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1.
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 相似文献
2.
The orientation of two closely related subspecies of Australian silvereye Zosterops lateralis was studied in captivity over 14 months. Migratory silvereyes Z. lateralis lateralis showed significant directional preferences during the spring and autumn migration periods and also displayed orientated behaviour during the breeding period. In contrast, the non-migratory subspecies Z. lateralis familiaris did not display any significant directional preferences at any time. This is the first time that the orientation behaviour of a migratory and non-migratory subspecies has been compared over the duration of an annual cycle, both during and outside the migratory periods. The results suggest that migratory silvereyes possess an endogenous program determining the timing and direction of autumn and spring migration and that this program is unique to the migratory subspecies. This is also the first comparison of this nature on southern hemisphere birds and demonstrates that the migratory behaviour of southern hemisphere migrants may be more similar to that of northern hemisphere migrants than previously thought.Submitted to Behavioral Ecology and Sociobiology: 23 Jan 2006. 相似文献
3.
Wind selectivity of migratory flight departures in birds 总被引:5,自引:0,他引:5
Optimal migration theory predicts that birds minimizing the overall time of migration should adjust stopover duration with
respect to the rate of fuel accumulation. Recent theoretical developments also take into account the wind situation and predict
that there is a time window (a set of days) during which birds should depart when assisted by winds but will not do so if
there are head winds. There is also a final day when birds will depart irrespective of wind conditions. Hence, the wind model
of optimal migration theory predicts that birds should be sensitive to winds and that there should be a correlation between
departures and winds blowing towards the intended migration direction. We tested this assumption by tracking the departures
of radio-tagged passerines during autumn migration in southern Sweden. Our birds were moderately to very fat when released
and therefore energetically ready for departure. There was a significant correlation between direction of departure and wind
direction. We also found that during days when birds departed there was a significantly larger tail wind component than during
days when birds were present but did not depart. Our results show that passerines do take the current wind situation into
account when departing on migratory flights. We also briefly discuss possible clues that birds use when estimating wind direction
and strength. The inclusion of wind is an important amendment to optimal migration theory of birds and should be explored
further.
Received: 1 March 1999 / Received in revised form: 4 October 1999 / Accepted: 16 October 1999 相似文献
4.
Carlos Palacín Juan C. Alonso Carlos A. Martín Javier A. Alonso 《Conservation biology》2017,31(1):106-115
Many bird populations have recently changed their migratory behavior in response to alterations of the environment. We collected data over 16 years on male Great Bustards (Otis tarda), a species showing a partial migratory pattern (sedentary and migratory birds coexisting in the same breeding groups). We conducted population counts and radio tracked 180 individuals to examine differences in survival rates between migratory and sedentary individuals and evaluate possible effects of these differences on the migratory pattern of the population. Overall, 65% of individuals migrated and 35% did not. The average distance between breeding and postbreeding areas of migrant individuals was 89.9 km, and the longest average movement of sedentary males was 3.8 km. Breeding group and migration distance had no effect on survival. However, mortality of migrants was 2.4 to 3.5 times higher than mortality of sedentary birds. For marked males, collision with power lines was the main cause of death from unnatural causes (37.6% of all deaths), and migratory birds died in collisions with power lines more frequently than sedentary birds (21.3% vs 6.3%). The percentage of sedentary individuals increased from 17% in 1997 to 45% in 2012. These results were consistent with data collected from radio‐tracked individuals: The proportion of migratory individuals decreased from 86% in 1997–1999 to 44% in 2006–2010. The observed decrease in the migratory tendency was not related to climatic changes (temperatures did not change over the study period) or improvements in habitat quality (dry cereal farmland area decreased in the main study area). Our findings suggest that human‐induced mortality during migration may be an important factor shaping the migration patterns of species inhabiting humanized landscapes. 相似文献
5.
Migrating birds often alternate between flight steps, when distance is covered and energy consumed, and stopover periods,
when energy reserves are restored. An alternative strategy is fly-and-forage migration, useful mainly for birds that hunt
or locate their prey in flight, and thus, enables birds to combine foraging with covering migration distance. The favourability
of this strategy in comparison with the traditional stopover strategy depends on costs of reduced effective travel speed and
benefits of offsetting energy consumption during migration flights. Evaluating these cost-benefit effects, we predict that
fly-and-forage migration is favourable under many conditions (increasing total migration speed), both as a pure strategy and
in combination with stopover behaviour. We used the osprey (Pandion haliaetus) as test case for investigating the importance of this strategy during spring and autumn migration at a lake in southern
Sweden. The majority, 78%, of passing ospreys behaved according to the fly-and-forage migration strategy by deviating from
their migratory track to visit or forage at the lake, while 12% migrated past the lake without response, and 10% made stopovers
at the lake. Foraging success of passing ospreys was almost as good as for birds on stopover. Timing of foraging demonstrated
that the birds adopted a genuine fly-and-forage strategy rather than intensified foraging before and after the daily travelling
period. We predict that fly-and-forage migration is widely used and important among many species besides the osprey, and the
exploration of its occurrence and consequences will be a challenging task in the field of optimal migration. 相似文献
6.
Håkan Karlsson Johan Bäckman Cecilia Nilsson Thomas Alerstam 《Behavioral ecology and sociobiology》2010,64(12):2021-2031
Using tracking radars, we investigated the variability of flight directions of long-distance nocturnal passerine migrants
across seasons (spring versus autumn migration) and sites at the southern (56° N) and northern (68° N) ends of the Scandinavian
Peninsula (Lund versus Abisko). Whilst most migrants at Lund are on passage to and from breeding sites in Fennoscandia, the
majority of the migrants at Abisko are close to their breeding sites, and migration at Abisko thus to a large degree reflects
initial departure from breeding sites (autumn) or final approach to breeding destinations (spring). The radar data were used
to test predictions about differences in orientation and wind drift effects between adult and juvenile birds (a large proportion
of autumn migrants consists of juvenile birds on their first journey), between situations far away from or near the goals
and between different phases of migration (initial departure, en route passage, final approach to goal). The concentrations
(both total and within-night concentrations) of flight directions differed significantly between seasons as well as sites,
with the highest concentration at Lund in spring (mean vector length of track directions, r = 0.79) and lowest at Abisko during spring (r = 0.35). Partial wind drift and partial compensation were recorded at Lund, with a similar effect size in spring and autumn,
whilst possible wind drift effects at Abisko were obscured by the large directional scatter at this site. The results from
Lund support the prediction that the high proportion of juveniles in autumn contributes to increase the directional scatter
during this season, whilst there was no support for predictions of differential wind drift effects between seasons and situations
with different goal distances. The most striking and surprising result was the exceedingly large scatter of flight directions
at Abisko, particularly in spring. We suggest that such an exaggerated scatter may be associated with final approach orientation,
where migrants reach their specific goals from all various directions by final navigation within a more wide-ranging goal
region. The larger scatter of autumn flight directions at Abisko compared to Lund may be due to exploratory flights in variable
directions being more common at initial departure from breeding sites than later during migratory passage. These surprising
results highlight the importance of studying and analysing orientation during final approach to (and initial departure from)
migratory goals for understanding the orientation systems of migratory birds. 相似文献
7.
S. Åkesson Lennart Karlsson Göran Walinder Thomas Alerstam 《Behavioral ecology and sociobiology》1996,38(5):293-302
Extensive ringing data from a coastal site (Falsterbo Bird Observatory) in southwesternmost Sweden were used to investigate
the occurrence of reverse autumn migration among 20 passerine bird species of widely different migration categories. The data
demonstrate that reverse migration is a widespread and regular phenomenon among nocturnal as well as diurnal migrants and
among irruptive migrants, temperate zone migrants, and long-distance migrants destined for tropical winter quarters. The reoriented
movements were directed approximately opposite to the normal migration direction, i.e. between NNW and ENE from the coast
and towards inland. Median distances of reverse movements varied between 9 and 65 km. Some individuals of irruptive and partial
migrants settled to winter in the reverse direction. Bird species with relatively small fat reserves at capture were more
likely to perform reverse migratory movements than species with larger fat deposits. In two species birds performing forward
migration were significantly heavier within 10 days after capture than individuals performing reverse movements. The reoriented
movements probably are of adaptive significance for birds confronted with the sea and pre-disposed to refuelling during migration.
A bimodal orientation mechanism will bring the birds from an area with high competition for food and high predation risk to
more suitable resting and feeding grounds before resuming migration in the forward direction and crossing the barrier.
Received: 11 July 1995/Accepted after revision: 19 November 1995 相似文献
8.
Peter Weindler Frank Böhme Varis Liepa Wolfgang Wiltschko 《Behavioral ecology and sociobiology》1998,42(4):289-294
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 相似文献
9.
It has been argued that the body mass levels achieved by birds are determined by the trade-off between risks of starvation
and predation. Birds have also been found to reduce body mass in response to an increased predation risk. During migration,
the need of extra fuel for flights is obvious and crucial. In this study, migratory blackcaps (Sylvia atricapilla) were subject to an experimental stopover situation where the predation risk was manipulated by exposure to a stuffed predator.
Blackcaps that perceived an imminent risk of predation increased their food intake and fuel deposition rate during the first
period of stopover compared with a control group. The pattern of night activity indicates that birds that were exposed to
the predator also chose to leave earlier than birds in the control group. Since there was no cover present at the stopover
site, birds might have perceived the risk of predation as higher regardless of whether they were foraging or not. Under such
circumstances it has been predicted that birds should increase their foraging activity. The findings in this study clearly
indicate that birds are able to adjust their stopover behaviour to perceived predation risk.
Received: 8 January 1997 / Accepted after revision: 11 April 1997 相似文献
10.
Roswitha Wiltschko Cordula Haugh Michael Walker Wolfgang Wiltschko 《Behavioral ecology and sociobiology》1998,43(4-5):297-300
Although the sun compass of birds is based on learning the sun's arc during development, it was unclear whether birds can
use the sun when its apparent movement is reversed, in particular, whether northern birds that have been introduced into the
southern hemisphere can use the southern sun. To answer this question, clock-shift experiments were performed with local homing
pigeons in Auckland, New Zealand (37°S). In three fast-shift tests and two slow-shift tests, the experimental birds showed
deflections from the untreated controls that were the mirror images of those observed in the northern hemisphere. These results
clearly show that homing pigeons in New Zealand use a sun compass that is adapted to the situation in the southern hemisphere.
The learning processes establishing the compensation mechanisms thus appear to be free of constraints concerning the direction
of the sun's movement. Differences from recent findings with migratory birds, where the direction of celestial rotation proved
of crucial importance for establishing the migratory direction, are discussed: the differences may arise from the different
orientation tasks, in particular, from the involvement of innate information in establishing the migratory direction.
Received: 13 November 1997 / Accepted after revision: 28 February 1998 相似文献
11.
Birds migrating along coastlines may be at increased risk if displacement occurs toward open-ocean. Eastern North America
experiences prevailing northwesterly winds during autumn, which could compel some migrants to drift eastward. Therefore, migrants
at stop-over sites along this route may be a mixture of on- and off-course individuals. We assessed whether orientation behavior
of juvenile yellow-rumped warblers (Dendroica coronata) captured at a stop-over site in southern Nova Scotia was related to where they originated from (i.e., likely on/off-course).
We hypothesized three scenarios after displacement: 1) continued orientation in the migratory direction selected before displacement,
2) orientation from the new location toward the previous destination, or 3) correction to regain the original pathway. Using
stable isotopes, we determined that stop-over migrants originated from nearby areas (and assumed on-course) and as far away
as western Hudson Bay, over 1,600 km northwest (and assumed off-course) of the site. We used video-based orientation registration
cages to determine an individuals’ migratory orientation. Because numerous factors influence migratory orientation (e.g.,
fuel reserves, celestial cues), we simultaneously assessed the influence of body condition and cloud cover, in addition to
geographic origin, on orientation behavior. Individuals that originated closer to the site tended to orient more southwesterly.
Orientation directions became increasingly more west-northwesterly the further away an individual originated from the site
(i.e., the more likely it was to have been displaced). The result is most consistent with scenario three: individuals from
northwest origins likely respond to easterly displacement by orienting westerly to reestablish their previous migratory route. 相似文献
12.
Time of departure and landing of nocturnal migrants are of great importance for understanding migratory strategy used by birds.
It allows us to estimate flying time and hence the distance that migrants cover during a single night. In this paper, I studied
the temporal schedule of nocturnal departures of European robins during spring migration. The study was done on the Courish
Spit on the Baltic Sea in 1998–2003 by retrapping 51 ringed birds in high mist nets during nocturnal migratory departure.
Take-offs of individual birds occurred between the first and tenth hour after sunset (median 176 min after sunset). Departure
time was not related to fuel stores at arrival and departure, stopover duration and progress of the season. The results suggest
that one reason for temporal variation in take-off time was differential response of European robins with high and low motivation
to depart to such triggers as air pressure and its trend. If these parameters reach a certain minimum threshold shortly before
sunset, robins with a high migratory motivation take off in the beginning of the night. When air pressure or its trend reaches
a maximum, it may trigger to take off later during the night birds with lower initial motivation for departure, including
those that have low refuelling efficiency. In regulation of timing of take-offs of robins, an important role is also played
by their individual endogenous circadian rhythm of activity which is related to the environment in a complex way. 相似文献
13.
Summary Reoriented autumn migration of chaffinches and bramblings occurs regularly in southernmost Sweden. The reoriented birds fly in a northeasterrly direction from the coast and inland, i.e. approximately opposite to the normal autumn migration direction. The daily peak of reoriented finch migration, as observed at inland sites 20–40 km from the coast, occurs on average 3.5 h later than the early morning departure in the normal migratory direction, and 1 h later than the peak of migration at the coast. According to trapping data the average weight of reoriented migrants and birds interrupting their migration at the coastline is significantly lower than the weight of migrants proceeding in the normal direction, and the proportion of yearlings seems to be larger in the former category. Censuses of flocks of resting finches showed that they mainly forage at stubble fields of summer rape Brassica napus, preferably fields surrounded by wooded vegetation offering shelter from predator attacks. Preferred food and habitats are mostly located inland, 20 km or more from the coast. These findings are consistent with the interpretation that reorientation constitutes an adaptive response by migrants with small fat reserves. When confronted with an ecological barrier, they return to suitable resting sites for restoring the fat reserves before crossing the barrier. 相似文献
14.
Frédéric Angelier Henri Weimerskirch Stéphanie Dano Olivier Chastel 《Behavioral ecology and sociobiology》2007,61(4):611-621
The ultimate factors influencing age-specific reproductive performance in birds have been widely discussed, and several hypotheses
have been suggested to explain why young/inexperienced breeders have lower reproductive success than older individuals. In
comparison, proximate factors and, particularly, hormonal mechanisms influencing age-related reproductive performance have
received lesser attention. In this paper, we examined how baseline levels of corticosterone and prolactin, two hormones involved
in reproduction, changed with age and experience in a long-lived bird, the Black-browed albatross (Thallasarche melanophris) during the brooding stage. Corticosterone levels were not linked to age, whereas prolactin levels increased until individuals
reached 15 years of age. First-time breeders had higher corticosterone levels and lower prolactin levels than experienced
ones. Corticosterone levels were not correlated with breeding experience among experienced birds, whereas prolactin levels
slightly increased with advancing experience. Among experienced breeders, there was no effect of individual quality on corticosterone
and prolactin levels. Baseline corticosterone and prolactin levels were respectively, positively and negatively correlated
to time spent fasting/brooding on the nest. Moreover, the probability of successfully fledging a chick was negatively related
to corticosterone levels, but not to prolactin levels. Elevated corticosterone levels in first time breeders may serve as
evidence for depleted body reserves resulting from lower foraging/brooding capabilities and therefore support the constraint
hypothesis. Low prolactin levels in young/inexperienced birds may be interpreted either as evidence for their lower breeding
capacities (constraint hypothesis) or for their limited breeding investment (restraint hypothesis). Finally, we report, for
the first time, the hormonal changes associated with the onset of senescence. The very old and most experienced birds, which
had the lowest probability of successfully fledging a young, displayed elevated corticosterone levels and low prolactin levels,
possibly indicating a degradation of breeding skills and/or a disruption of the endocrine system in senescent birds. 相似文献
15.
Kenneth P. Able William F. Gergits Jeffrey D. Cherry Scott B. Terrill 《Behavioral ecology and sociobiology》1984,15(1):39-43
Summary Using the wood thrush (Hylocichla mustelina) as an example of a typical nocturnal migrant, we employed radio telemetry to follow breeding birds during the course of homing movements following displacements. Seven thrushes were displaced over distances of 6.5 to 17.3 km in a variety of directions from their nesting territories. The thrushes moved in a series of short flights (mean = 2.1 km) performed primarily at dawn. Consequently, the birds took several days to home from even these relatively short displacements. Thrushes flew under clear and solid overcast skies and even in light rain. The pooled individual flights of the birds were significantly oriented in the homeward direction (Fig. 2). Their orientation relative to home did not improve significantly as they progressed toward the goal. Three of the birds were documented to have returned to their home territories. The detailed tracks of the birds preclude the possibility that they homed by random search. 相似文献
16.
Zachary M. Laubach Daniel T. Blumstein L. Michael Romero Greg Sampson Johannes Foufopoulos 《Behavioral ecology and sociobiology》2013,67(3):481-492
Status badges, such as crown plumage, mediate intraspecific interactions. The reliability of crown morphology as a status badge in male mountain white-crowned sparrows (Zonotrichia leucophrys oriantha) is uncertain. We examined morphological and physiological correlates of the proportion of crown that was white (“crown-white”) in 178 male mountain white-crowned sparrows during the 2008–2009 breeding seasons. Using a paired experimental design, we presented territory-holding males with white-enhanced and white-reduced decoys and recorded aggressive behaviors. To assess physiological constraints on signal bluffing, a subsample of birds was captured and released after manipulating natural crowns to simulate bluffed white-enhanced or white-reduced crowns; corticosterone concentrations were assayed from blood drawn upon recapture and after a restraint-induced stressor. We found a significant positive association between crown-white and a measure of body size—tarsus length—which is an established indicator of resource-holding potential. In the decoy challenge, males responded more aggressively toward white-enhanced than white-reduced decoys. In the hormone experiment, white-enhanced birds had higher baseline corticosterone levels, whereas white-reduced birds had similar concentrations to controls. Furthermore, white-enhanced birds had an attenuated restraint-induced corticosterone response, while white-reduced birds mounted a significantly larger increase in corticosterone than controls. Taken together, these findings indicate that crown-white is a reliable status badge of resource-holding potential in male mountain white-crowned sparrows during the breeding season. 相似文献
17.
Jannika E. Boström Thord Fransson Ian Henshaw Sven Jakobsson Cecilia Kullberg Susanne Åkesson 《Behavioral ecology and sociobiology》2010,64(11):1725-1732
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. 相似文献
18.
Ian Henshaw Thord Fransson Sven Jakobsson Cecilia Kullberg 《Behavioral ecology and sociobiology》2010,64(8):1317-1323
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. 相似文献
19.
Jacob Kiepenheuer 《Behavioral ecology and sociobiology》1984,14(2):81-99
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. 相似文献
20.
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 相似文献