Climate and the complexity of migratory phenology: sexes,migratory distance,and arrival distributions

The intra- and inter-season complexity of bird migration has received limited attention in climatic change research. Our phenological analysis of 22 species collected in Chicago, USA, (1979–2002) evaluates the relationship between multi-scalar climate variables and differences (1) in arrival timing between sexes, (2) in arrival distributions among species, and (3) between spring and fall migration. The early migratory period for earliest arriving species (i.e., short-distance migrants) and earliest arriving individuals of a species (i.e., males) most frequently correlate with climate variables. Compared to long-distance migrant species, four times as many short-distance migrants correlate with spring temperature, while 8 of 11 (73%) of long-distance migrant species’ arrival is correlated with the North Atlantic Oscillation (NAO). While migratory phenology has been correlated with NAO in Europe, we believe that this is the first documentation of a significant association in North America. Geographically proximate conditions apparently influence migratory timing for short-distance migrants while continental-scale climate (e.g., NAO) seemingly influences the phenology of Neotropical migrants. The preponderance of climate correlations is with the early migratory period, not the median of arrival, suggesting that early spring conditions constrain the onset or rate of migration for some species. The seasonal arrival distribution provides considerable information about migratory passage beyond what is apparent from statistical analyses of phenology. A relationship between climate and fall phenology is not detected at this location. Analysis of the within-season complexity of migration, including multiple metrics of arrival, is essential to detect species’ responses to changing climate as well as evaluate the underlying biological mechanisms.     

Climate change effects on migration phenology may mismatch brood parasitic cuckoos and their hosts

Phenological responses to climate change vary among taxa and across trophic levels. This can lead to a mismatch between the life cycles of ecologically interrelated populations (e.g. predators and prey), with negative consequences for population dynamics of some of the interacting species. Here we provide, to our knowledge, the first evidence that climate change might disrupt the association between the life cycles of the common cuckoo (Cuculus canorus), a migratory brood parasitic bird, and its hosts. We investigated changes in timing of spring arrival of the cuckoo and its hosts throughout Europe over six decades, and found that short-distance, but not long-distance, migratory hosts have advanced their arrival more than the cuckoo. Hence, cuckoos may keep track of phenological changes of long-distance, but not short-distance migrant hosts, with potential consequences for breeding of both cuckoo and hosts. The mismatch to some of the important hosts may contribute to the decline of cuckoo populations and explain some of the observed local changes in parasitism rates of migratory hosts.     

Settling down of seasonal migrants promotes bird diversification

How seasonal migration originated and impacted diversification in birds remains largely unknown. Although migratory behaviour is likely to affect bird diversification, previous studies have not detected any effect. Here, we infer ancestral migratory behaviour and the effect of seasonal migration on speciation and extinction dynamics using a complete bird tree of life. Our analyses infer that sedentary behaviour is ancestral, and that migratory behaviour evolved independently multiple times during the evolutionary history of birds. Speciation of a sedentary species into two sedentary daughter species is more frequent than speciation of a migratory species into two migratory daughter species. However, migratory species often diversify by generating a sedentary daughter species in addition to the ancestral migratory one. This leads to an overall higher migratory speciation rate. Migratory species also experience lower extinction rates. Hence, although migratory species represent a minority (18.5%) of all extant birds, they have a higher net diversification rate than sedentary species. These results suggest that the evolution of seasonal migration in birds has facilitated diversification through the divergence of migratory subpopulations that become sedentary, and illustrate asymmetrical diversification as a mechanism by which diversification rates are decoupled from species richness.     

A comparative study of migratory behaviour and body mass as determinants of moult duration in passerines

Birds moult to maintain plumage function through life, but the factors that determine moult duration are poorly understood. In temperate areas, variation in moult duration could be largely associated with between-species differences in migratory behaviour (migrants have less time for moulting after breeding), and body mass (because the aerodynamic cost of rapid moult increases allometrically with body size). Moreover, if the energetic cost of transport favours a smaller body size in migratory species, then the effects of migratory behaviour and body mass on moult duration could be confounded. We conducted a comparative study of the duration of adult complete moult in 48 European passerine species, in relation to body mass and migratory behaviour (sedentary, short-distance migrants and long-distance migrants). Lighter and more migratory species moulted faster than heavier and more sedentary species, but migration was not associated with body mass. If accelerated moult compromises the success of migration, changes in the physiology or phenology of moult in migratory birds are better interpreted as adaptive responses to compensate for such costs.     

Constraints on dispersal and the evolution of the avifauna of the Northern Hemisphere

One could predict that the capacity for travelling great distances might predispose long-distance migratory birds to be good colonists and to exhibit wider geographic distributions than their non-migratory or short-distance migratory relatives or non-volant mammals. This prediction is not supported by the data on avian biogeography. The distributions of species, genera and families of North American and Eurasian birds and mammals are indeed related to migratory status, but long-distance migratory birds exhibit a great deal of biogeographic regionalism. In particular, at all taxonomic levels their distributions tend to be confined to either the Eastern or Western Hemisphere, suggesting that there has been little successful east–west dispersal between North America and Eurasia. Compared to non-migratory birds, short-distance migratory birds and non-volant mammals, long-distance migrants appear to be subject to severe constraints on their physiology, behaviour and ecology, w hich have prevented colonization of distant regions.     

Annual and seasonal survival of trumpeter swans in the upper midwest

The reintroduction of trumpeter swans to the north central United States appears to be a conservation success story. For the most part, population management goals have been met or exceeded. The population cannot be considered self-sustaining, however, because 90% of the swans migrate short distances to wintering sites where supplemental feeding occurs. The remaining 10% migrate longer distances to areas where adequate open water and forage occur naturally. To determine how these 2 different wintering habits might affect mortality, we used mark-resight data gathered between 2000 and 2008 to estimate and compare annual survival rates for long- and short-distance migrant swans marked in Wisconsin. Apparent annual survival rates were similar for long- (0.81, SE = 0.019) and short- (0.81, SE = 0.022) distant adult migrants but were higher for long-distance sub-adult (0.86, SE = 0.036) migrants than for short-distance sub-adult migrants (0.7, SE = 0.046). We also estimated seasonal survival of long-distance migrants to determine if the migratory periods are a time of high mortality. We found little evidence for seasonal variation in survival and estimates for both migratory and non-migratory seasons were very high (>0.97). Overall, the results suggest that little mortality occurs during migration and long-distance migrants are able to survive at rates at least equal to, but probably higher than, short-distance migrants. © 2011 The Wildlife Society.     

Postglacial population expansion drives the evolution of long-distance migration in a songbird

The evolution of long-distance migratory behavior from sedentary populations is a central problem in studies of animal migration. Three crucial issues that remain unresolved are: (1) the biotic and abiotic factors promoting evolution of migratory behavior, (2) the geographic origin of ancestral sedentary populations, and (3) the time scale over which migration evolves. We test the role of postglacial population expansions during the Quaternary in driving the evolution of songbird migration against prevailing views favoring the role of intraspecific competition. In contrast to previous attempts to investigate these questions using interspecific phylogenies, we adopt an intraspecific approach and examine the phylogeography of a North American songbird, the chipping sparrow (Spizella passerina), which exhibits both long-distance migratory behavior in temperate North America and sedentary behavior in Mexico and Central America. We show that migratory populations descend from sedentary populations in southern Mexico and that migration has evolved as a result of a northward population expansion into temperate North America since the last glacial maximum 18,000 years ago. Migration appears to have evolved rapidly in some species as populations colonized areas of high seasonality in the temperate zone. The phylogeography of the yellow-eyed junco (Junco phaeonotus), a strictly sedentary species, provides a null model supporting the view that northward range expansions were driven solely by environmental factors and not by a predisposition to evolve migratory behavior. These results provide the strongest evidence to date that historical climate patterns can drive the rapid evolution of avian migration in natural populations, and they suggest a general mechanism for the repeated evolution of migration within and across bird lineages.     

Seasonal niche-tracking behaviour of two partially migratory passerines

Migratory birds spend periods of the year in different locations as a response to seasonal changes in environmental suitability. They are classified as either ‘niche-trackers’ or ‘niche-switchers’, depending on whether they track or switch environmental conditions throughout the year. However, the relationship between these strategies and their migratory behaviour is still unclear. Here we examine whether migratory European Robins Erithacus rubecula and Eurasian Blackcaps Sylvia atricapilla track environmental conditions between breeding and wintering areas and whether their behaviour differs from that of their sedentary counterparts. We used ringing data for both migratory and sedentary individuals wintering sympatrically in the Iberian Peninsula to assess the environmental conditions relating to their seasonal distributions. We explored seasonal niche-tracking using two multivariate analyses with alternative sets of predictor variables (landscape and climate) to generate different environmental scenarios. Our results show that migratory individuals track similar climatic conditions throughout their seasonal distributions, whereas sedentary birds cope with great variation in climate over the course of the year. In addition, migratory birds show less seasonal overlap in the landscape structure of their chosen habitats compared with sedentary individuals. These results suggest that there is a trade-off between, on the one hand, the cost for migrants of travelling long distances and, on the other, the increased flexibility required by sedentary birds if they are to tolerate a wider suite of environmental conditions within their permanent ranges. Given that sedentary populations in the southern Iberian Peninsula seem to be linked to populations of migratory individuals of these two species that started to move northwards after the last glacial cycle, the observed patterns suggest that migrant birds represent a fraction of the southern population that is specialized in the exploitation of a narrower range of environmental conditions.     

Avian population consequences of climate change are most severe for long-distance migrants in seasonal habitats

One consequence of climate change is an increasing mismatch between timing of food requirements and food availability. Such a mismatch is primarily expected in avian long-distance migrants because of their complex annual cycle, and in habitats with a seasonal food peak. Here we show that insectivorous long-distance migrant species in The Netherlands declined strongly (1984–2004) in forests, a habitat characterized by a short spring food peak, but that they did not decline in less seasonal marshes. Also, within generalist long-distance migrant species, populations declined more strongly in forests than in marshes. Forest-inhabiting migrant species arriving latest in spring declined most sharply, probably because their mismatch with the peak in food supply is greatest. Residents and short-distance migrants had non-declining populations in both habitats, suggesting that habitat quality did not deteriorate. Habitat-related differences in trends were most probably caused by climate change because at a European scale, long-distance migrants in forests declined more severely in western Europe, where springs have become considerably warmer, when compared with northern Europe, where temperatures during spring arrival and breeding have increased less. Our results suggest that trophic mismatches may have become a major cause for population declines in long-distance migrants in highly seasonal habitats.     

Long-distance migrating species of birds travel in larger groups

How individuals migrate over long distances is an enduring mystery of animal migration. Strong selection pressure for travelling in groups has been suggested in long-distance migrating species. Travelling in groups can reduce the energetic demands of long migration, increase navigational accuracy and favour group foraging at migratory halts. Nevertheless, this hypothesis has received scant attention. I examined evolutionary transitions in migration distance in all North American breeding species of birds. I documented 72 evolutionary shifts in migration distance in the pool of 409 species. In contrasting clades, long-distance migration, as opposed to short-distance migration, was associated with a larger travelling group size. No other transitions occurred alongside in other traits such as group size in the non-breeding season or body mass. The results suggest that larger group sizes have been beneficial in the evolution of long-distance migration in a large clade of birds.     

Abundance – distribution relationships in fish assembly of the Amazonas floodplain lakes

This study shows that patterns in some community assemblages are not mainly governed by local factors but also by regional ones. Using field data from 36 floodplain forest lakes in the Amazon basin, we present transect count data on the richness, abundance and distribution of floodplain lake fish species. A total of 194 fish species were collected, of which 43 were classified as short-distance migrants. A positive relation was found for local migratory and sedentary species abundances with distribution at a regional scale. The study also suggests that the probability of the presence of a migratory species is more affected by aspects of river-lake connectivity than sedentary species. Our results seem to indicate that migratory species play an important role in local dynamics of floodplain lakes.     

Rapid change in host use of the common cuckoo Cuculus canorus linked to climate change

Parasites require synchrony with their hosts so if host timing changes with climate change, some parasites may decline and eventually go extinct. Residents and short-distance migrant hosts of the brood parasitic common cuckoo, Cuculus canorus, have advanced their phenology in response to climate change more than long-distance migrants, including the cuckoo itself. Because different parts of Europe show different degrees of climate change, we predicted that use of residents or short-distance migrants as hosts should have declined in areas with greater increase in spring temperature. Comparing relative frequency of parasitism of the two host categories in 23 European countries before and after 1990, when spring temperatures in many areas had started to increase, we found that relative parasitism of residents and short-distance migrants decreased. This change in host use was positively related to increase in spring temperature, consistent with the prediction that relative change in phenology for different migrant classes drives host-use patterns. These findings are consistent with the hypothesis that climate change affects the relative abundance of different host races of the common cuckoo.     

Multiple Weather Factors Affect Apparent Survival of European Passerine Birds

Weather affects the demography of animals and thus climate change will cause local changes in demographic rates. In birds numerous studies have correlated demographic factors with weather but few of those examined variation in the impacts of weather in different seasons and, in the case of migrants, in different regions. Using capture-recapture models we correlated weather with apparent survival of seven passerine bird species with different migration strategies to assess the importance of selected facets of weather throughout the year on apparent survival. Contrary to our expectations weather experienced during the breeding season did not affect apparent survival of the target species. However, measures for winter severity were associated with apparent survival of a resident species, two short-distance/partial migrants and a long-distance migrant. Apparent survival of two short distance migrants as well as two long-distance migrants was further correlated with conditions experienced during the non-breeding season in Spain. Conditions in Africa had statistically significant but relatively minor effects on the apparent survival of the two long-distance migrants but also of a presumably short-distance migrant and a short-distance/partial migrant. In general several weather effects independently explained similar amounts of variation in apparent survival for the majority of species and single factors explained only relatively low amounts of temporal variation of apparent survival. Although the directions of the effects on apparent survival mostly met our expectations and there are clear predictions for effects of future climate we caution against simple extrapolations of present conditions to predict future population dynamics. Not only did weather explains limited amounts of variation in apparent survival, but future demographics will likely be affected by changing interspecific interactions, opposing effects of weather in different seasons, and the potential for phenotypic and microevolutionary adaptations.     

Long-term population declines in Danish trans-Saharan migrant birds

Capsule Long-distance migrant birds show less favourable trends than sedentary/short-distance species.

Aims To use breeding bird surveys to contrast population trends amongst common species according to their migration pattern.

Methods Changes in abundance of 62 Danish breeding sedentary, short-distance (Europe/North Africa) or long-distance (trans-Saharan) migrants were described by fitting log linear regression models to point-count census data gathered during 1976–2005.

Results Trans-Saharan migrants declined by 1.3% per annum during this period, while short-distance migrants and sedentary species increased by 1.4% and 1.0% per annum, respectively. There were no significant decadal declines amongst species using different summer breeding habitats, except for wetlands, and there was no consistent variation in trends associated with wintering regions or habitats or diet.

Conclusions More information is urgently needed on diet, feeding ecology, habitat requirements, winter distribution and intra-African movements of the commoner European summer visitors to identify causes of the declines and highlight when in the annual cycle detrimental effects occur. Studies linking individuals on their breeding, staging and wintering grounds are especially needed. Danish trends resemble those from elsewhere in Europe, confirming that restoration to favourable conservation status requires inter-continental action to meet European and global targets to reduce or halt biodiversity loss.     

Ancestry and evolution of seasonal migration in the Parulidae

Seasonal migration in birds is known to be highly labile and subject to rapid change in response to selection, such that researchers have hypothesized that phylogenetic relationships should neither predict nor constrain the migratory behaviour of a species. Many theories on the evolution of bird migration assume a framework that extant migratory species have evolved repeatedly and relatively recently from sedentary tropical or subtropical ancestors. We performed ancestral state reconstructions of migratory behaviour using a comprehensive, well-supported phylogeny of the Parulidae (the 'wood-warblers'), a large family of Neotropical and Nearctic migratory and sedentary songbirds, and examined the rates of gain and loss of migration throughout the Parulidae. Counter to traditional hypotheses, our results suggest that the ancestral wood-warbler was migratory and that losses of migration have been at least as prevalent as gains throughout the history of Parulidae. Therefore, extant sedentary tropical radiations in the Parulidae represent losses of latitudinal migration and colonization of the tropics from temperate regions. We also tested for phylogenetic signal in migratory behaviour, and our results indicate that although migratory behaviour is variable within some wood-warbler species and clades, phylogeny significantly predicts the migratory distance of species in the Parulidae.     

Light enough to travel: migratory bats have smaller brains,but not larger hippocampi,than sedentary species

Migratory bird species have smaller brains than non-migratory species. The behavioural flexibility/migratory precursor hypothesis suggests that sedentary birds have larger brains to allow the behavioural flexibility required in a seasonally variable habitat. The energy trade-off hypothesis proposes that brains are heavy, energetically expensive and therefore, incompatible with migration. Here, we compared relative brain, neocortex and hippocampus volume between migratory and sedentary bats at the species-level and using phylogenetically independent contrasts. We found that migratory bats had relatively smaller brains and neocortices than sedentary species. Our results support the energy trade-off hypothesis because bats do not exhibit the same degree of flexibility in diet selection as sedentary birds. Our results also suggest that bat brain size differences are subtler than those found in birds, perhaps owing to bats'' shorter migration distances. Conversely, we found no difference in relative hippocampus volume between migratory and sedentary species, underscoring our limited understanding of the role of the hippocampus in bats.     

Evolutionary aspects of migratory behavior in European warblers

During the past 20 years, European Sylvia warblers have been used for a model study of the control mechanisms of bird migration and of evolutionary aspects of migratory behavior. Endogenous annual rhythms (‘circannual’ rhythms) and photo-period have proved to be the essential internal and external controlling factors. It is unknown whether this basic system, that also controls migration in other bird species, is currently evolutionarily stable or is instead adapting birds to the present slightly changing environmental conditions. Using the Blackcap, the control of partial migration in a bird species was analyzed. Two-way selective breeding experiments demonstrated a large selection response and high heritability values. These experiments have also indicated that a partially migratory population can become either almost completely migratory, or sedentary, in two to five generations. Hence, genetic influences are very important and presumably dominant over environmental factors in the expression of migratory or sedentary behavior. The large selection response implies a strikingly high evolutionary potential with respect to strong selection pressures. Further, in the Blackcap, migratory orientation behavior (in addition to migratory activity) was immediately transmitted into a F₁-generation when a cross-breeding experiment was performed using birds from a migratory and a resident population. The hybrids displayed their migratory activity along an axis that is used by their migratory parents. Finally, a rapidly developing novel migratory habit (new migratory direction to new wintering areas) in the Blackcap is discussed with respect to a positive feedback-mechanism, possibly including a series of advantages leading to above-average fitness.     

Does the migration programme constrain dispersal and range sizes of migratory birds?

Aim  It is generally believed that the migration programme constrains the dispersal and hence range sizes of migratory bird species. This conclusion is based on analyses of breeding ranges of migratory versus non-migratory (resident) terrestrial bird species, and rests on the assumption that there are no ecological or evolutionary constraints on extending the non-breeding range. To investigate this assumption, the abilities of migrant and resident terrestrial species to colonize new wintering areas were compared.
Location  Three major wintering regions of long-distance migrants: South America, sub-Saharan Africa and the Indian Subcontinent.
Methods  It was determined whether the relative numbers of residents and short- and long-distance migrants were the same in those species that have dispersed to a novel wintering region as in the source species pools.
Results  At the species level, long-distance migratory species are more likely to have non-breeding ranges that include more than one of the above regions than resident species. This indicates that the dispersal of migratory species is less constrained than that of resident species. The pattern holds irrespective of the inclusion or exclusion of species associated with coastal, freshwater and wetland habitats, and also holds for ecological groups such as aerial feeders. The pattern is most pronounced between the regions separated by the strongest dispersal barriers (South America and sub-Saharan Africa).
Main conclusions  It is unlikely that the migration programme per se constrains dispersal, but rather that difficulties in establishing new non-breeding areas prevent range expansions in migrant species.     

Fuelling decisions in migratory birds: geomagnetic cues override the seasonal effect

Recent evaluations of both temporal and spatial precision in bird migration have called for external cues in addition to the inherited programme defining the migratory journey in terms of direction, distance and fuelling behaviour along the route. We used juvenile European robins (Erithacus rubecula) to study whether geomagnetic cues affect fuel deposition in a medium-distance migrant by simulating a migratory journey from southeast Sweden to the wintering area in southern Spain. In the late phase of the onset of autumn migration, robins exposed to the magnetic treatment attained a lower fuel load than control birds exposed to the ambient magnetic field of southeast Sweden. In contrast, robins captured in the early phase of the onset of autumn migration all showed low fuel deposition irrespective of experimental treatment. These results are, as expected, the inverse of what we have found in similar studies in a long-distance migrant, the thrush nightingale (Luscinia luscinia), indicating that the reaction in terms of fuelling behaviour to a simulated southward migration varies depending on the relevance for the species. Furthermore, we suggest that information from the geomagnetic field act as an important external cue overriding the seasonal effect on fuelling behaviour in migratory birds.     

Speciation and rapid phenotypic differentiation in the yellow-rumped warbler Dendroica coronata complex

The relative importance of the Pleistocene glacial cycles in driving avian speciation remains controversial, partly because species limits in many groups remain poorly understood, and because current taxonomic designations are often based on phenotypic characteristics of uncertain phylogenetic significance. We use mtDNA sequence data to examine patterns of genetic variation, sequence divergence and phylogenetic relationships between phenotypically distinct groups of the yellow-rumped warbler complex. Currently classified as a single species, the complex is composed of two North American migratory forms (myrtle warbler Dendroica coronata coronata and Audubon's warbler Dendroica coronata auduboni), and two largely sedentary forms: Dendroica coronata nigrifrons of Mexico, and Dendroica coronata goldmani of Guatemala. The latter are typically considered to be races of the Audubon's warbler based on plumage characteristics. However, mtDNA sequence data reveal that sedentary Mesoamerican forms are reciprocally monophyletic to each other and to migratory forms, from which they show a long history of isolation. In contrast, migratory myrtle and Audubon's warblers form a single cluster due to high levels of shared ancestral polymorphism as evidenced by widespread sharing of mtDNA haplotypes despite marked phenotypic differentiation. Sedentary and migratory forms diverged in the early Pleistocene, whereas phenotypic differentiation between the two migratory forms has occurred in the Holocene and is likely the result of geographical isolation and subsequent range expansion since the last glaciation. Our results underscore the importance of Quaternary climatic events in driving songbird speciation and indicate that plumage traits can evolve remarkably fast, thus rendering them potentially misleading for inferring systematic relationships.