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     

Prior residency as a key determinant of social dominance in the willow tit (Parus montanus)

Summary We studied how age, body size and prior residency affected social dominance in the willow tit (Parus montanus) groups. The contribution of each variable was experimentally tested in unisexual two-bird trials, in which the birds were matched for all variables except the one studied. Large birds were dominant over smaller ones (Fig. 1). The effect of body size was more prominent in males than in females. Age had no influence on dominance. Residents became dominant more often than newcomers (Fig. 2). Adulthood or larger body size did not override the advantage of prior residency (Fig. 2). Therefore, the proximate reason for the age-dependent dominance seen in natural willow tit flocks is most likely the prior residency advantage of the adults. Factors connected with fighting ability (body size and age) seem to be less important than the time of establishment of rank, which may reflect the importance of resource value differences between residents and newcomers in this context. The advantage of residency might make it advantageous to be a member of a flock even as a subordinate, rather than being solitary.     

Temporal performance roles during vocal interactions in nightingales (Luscinia megarhynchos B.)

Summary The temporal patterning of vocal interactions between territorial nightingales (Luscinia megarhynchos B). was investigated during nocturnal dyadic singing. We distinguished three forms of temporal performance roles (Table 1; Fig. 3): inserter (preferred song start 0.5–1.0 s after offset of a neighbour's song), overlapper (preferred song start 0.5–1.0 s after onset of a neighbour's song), and Autonomous bird (no evidence for temporal responses adjusted to the timing of a neighbour's singing; Fig. 4). With the beginning of the daily dawn chorus, mutual temporal adjustment of song performances could no longer be ascertained (Fig. 2). To test the flexibility of song onset timing, we presented subjects with three playback programs (I, II, III) simulating a non-flexible conspecific bird. Sequences of songs recorded from a stranger that shared no song types with the subjects were used; intersong pauses were unaltered in I, lengtened in II, and shortened in III. Results confirmed that the timing of song onset can be influenced by the temporal patterning of auditory stimuli (Table 1; Fig. 5): Inserters and overlappers sang at a slower rate during program II than during program I. During program III, which by accelerated stimulus succession caused frequent overlap of songs, inserters and overlappers interrupted their singing behaviour (Fig. 6). This suggests that overlapping, which results in signal interference, is of some functional significance in vocal interactions.     

Sympatric corvids

Summary Overlap in winter resource use by four farmland corvid species was insufficient to predict any costly interspecific aggression. Rates of inexpensive displacement aggression were too low to affect intake rates of prey, and were not related to flock size. Eurasian crows also chase-attacked corvids and other species. This more costly form of aggression is interpreted not as interference aggression against food competitors, but as a response to the disturbance of earthworm prey into unavailability by the foraging movements of such flocks (Fig. 1). Rooks and jackdaws were less likely to leave the field following an attack if they were in larger flocks, but such flocks were attacked more frequently (Table 1).     

Ecological and social determinants of spacing behaviour in desert baboon groups

This paper investigates the determinants of individual spacing behaviour in a desert baboon population (Papio cynocephalus ursinus). Patterns of neighbour proximity and neighbour density were examined among adults in four groups under different ecological and social conditions (through instantaneous sampling during focal follows). Initial analysis of these data shows that (1) the use of vertical substrates (refuges such as tall trees and cliff faces) can confound patterns of spacing, and (2) individual differences in spacing can depend on the spatial scale over which it is measured. To minimise these substrate and scale effects, this analysis focuses on animals which are off refuges and examines spacing behaviour through its underlying statistical `dimensions' (identified through factor analysis). Analysis of these dimensions indicates that sex, group size, activity-habitat and female reproductive state can all have independent effects on spacing: (1) males are more dispersed than females in smaller groups, (2) male and female dispersion increases with time spent in foraging habitats, and (3) female dispersion is reduced during lactation. According to the hypotheses tested, these results indicate that feeding competition only affects spacing behaviour during foraging while predation risk plays little or no role in spacing. Most aspects of spacing behaviour are best explained by male reproductive strategies and their social repercussions. Received: 25 May 1998 / Accepted after revision: 18 July 1998     

Social influences on the whistled songs of starlings

Summary Playback experiments on individuals singing near their nest show that starlings tend to repeat the theme they have just heard even if it is an unfamiliar dialect and not programmed in their own current sequence. The type of response and its temporal characteristics could be related to internal sequential organization rules. Moreover, starlings discriminate between two dialectal variants of a given theme and respond more often and more quickly to the familiar dialect. A social influence on the choice of theme by the individual can also be observed in foraging flocks and roosts where song is frequently given. Observation of such groups reveal that one theme predominates on each given occasion and that it depends on the number of individuals present and on the type of group (flock or roost). This is probably a function of the tendency to match. Previous observations of flights toward the roost showed that birds originating from different dialectal areas congregate in such groups. A few analyses of songs in flocks and roosts confirm the presence of different dialects simultaneously.     

Individual variation in mate care by alpha males in crested tit winter flocks

Summary Winter flocks of crested tits Parus cristatus, typically consisting of two adults and one or two non-kin 1st-year birds, were observed to split up into subflocks in a way related to ambient temperature. On warm days, when they were foraging in subflocks without 1 st-year birds, alpha males always occupied the most preferred upper foraging sites, as expected from their top dominance rank. On cold days, when foraging in flocks with 1st-year birds, 8 out of 13 alpha males shifted to lower (less preferred) positions below their alpha mates while allowing the latter to forage at the best sites. As enhanced access to preferred microsites on days with high energy stress is believed to increase overwinter survival probability, this shifting behaviour of alpha males can be considered as a form of mate care. Out of 13 alpha males, however, 5 did not shift and always occupied the best foraging sites irrespective of flock composition. As (i) these non-shifting males were in poorer physical condition than shifting males, (ii) they scanned significantly less for predators than either females or shifting males when foraging in the uppermost tree parts, and (iii) four out of five non-shifting males were replaced by immigrants in early spring, absence of mate care during winter may be caused by constraints due to condition. High-quality-territory owners in poor condition at the end of autumn were most vulnerable to replacement by immigrants. Therefore, as four out of five replacements affected high-quality territories, selective intrusion by immigrants is suggested. Correspondence to: L. Lens     

Pigeon homing: the effect of a clock-shift is often smaller than predicted

This analysis is based on 103 releases with 6-h clock-shifted pigeons of various ages and experiences. Resetting the internal clock normally leads to a significant change in initial orientation; however, in half of the cases, the induced deflections are significantly smaller than predicted by the sun compass hypothesis. The relative size of the deflections decreases with increasing age and experience (Fig. 3). Only young pigeons with limited experience respond as expected, while old birds show deflections which are, on the average, only slightly more than half of the predicted size, except at extremely familiar sites (Table 2). There is no difference between fast and slow shifts (Fig. 4). It is not possible to clearly specify under what circumstances smaller deflections occur; previous clock-shifts (Fig. 5), familiarity with the release site (Table 4) and duration of the shifting procedure (Table 5) do not seem to be the reasons. Clock-shifting also tends to decrease the vector lengths and has a marked effect on homing performance (Table 7). Nevertheless, considerable numbers of clock-shifted birds return on the day of release before their internal clock has begun to be reset back to normal. The general role of the sun compass in bird orientation is considered and theoretical implications of our findings are discussed in view of the map and compass-model and the possibility that an alternative, non-time-compensating compass is used in parallel with the sun compass.     

Information transfer and gain in flocks: the effects of quality and quantity of social information at different neighbour distances

We assessed experimentally how the quality and quantity of social information affected foraging decisions of starlings (Sturnus vulgaris) at different neighbour distances, and how individuals gained social information as a function of head position. Our experimental set up comprised three bottomless enclosures, each housing one individual placed on a line at different distances. The birds in the extreme enclosures were labelled senders and the one in the centre receiver. We manipulated the foraging opportunities of senders (enhanced, natural, no-foraging), and recorded the behaviour of the receiver. In the first experiment, receivers responded to the condition of senders. Their searching rate and food intake increased when senders foraged in enhanced conditions, and decreased in no-foraging conditions, in relation to natural conditions. Scanning was oriented more in the direction of conspecifics when senders behaviour departed from normal. In the second experiment, responses were dose dependent: receivers increased their searching rate and orientated their gaze more towards conspecifics with the number of senders foraging in enhanced food conditions. In no-foraging conditions, receivers decreased their searching and intake rates with the number of senders, but no variation was found in scanning towards conspecifics. Differences in foraging and scanning behaviour between enhanced and no-foraging conditions were much lower when neighbours were separated farther. Overall, information transfer within starling flocks affects individual foraging and scanning behaviour, with receivers monitoring and copying senders behaviour mainly when neighbours are close. Information transfer may be related to predation information (responding to the vigilance of conspecifics) and foraging information (responding to the feeding success of conspecifics). Both sources of information, balanced by neighbour distance, may simultaneously affect the behaviour of individuals in natural conditions.Communicated by H. Kokko     

A comparison of sound propagation and song frequency in temperate marsh and grassland habitats

Summary Attenuation of pure tones was measured in marsh and grassland habitat. At surface level, in grassland, the ground effect strongly attenuated frequencies below 2.0 kHz (Fig. 2). The ground effect was reduced by increasing source-receiver elevation. In marsh habitat the ground effect did not occur, and low frequencies were optimal for sound propagation (Fig. 3). As predicted from sound propagation tests, analysis of recorded songs of seven grassland and six marsh species indicated that minimum and emphasized frequency were significantly lower in songs of marsh birds. Maximum frequency did not differ between habitats (Table 4). Buzzing songs of yellow-headed blackbirds were broadcast and re-recorded at 1 and 50 m from the speaker in marsh and grassland habitats. Low frequency components attenuated more rapidly than higher frequency components in grassland, and the reverse occurred in marsh. Results suggested that the ground effect restricts the use of low frequencies by grassland birds. This evidence is consistent with the sound window hypothesis of Morton.     

The evolution of marmot sociality: I. Why disperse late?

Summary Prolonged toleration of offspring in marmots was hypothesized to be (1) a means of preventing dispersal of undersized young (Barash 1974 a) or more generally (2) continued parental investment, increasing the probability of descendant survival and reproduction (Armitage 1981, 1987). These hypotheses are tested in this paper for one of the most social of marmot species, the alpine marmot. The animals studied lived in groups within territories defended by a dominant male and female, or as floaters lacking a well-defined home range. Offspring did not disperse before sexual maturity at age 2 (Fig. 1). Only territorial females bred, whereas territorial males were not able to monopolize reproduction likewise (Table 2). Dispersers had similar spring mass to nondispersers (Table 4). Hence, hypothesis 1 is not supported, at least not for adult-sized, > 2 years old animals. During their residency, 19% of subordinates obtained their natal territory or a neighboring one (Fig. 2). Long distance dispersal bore a high mortality risk. Thus, toleration of mature offspring could well represent parental investment. Other results, however, suggest additional influences on the timing of dispersal. (i) Males dispersed later than females (Fig. 3), possibly because of mate sharing by territorial males (see Emlen 1982). (ii) The higher mass loss of dispersers during the previous winter indicates that weak animals were forced to leave (Table 5) despite presumably lower chances of becoming territorial (Table 3). (iii) Subordinate animals which could not be the offspring of both territorials present were not more likely to disperse (Fig. 3). (iv) Lower dispersal rates when immatures lived in the group (Fig. 3) may indicate benefits from the subordinates' presence for rearing young.     

Sexual chases in sand martins (Riparia riparia): cues for males to increase their reproductive success

Summary During the pre-laying and laying stages of the breeding cycle, female sand martins are guarded by their mates and chased by other males seeking promiscuous copulations. Because females become exceptionally heavy when they are most likely to be fertile, their increased mass was thought to present cues during flight to males seeking promiscuous copulations. Heavy female sand martins released from the hand were selectively chased in sexual chases (Figs. 1, 2). Breeding females were heaviest during laying and pre-laying (Fig. 3), exceeding any masses normally achieved by breeding males (Fig. 3). A sample of naturally heavy females and birds whose mass had been experimentally increased to that of laying and prelaying females took longer to reach ascending flight, as determined by analysis of video recordings, than a sample of lighter birds (Table 1). It was concluded that this and other flight cues may be detected by males so that they may achieve extra-pair copulations and hence increase their reproductive success.     

Alarm calls of Belding's ground squirrels to aerial predators: nepotism or self-preservation?

Summary Belding's ground squirrels (Spermophilus beldingi) give acoustically distinct alarm calls to aerial and terrestrial predators. The animals typically give multiple-note trills to predatory mammals, and single-note whistles to flying hawks. During a 9-year study of free-living S. beldingi at Tioga Pass, California, the adaptive significance of the whistle call was investigated. Data were gathered on 664 ground squirrel-hawk interactions, most of which were induced by flying trained raptors over individually marked study animals of known sex and age. The sight of a flying hawk and the sound of whistles stimulated widespread calling and running to shelter by the ground squirrels (Fig. 1). Wild raptors were rarely successful at capturing the rodents once a whistle had been given, and fewer callers than noncallers were killed (Table 1). Individuals of both sexes and all ages whistled equally often (Fig. 4), and females' tendencies to whistle were not affected by the presence of relatives, including offspring (Fig. 5). The most frequent callers were animals in exposed positions: far from cover and close to the predatory bird (Table 2). Taken together the data suggest that unlike trills, which increase vulnerability to terrestrial predators (Table 1) and function to warn relatives, whistle directly benefit callers by increasing their chances of escaping from hawks.     

Erratum to: Beak colour reflects circulating carotenoid and vitamin A levels in spotless starlings (Sturnus unicolor)

Many colourful sexually selected signals in animals are carotenoid-dependent and, because carotenoids function as antiradicals and immunostimulating molecules, carotenoid-dependent signals may honestly reflect the health state of individuals. Some others nutrients like vitamin A may also enhance health and colouration, but these have rarely been tested alongside carotenoids in colourful birds. Here, we examined whether beak colour of the spotless starling (Sturnus unicolor) reflected circulating levels of carotenoids and/or vitamin A (retinol). Spotless starlings are polygynous, sexually dimorphic birds (i.e. length of chest feathers). The tip of the beaks of male and female spotless starlings is more intensely coloured at the beginning of the breeding season and becomes dull after mating, which may suggest a sexual function. We found that females have a more intensely coloured beak and higher plasma carotenoid concentration than males during mating, and, despite the finding that carotenoid and vitamin A levels were not significantly related; colour intensity was positively correlated with plasma concentration of carotenoids and vitamin A in both sexes. However, adult beak coloration was not associated with carotenoid and vitamin A concentrations after nestlings were hatched. Therefore, beak colouration of spotless starlings provides information about circulating levels of carotenoids and vitamins during the mating season and may potentially function as a reliable signal of physiological status in the context of sexual selection.     

Food abundance and parental care in yellow warblers (Dendroica petechia)

Emlen and Oring (1977) suggested that monogamy in birds is maintained because of the need for strict biparental care. A corollary of their suggestion is that paternal care should decrease under conditions of high food abundance. An alternative is that paternal care would increase if males take advantage of the higher food abundance by trying to reduce the length of the nestling feeding period. We tested these two ideas using yellow warblers (Dendroica petechia) by providing some pairs with supplemental food, thereby reducing the importance of biparental care. However, the extra food did not decrease paternal effort, nor did it increase it (Fig. 2). Early in the nestling period experimental females brooded more but visited their nestlings less than did control females, but later, when brooding times decreased, experimental females fed their nestlings more than did control females (Fig. 3). There were no significant differences in nestling survival (Fig. 5), but nestlings in the control treatment were larger and heavier up to 6 days old (Fig. 6). The main effect of supplemental food was on maternal, not paternal behaviour. Models of biparental care assume interdependence between the parental effort of both parents. In this species, however, males and females provide for their brood independently from each other.     

Inheritance of migratory direction in a bird species: a cross-breeding experiment with SE- and SW-migrating blackcaps (Sylvia atricapilla)

Summary Young avian migrants of many species are able to find their species- or population-specific wintering area without the help of conspecifics. In orientation tests hand-raised birds have been demonstrated to choose appropriate population-specific migratory directions, suggesting a genetic basis to this behaviour. I here report results of a cross-breeding experiment between individuals of two blackcap (Sylvia atricapilla) populations with widely different migratory directions. The orientation of the F1 offspring was intermediate between and significantly different from that of both parental populations (Fig. 2). The variance of individual mean directions in the F1 generation did not increase compared with the parental groups, and the inheritance of migratory directions was not sex-linked. The data provide direct evidence for a genetic basis of migratory directions in birds and demonstrate a phenotypically intermediate mode of inheritance.     

Female choice for complex song in the European starling: a field experiment

Male European starlings Sturnus vulgaris sing long complex songs that appear to be important in the courtship of females but which also influence competitive interactions between males. We tested the hypothesis that females choose mates on the basis of the complexity of their songs, rather than on the quality of the territories the males defended. In order to determine whether certain territories were preferred over others, the first set of birds to settle in the experimental nest-boxes was removed and a second set allowed to settle. Consistent preferences for certain nest-boxes were indicated by correlations between the settlement patterns of the first and second sets of birds. However, males with the most complex song did not necessarily occupy the most preferred nest sites. Males with more complex song acquired mates faster. This relationship remained significant when nest-site preference was statistically controlled, indicating that female starlings chose males with complex song rather than those that defended preferred nest sites. A number of morphological variables were also found to be uncorrelated with female choice. Song complexity in European starlings increases with age, and the evolution of song complexity in this species is consistent with an age-indicator model of sexual selection. Males with larger repertoires were also in better condition, indicating that females obtain high-quality mates by choosing on the basis of male song. Received: 29 April 1995/Accepted after revision: 9 September 1995     

Infrasonic calls of the Asian elephant (Elephas maximus)

Summary Calls at frequencies below the range of human hearing were recorded from two groups of captive Asian elephants (Elephas maximus). Most of the calls ranged in frequency from 14 to 24 Hz, with durations of 10–15 s (Fig. 1). With the nearest elephant 5 m from the microphone, sound pressure levels were 85 to 90 dB (re 20 Pa). These calls occurred in a variety of circumstances. Elephants are the first terrestrial mammals reported to produce infrasound. These calls may be important in the coordination of behavior in thick vegetation or among separated groups of elephants.     

The evolution of marmot sociality: II. Costs and benefits of joint hibernation

Summary Social groups of alpine marmots (Marmota marmota) were studied for 7 years. The groups consisted of a territorial pair and up to 18 lower ranking animals of various ages, mostly the pair's offspring (Tables 1, 2). Group members lived in a common home range and always hibernated together in one hibernaculum. Groups with older, subordinate animals experienced slightly higher summer mortality but significantly reduced winter mortality (Fig. 1). Infant winter mortality was further decreased if most older subordinates were potentially their full sibs (Fig. 2). Subordinate group members lost less mass during winter with increasing size of the hibernating group, but this trend was reversed when infants were present. Furthermore, augmented mass loss due to low hibernaculum quality became evident (Fig. 3). Apart from these effects, the presence of infants caused additional mass loss in potential full sibs. The opposite was found in subordinates certainly descending from other parents than those of the infants (Table 3).Winter mortality and mass loss data revealed (i) a general benefit of joint hibernation, (ii) an unavoidable cost of infants' presence to other group members, (iii) that only potential full sibs helped in warming infants, (iv) that helping was energetically expensive and increased infant survival. The evolution of postponed dispersal in ground-dwelling squirrels has been attributed to the direct fitness gained by enhanced reproductive chances of offspring when not expelled from the natal territory (Armitage 1981, 1987, 1988). This study shows that group living in alpine marmots has benefits during winter and indicates the additional importance of kin selection in marmot social evolution.