Individuality in bird migration: routes and timing

The exploration of animal migration has entered a new era with individual-based tracking during multiple years. Here, we investigated repeated migratory journeys of a long-distance migrating bird, the marsh harrier Circus aeruginosus, in order to analyse the variation within and between individuals with respect to routes and timing. We found that there was a stronger individual repeatability in time than in space. Thus, the annual timing of migration varied much less between repeated journeys of the same individual than between different individuals, while there was considerable variation in the routes of the same individual on repeated journeys. The overall contrast in repeatability between time and space was unexpected and may be owing to strong endogenous control of timing, while short-term variation in environmental conditions (weather and habitat) might promote route flexibility. The individual variation in migration routes indicates that the birds navigate mainly by other means than detailed route recapitulation based on landmark recognition.     

Breeding latitude predicts timing but not rate of spring migration in a widespread migratory bird in South America

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Repeatability of individual migration routes,wintering sites,and timing in a long‐distance migrant bird

Migratory birds are often faithful to wintering (nonbreeding) sites, and also migration timing is usually remarkably consistent, that is, highly repeatable. Spatiotemporal repeatability can be of advantage for multiple reasons, including familiarity with local resources and predators as well as avoiding the costs of finding a new place, for example, nesting grounds. However, when the environment is variable in space and time, variable site selection and timing might be more rewarding. To date, studies on spatial and temporal repeatability in short‐lived long‐distance migrants are scarce, most notably of first‐time and subsequent migrations. Here, we investigated repeatability in autumn migration directions, wintering sites, and annual migration timing in Hoopoes (Upupa epops), a long‐distance migrant, using repeated tracks of adult and first‐time migrants. Even though autumn migration directions were mostly the same, individual wintering sites often changed from year to year with distances between wintering sites exceeding 1,000 km. The timing of migration was repeatable within an individual during autumn, but not during spring migration. We suggest that Hoopoes respond to variable environmental conditions such as north–south shifts in rainfall during winter and differing onset of the food availability during spring migration.     

Male response to historical and geographical variation in bird song

In many species, individuals discriminate among sexual signals of conspecific populations in the contexts of mate choice and male–male competition. Differences in signals among populations (geographical variation) are in part the result of signal evolution within populations (temporal variation). Understanding the relative effect of temporal and geographical signal variation on signal salience may therefore provide insight into the evolution of behavioural discrimination. However, no study, to my knowledge, has compared behavioural response to historical signals with response to current signal variation among populations. Here, I measured the response of male white-crowned sparrows (Zonotrichia leucophrys) to historical songs compared with current songs from their local population, a nearby non-local population and a distant population. Males responded most strongly to current local songs, less, but equally, to historical local and current non-local songs, and least to songs of the distant population. Moreover, response to both temporal and geographical variation in song was proportional to how much songs differed acoustically from current local songs. Signal evolution on an ecological time scale appears to have an effect on signal salience comparable to differences found between current neighbouring populations, supporting the idea that behavioural discrimination among learned signals of conspecific populations can evolve relatively rapidly.     

Pairing context determines condition-dependence of song rate in a monogamous passerine bird

Condition-dependence of male ornaments is thought to provide honest signals on which females can base their sexual choice for genetic quality. Recent studies show that condition-dependence patterns can vary within populations. Although long-term association is thought to promote honest signalling, no study has explored the influence of pairing context on the condition-dependence of male ornaments. In this study, we assessed the influence of natural variation in body condition on song rate in zebra finches (Taeniopygia guttata) in three different situations: during short and long encounters with an unfamiliar female, and within heterosexual mated pairs. We found consistent individual differences in male directed and undirected song rate. Moreover, body condition had a positive effect on song rate in paired males. However, male song rate was not influenced by body condition during short or long encounters with unfamiliar females. Song rate appears to be an unreliable signal of condition to prospective females as even poor-condition birds can cheat and sing at a high rate. By contrast, paired females can reliably use song rate to assess their mate''s body condition, and possibly the genetic quality. We propose that species'' characteristics, such as mating system, should be systematically taken into account to generate relevant hypotheses about the evolution of condition-dependent male ornaments.     

Rainfall at African wintering grounds predicts age‐specific probability of haemosporidian infection in a migratory passerine bird

In migratory species breeding in temperate zones and wintering in tropical areas, the prevalence of blood parasites may be affected by migratory strategies and winter habitat choice. We explored whether African winter habitat was linked to the probability of haemosporidian infection in the House Martin Delichon urbicum breeding in Spain, and tested for potential differences between age‐classes. As a proxy for winter habitat features, we analysed stable isotope (δ²H, δ¹³C and δ¹⁵N) values of winter‐grown feathers moulted in tropical Africa. Rainfall at the African winter grounds was related to the probability of being infected with haemosporidians and this effect differed among age‐classes. We found that haemosporidian prevalence was similar for young and experienced birds wintering in habitats of higher rainfall (²H‐depleted), whereas there were great differences in winter habitats of lower rainfall (²H‐enriched), with young having a much higher prevalence compared with experienced birds. Likewise, experienced birds wintering in habitats of higher rainfall had a higher probability of haemosporidian infection compared with experienced birds wintering in habitats of lower rainfall. By contrast, young birds wintering in habitats of lower rainfall had a higher probability of haemosporidian infection compared with young birds wintering in habitats of higher rainfall. These outcomes highlight the interaction of age with haemosporidian infection in the migratory ecology of the House Martin, which may drive carry‐over effects in this long‐distance aerial insectivore.     

Climate change leads to differential shifts in the timing of annual cycle stages in a migratory bird

Shifts in reproductive phenology due to climate change have been well documented in many species but how, within the same species, other annual cycle stages (e.g. moult, migration) shift relative to the timing of breeding has rarely been studied. When stages shift at different rates, the interval between stages may change resulting in overlaps, and as each stage is energetically demanding, these overlaps may have negative fitness consequences. We used long‐term data of a population of European pied flycatchers (Ficedula hypoleuca) to investigate phenological shifts in three annual cycle stages: spring migration (arrival dates), breeding (egg‐laying and hatching dates) and the onset of postbreeding moult. We found different advancements in the timing of breeding compared with moult (moult advances faster) and no advancement in arrival dates. To understand these differential shifts, we explored which temperatures best explain the year‐to‐year variation in the timing of these stages, and show that they respond differently to temperature increases in the Netherlands, causing the intervals between arrival and breeding and between breeding and moult to decrease. Next, we tested the fitness consequences of these shortened intervals. We found no effect on clutch size, but the probability of a fledged chick to recruit increased with a shorter arrival‐breeding interval (earlier breeding). Finally, mark–recapture analyses did not detect an effect of shortened intervals on adult survival. Our results suggest that the advancement of breeding allows more time for fledgling development, increasing their probability to recruit. This may incur costs to other parts of the annual cycle, but, despite the shorter intervals, there was no effect on adult survival. Our results show that to fully understand the consequences of climate change, it is necessary to look carefully at different annual cycle stages, especially for organisms with complex cycles, such as migratory birds.     

Roles of photoperiod and testosterone in seasonal plasticity of the avian song control system

The song control nuclei of songbirds undergo pronounced seasonal changes in size and neuronal attributes. The mechanisms by which seasonal changes in environmental variables such as photoperiod mediate seasonal changes in these brain regions are not known. Manipulations of photoperiod and/or testosterone in captive songbirds induce seasonal changes in the size of song nuclei comparable to those observed in wild songbirds. It is unclear, however, whether the effects of photoperiod on the song nuclei are mediated by testosterone or by steroid-independent mechanisms. We independently manipulated photoperiod and testosterone in castrated male Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) to determine the contributions of steroid-dependent and -independent actions of photoperiod to seasonal changes in the size and neuronal attributes of song nuclei. Testosterone implants increased the size of several song nuclei, regardless of photoperiod. Photoperiod exerted small but significant steroid-independent effects on the volume of the higher vocal center and the size of neurons in the robust nucleus of the archistriatum. Photoperiod also modulated the effect of testosterone on the size of area X; testosterone treatment had a more pronounced effect on the size of area X on short days than on long days. These results suggest that although testosterone is the primary factor mediating seasonal changes in neural attributes of the song nuclei, photoperiod may act via mechanisms that are independent of steroid levels to supplement or modulate the actions of testosterone. © 1997 John Wiley & Sons, Inc. J Neurobiol 32: 426–442, 1997.     

Migratory connectivity and population-specific migration routes in a long-distance migratory bird

Knowledge about migratory connectivity, the degree to which individuals from the same breeding site migrate to the same wintering site, is essential to understand processes affecting populations of migrants throughout the annual cycle. Here, we study the migration system of a long-distance migratory bird, the Montagu''s harrier Circus pygargus, by tracking individuals from different breeding populations throughout northern Europe. We identified three main migration routes towards wintering areas in sub-Saharan Africa. Wintering areas and migration routes of different breeding populations overlapped, a pattern best described by ‘weak (diffuse) connectivity’. Migratory performance, i.e. timing, duration, distance and speed of migration, was surprisingly similar for the three routes despite differences in habitat characteristics. This study provides, to our knowledge, a first comprehensive overview of the migration system of a Palaearctic-African long-distance migrant. We emphasize the importance of spatial scale (e.g. distances between breeding populations) in defining patterns of connectivity and suggest that knowledge about fundamental aspects determining distribution patterns, such as the among-individual variation in mean migration directions, is required to ultimately understand migratory connectivity. Furthermore, we stress that for conservation purposes it is pivotal to consider wintering areas as well as migration routes and in particular stopover sites.     

中国侯鸟环志的研究

环志是研究鸟类迁徙最简单有效的科学方法。中国从１９８２年开始建立全国鸟类环志中心以来,在林业部的领导下至１９９０年先后建立了候鸟环志站、点６０处,共计环志了候鸟１８６种６２７５５只,同期回收到国内外环志鸟４１３只。回收环志鸟提供了东亚地区候鸟迁徙的宝贵信息,证实了：１．候鸟迁徙可自喜马拉雅山脉飞越;２．中国候鸟迁徙路线和推论是正确的;３．中国东北繁殖的白枕鹤飞往日本九洲南部越冬,丹顶鹤等迁往东部沿海、长江中下游几处湿地越冬,与来自俄罗斯西伯利亚的部分鹤在相同地区越冬;４．某些鸟种迁徙路线在与日本同种候鸟迁徙路线比较后,表明在西太平洋地区,通过中国和日本存在着两条平行的自南向北迁徙的路线。其他信息也指出了更多的情况,同时也表明中国环志研究在西太平洋迁徙候鸟研究中处于关键性的地位。     

Experimental evidence for real-time song frequency shift in response to urban noise in a passerine bird

Research has shown that bird songs are modified in different ways to deal with urban noise and promote signal transmission through noisy environments. Urban noise is composed of low frequencies, thus the observation that songs have a higher minimum frequency in noisy places suggests this is a way of avoiding noise masking. Most studies are correlative and there is as yet little experimental evidence that this is a short-term mechanism owing to individual plasticity. Here we experimentally test if house finches (Carpodacus mexicanus) can modulate the minimum frequency of their songs in response to different noise levels. We exposed singing males to three continuous treatments: low–high–low noise levels. We found a significant increase in minimum frequency from low to high and a decrement from high to low treatments. We also found that this was mostly achieved by modifying the frequency of the same low-frequency syllable types used in the different treatments. When different low-frequency syllables were used, those sung during the noisy condition were longer than the ones sang during the quiet condition. We conclude that house finches modify their songs in several ways in response to urban noise, thus providing evidence of a short-term acoustic adaptation.     

The Song Overlap Null model Generator (SONG): a new tool for distinguishing between random and non-random song overlap

Song overlapping, a behaviour in which an individual begins singing before its counterpart has completed its song, has been the subject of recent debate. Although many studies have suggested that song overlapping functions as a signal, the majority of these studies fail to address the possibility that overlapping is a chance occurrence. Part of the difficulty in determining whether overlap is intentional or accidental lies in the lack of compelling null models for estimating chance levels of song overlap. We have developed the Song Overlap Null model Generator (SONG), a software package for R. SONG uses resampling randomization to predict the expected amount of overlap due to chance, and is applicable to any system in which individuals engage in signalling interactions. To evaluate the effectiveness of SONG, we examined the overlapping behaviour of three avian species: black-capped chickadees (Poecile atricapillus), rufous-and-white wrens (Thryophilus rufalbus) and long-tailed manakins (Chiroxiphia linearis). Our analyses revealed that black-capped chickadees avoided overlapping the songs of playback-simulated intruders, duetting wrens overlapped the songs of their mates and manakins avoided overlapping the duets of their neighbours. We believe that SONG will prove to be a valuable tool for understanding signal timing in songbirds as well as other taxa.     

Global change and the distributional dynamics of migratory bird populations wintering in Central America

Understanding the susceptibility of highly mobile taxa such as migratory birds to global change requires information on geographic patterns of occurrence across the annual cycle. Neotropical migrants that breed in North America and winter in Central America occur in high concentrations on their non‐breeding grounds where they spend the majority of the year and where habitat loss has been associated with population declines. Here, we use eBird data to model weekly patterns of abundance and occurrence for 21 forest passerine species that winter in Central America. We estimate species’ distributional dynamics across the annual cycle, which we use to determine how species are currently associated with public protected areas and projected changes in climate and land‐use. The effects of global change on the non‐breeding grounds is characterized by decreasing precipitation, especially during the summer, and the conversion of forest to cropland, grassland, or peri‐urban. The effects of global change on the breeding grounds are characterized by increasing winter precipitation, higher temperatures, and the conversion of forest to peri‐urban. During spring and autumn migration, species are projected to encounter higher temperatures, forests that have been converted to peri‐urban, and increased precipitation during spring migration. Based on current distributional dynamics, susceptibility to global change is characterized by the loss of forested habitats on the non‐breeding grounds, warming temperatures during migration and on the breeding grounds, and declining summer rainfall on the non‐breeding grounds. Public protected areas with low and medium protection status are more prevalent on the non‐breeding grounds, suggesting that management opportunities currently exist to mitigate near‐term non‐breeding habitat losses. These efforts would affect more individuals of more species during a longer period of the annual cycle, which may create additional opportunities for species to respond to changes in habitat or phenology that are likely to develop under climate change.