrants

Summary Many birds of the northern hemisphere shift their migratory course to more southerly directions when moving from northern to southern latitudes. Birds from Central Europe, for example, change their course from SW to S or from SE to S respectively (Fig. 1). This also seems to apply to some other animals.The hypothesis presented here explains the observed shifts in migratory direction on the basis of changes in the parameters of the earth's magnetic field and hence would make a genetic fixation of shifts in the migratory direction unnecessary.To determine the direction of migration birds do not refer to the polarity of the magnetic field but to its dip (=). According to the hypothesis presented here, the birds, however, do not refer to the direction of dip as previously believed but to the individual apparent angle of dip (=), this angle changes depending on the heading of the bird (see Fig. 3 and Eq. 1). Maintaining a species specific or population specific the bird will move in its predetermined migratory direction. Changes in the dip of the earth's magnetic field correspond to changes in latitude. According to the hypothesis with fixed, the migratory direction will change when the dip changes. Given the hypothesis and the parameters of the earth's field theoretical migratory paths of birds between summer and winter quarters may be calculated (Figs. 8–11). The calculated tracks and the actually observed migratory routes agree well. This is also confirmed by radar and other observations of migratory directions in areas of different dip angles (Fig. 13). Displacing migrating birds to areas of smaller dip angles (= lower magnetic latitudes) results in predeterminable shifts in the birds migratory direction (Figs. 5, 6). The hypothesis also accounts for the so far unexplained orientation behaviour of transequatorial migrants under the magnetic equator.A very simple model of this hypothetical compass mechanism may be based on the assumption of the sensor axis is supposed to correspond to the apparent angle of dip when moving in the migratory direction. In this position the difference between the apparent angle of dip and the angle of the sensor is zero. Any change in the direction of movement, however, will result in a difference leading to a response of an assumed receptor. When maintaining the zero difference the bird invariably sticks to its migratory course. The proposed mechanism is a null instrument unaffected by changes in field intensity and not depending on the measurement of absolute values.     

Stopover behaviour and departure decision of northern wheatears, Oenanthe oenanthe, facing different onward non-stop flight distances

On the small North Sea island Helgoland (54°11' N, 07°55' E) we studied the stopover ecology of two subspecies of northern wheatear, Oenanthe oenanthe, during spring migration. Birds heading for Scandinavia (O. o. oenanthe) face only short flights across an ecological barrier (50-500 km) whereas those originating from Greenland and Iceland (O. o. leucorhoa) have to cover between 1,000 and 2,500 km in the impending flight. Colour-ringed individuals showed that 90% of Scandinavian birds left on the day of ringing while 40% (males) and 30% (females) of Greenland/Icelandic birds stayed at least 1 night. The birds who remained were thus mostly O. o. leucorhoa. They often established desirable feeding territories on the beach and had a high rate of body mass increase (1.7 g/day). However, subspecies did not differ in habitat choice and in foraging effort, but O. o. leucorhoa had a higher success rate in pecking. Departure decisions were analysed by comparing (a) conditions on the day of ringing between departing and staying birds and (b) for birds staying between the day of departure and the preceding day. The factors that were probably important in the decision to depart differed between subspecies. In O. o. leucorhoa, few birds departed with bad or deteriorating weather conditions (tailwind component, cloud cover), whereas departures of O. o. oenanthe seemed to be little affected by those factors. A few O. o. oenanthe stayed early in the spring migration season and/or had low fat reserves. Interference during foraging seemed to play a role because both subspecies tended to leave when the densities of northern wheatears were high. Other factors related to refuelling conditions (food supply, foraging effort, predation risk) failed to show differences between staying and departing individuals. In summary, almost all Scandinavian birds departed quickly and irrespective of refuelling and weather conditions, whereas many (but not all) Greenland/Icelandic birds seemed to prepare for a long-distance flight and carefully adjusted departure to weather conditions. The observed differences in stopover behaviour and departure decisions in the two subspecies of northern wheatear indicate that the distance to the next stopover site or to the goal area has to be considered when applying optimal migration models.     

Male choice,fighting ability,assortative mating and the intensity of sexual selection in the milkweed longhorn beetle,Tetraopes tetraophthalmus (Coleoptera,Cerambycidae)

Summary Over five hundred adult longhorn milkweed beetles, Tetraopes tetraophthalmus, were individually marked and their copulatory success followed for one month in a pasture of Asclepias syriaca in northern Indiana, USA. Migration of beetles from the field site was greatest from areas of low population density. Dispersal was significantly greater for males experiencing low copulatory success; a similar but nonsignificant trend was observed for females. Large males, which displayed greater site tenacity than small males, copulated more frequently than small males because of their ability to displace small males from females. Both large and small males demonstrated a preference for large females in laboratory tests. Male preference in combination with aggressive displacement of small males results in size-assortative mating which was much stronger under conditions of high population density. It contributes to variance in male reproductive success since female size is known to be correlated with fecundity and offspring viability. Variance in copulatory success is similar for males and females, suggesting that both sexes experience similar intensities of sexual selection with respect to this component of reproductive success. Futhermore, comparison of this with other studies suggests that the intensity of sexual selection among males is positively correlated with the variance in body size which appears to be under both stabilizing and directional sexual selection in males but not in females.     

Love thy neighbour? Social nesting pattern,host mass and nest size affect ectoparasite intensity in Darwin’s tree finches

Social nesting behaviour is commonly associated with high prevalence and intensity of parasites in intraspecific comparisons. Little is known about the effects of interspecific host breeding density for parasite intensity in generalist host–parasite systems. Darwin’s small tree finch (Camarhynchus parvulus) on Santa Cruz Island, Galápagos Islands, nests in both heterospecific aggregations and at solitary sites. All Darwin finch species on Santa Cruz Island are infested with larvae of the invasive blood-sucking fly Philornis downsi. In this study, we test the prediction that total P. downsi intensity (the number of parasites per nest) is higher for nests in heterospecific aggregations than at solitary nests. We also examine variation in P. downsi intensity in relation to three predictor variables: (1) nest size, (2) nest bottom thickness and (3) host adult body mass, both within and across finch species. The results show that (1) total P. downsi intensity was significantly higher for small tree finch nests with many close neighbours; (2) finches with increased adult body mass built larger nests (inter- and intraspecific comparison); (3) parasite intensity increased significantly with nest size across species and in the small tree finch alone; and (4) nest bottom thickness did not vary with nest size or parasite intensity. These results provide evidence for an interaction between social nesting behaviour, nest characteristics and host mass that influences the distribution and potential impact of mobile ectoparasites in birds.     

Maternal effects on post-weaning physical and social development in juvenile mountain goats (Oreamnos americanus)

Little is known about maternal effects on post-weaning development, yet they may be important because maternal care could have long-term consequences only evident when offspring approach adulthood. We have assessed the effects of maternal age, current reproduction (presence of a kid of the year) and social rank on the body mass, horn length and social rank of 1- and 2-year-old mountain goats (Oreamnos americanus). Maternal reproductive status and social rank did not affect the mass or horn length of either yearlings or 2-year-olds. Maternal age was positively correlated with yearling body mass for males but not females. We could not detect any maternal age effects on body mass of 2-year-olds. Maternal age and spring forage quality were positively correlated with horn length of yearlings of both sexes, but not of 2-year-olds. Juvenile females showed compensatory growth in mass between 1 and 2 years of age, but males did not. Neither sex showed compensatory growth in horn length. None of the maternal characteristics we examined directly affected the social rank of juveniles, which increased with body mass. Social rank in female mountain goats seems to be established early in life and maintained to adulthood. By affecting yearling development, maternal age could affect the reproductive success of males.     

Copepod photobehavior in a simulated natural light environment and its relation to nocturnal vertical migration

To investigate the roles of light in initiating, controlling and directing nocturnal vertical migration, photoresponses of the adult, female copepod Acartia tonsa Dana were measured under simulated natural underwater light conditions using a video system. Copepods were adapted to a range of background light levels and tested with the following stimuli: absolute quantal intensity, absolute change in quantal intensity and relative (%) change in quantal intensity. The stimulus initiating vertical movements was relative change in quantal intensity, while responsiveness was controlled by the level of light adaptation. A. tonsa swam upward in response to decreases. Response with minimal stimulation occurred at an adaptation intensity close to that in the copepod's natural habitat at the time of the migratory ascent (near the bottom of the Newport River estuary, North Carolina, near sunset). Analysis of the angles of upward movement showed that light is not a directional cue. Relative increases in intensity resulted in sinking, with minimal stimulation required at an adaptation intensity close to that in the field when the migratory descent occurs near sunrise. These results offer a reasonable explanation of how light cues may shape nocturnal vertical migratory patterns.     

Clock-shift experiments with Savannah sparrows, Passerculus sandwichensis, at high northern latitudes

Orientation can be difficult for nocturnal bird migrants at high northern latitudes because of the large changes of magnetic declinations, rapid longitudinal time-shifts experienced during a long-distance flight and the invisibility of stars during the polar summer. Both sunset cues as well as geomagnetic cues have been shown to be of great importance in the orientation system of Savannah sparrows, Passerculus sandwichensis. We used clock-shift experiments to investigate whether geomagnetic and sunset cues were used for migratory orientation by wild-caught young Savannah sparrows at high geomagnetic latitudes in Northern Canada. We exposed birds to a 4-h slow clock-shift, expecting a 60° clock-wise shift in orientation after the treatment. Under natural clear skies in the local geomagnetic field, the birds responded by showing a significant axial mean orientation directed towards the position of the setting sun in the NW and towards their preferred migratory direction in the SE. After exposure to the clock-shift for 6 days and nights the birds showed a clear response to the treatment and shifted significantly towards NNE. Birds that first oriented towards NW in the experiments before clock-shift tended to shift clock-wise, thus reacted to the clock-shift in the expected way. The reaction of the individual birds that originally oriented towards SE seems to vary. In summary, our birds did not select a constant angle (menotaxis) in relation to the sun's position during the experiments, but presumably were affected by the sun showing phototaxis or followed their magnetic compass. Possible explanations of the unexpected experimental results are discussed. Electronic Publication     

Vertical distribution and nocturnal migration of zooplankton in relation to the development of the seasonal thermocline in Patraikos Gulf

Vertical distribution and nocturnal migration of zooplankton species in relation to the development of the seasonal thermocline in the shallow waters (90 m) of Patraikos Gulf (Ionian Sea, Greece) were investigated using a WP-2 closing net. Juvenile and adult copepods accounted for a mean of 91% of the total collected in three sampling periods, i.e. May, July and September 1985.Ctenocalananus vanus, Paracalanus parvus andOithona plumifera were the dominant copepods. The majority of the zooplankton tend to aggregate at the thermocline layer. Among copepods the two congeneric speciesClausocalanus pergens andC. furcatus exhibited different migratory responses to the development of the thermocline.C. pergens occurred in the lower part of the thermocline andC. furcatus in the upper region or above. The diel vertical migration of all species could be divided into four types: (1) no vertical migration; (2) upward migration at night; (3) occasional migration; and (4) reverse migration (down at night). In July when the strongest thermocline developed, most zooplankters rose close to the surface at night. For most species, temperature discontinuity did not limit their diel migration.Please address all correspondence and requests for reprints to Dr J.J. Lykakis     

Reproductive success of male bank voles (<Emphasis Type="Italic">Clethrionomys glareolus</Emphasis>): the effect of operational sex ratio and body size

The operational sex ratio (OSR) may influence the intensity of competition for mates and mate choice and is therefore thought to be a major factor predicting the intensity and direction of sexual selection. We studied the opportunity for sexual selection, i.e., the variance in male reproductive success and the direction and intensity of sexual selection on male body mass in bank vole (Clethrionomys glareolus) enclosure populations with experimentally manipulated sex ratios. The opportunity for sexual selection was high among male-biased OSRs and decreased towards female-biased OSRs. Paradoxically, selection for large male body mass was strongest in female-biased OSRs and also considerable at intermediate OSRs, whereas at male-biased OSRs, only a weak relationship between male size and reproductive success was found. Litters in male-biased OSRs were more likely to be sired by multiple males than litters in female-biased OSRs. Our results suggest that the intensity and direction of sexual selection in males differs among different OSRs. Although the direction of sexual selection on male body mass was opposite than predicted, large body mass can be favored by sexual selection. Naturally varying OSRs may therefore contribute to maintain variation in male sexually selected traits.     

Migratory orientation of red-eyed vireos,Vireo olivaceus,in relation to energetic condition and ecological context

How and when migrants integrate directional information from different sources may depend not only on the bird’s internal state, including fat stores, but also on the ecological context during passage. We designed experiments to (1) examine the influence of stored fat on the decision to migrate and on the choice of migratory direction and (2) investigate how the integration of orientation cue information is tied to energetic status in relation to migration across an ecological barrier. Migratory orientation of red-eyed vireos (Vireo olivaceus) at twilight was recorded using two different techniques, orientation cage experiments and free-flight release tests, during both fall and spring migration. During fall migration, the amount of stored fat proved decisive for directional selections of the vireos. Fat birds chose directions in accordance with migration across the Gulf of Mexico. Lean birds oriented either parallel to the coast line (cage tests) or moved inland (free-flight releases). Whereas only fat birds showed significant responses to experimental deflections of the geomagnetic field, lean birds displayed a tendency to shift their activity in the expected direction, making it difficult to evaluate the prediction that use of the magnetic compass is context dependent. Fat loads also had a significant effect on the decision to migrate, i.e., fat individuals were more likely to embark on migration than were lean birds (true for both cage and release experiments). During spring migration, a majority of experimental subjects were classified as lean, following their arrival after crossing the Gulf of Mexico, and oriented in seasonally appropriate directions. The vireos also showed significant responses to experimental deflections of the geomagnetic field regardless of their energetic status. Free-flight release experiments during spring migration revealed a significant difference in mean directions between clear sky and overcast tests. The difference may indicate a compensatory response to wind drift or possibly a need for celestial cues to calibrate the magnetic compass. Finally, this is the first demonstration of magnetic compass orientation in a North American vireo. Received: 15 December 1995/Accepted after revision: 24 March 1996