Franz Bairlein

When and where does mortality occur in migratory birds? Direct evidence from long‐term satellite tracking of raptors

Information about when and where animals die is important to understand population regulation. In migratory animals, mortality might occur not only during the stationary periods (e.g. breeding and wintering) but also during the migration seasons. However, the relative importance of population limiting factors during different periods of the year remains poorly understood, and previous studies mainly relied on indirect evidence. Here, we provide direct evidence about when and where migrants die by identifying cases of confirmed and probable deaths in three species of long-distance migratory raptors tracked by satellite telemetry. We show that mortality rate was about six times higher during migration seasons than during stationary periods. However, total mortality was surprisingly similar between periods, which can be explained by the fact that risky migration periods are shorter than safer stationary periods. Nevertheless, more than half of the annual mortality occurred during migration. We also found spatiotemporal patterns in mortality: spring mortality occurred mainly in Africa in association with the crossing of the Sahara desert, while most mortality during autumn took place in Europe. Our results strongly suggest that events during the migration seasons have an important impact on the population dynamics of long-distance migrants. We speculate that mortality during spring migration may account for short-term annual variation in survival and population sizes, while mortality during autumn migration may be more important for long-term population regulation (through density-dependent effects).

Cross-hemisphere migration of a 25 g songbird

The northern wheatear (Oenanthe oenanthe) is a small (approx. 25 g), insectivorous migrant with one of the largest ranges of any songbird in the world, breeding from the eastern Canadian Arctic across Greenland, Eurasia and into Alaska (AK). However, there is no evidence that breeding populations in the New World have established overwintering sites in the Western Hemisphere. Using light-level geolocators, we demonstrate that individuals from these New World regions overwinter in northern sub-Sahara Africa, with Alaskan birds travelling approximately 14 500 km each way and an eastern Canadian Arctic bird crossing a wide stretch of the North Atlantic (approx. 3500 km). These remarkable journeys, particularly for a bird of this size, last between one to three months depending on breeding location and season (autumn/spring) and result in mean overall migration speeds of up to 290 km d−1. Stable-hydrogen isotope analysis of winter-grown feathers sampled from breeding birds generally support the notion that Alaskan birds overwinter primarily in eastern Africa and eastern Canadian Arctic birds overwinter mainly in western Africa. Our results provide the first evidence of a migratory songbird capable of linking African ecosystems of the Old World with Arctic regions of the New World.

Basal and stress-induced corticosterone levels of garden warblers, Sylvia borin, during migration

SummaryPlasma levels of the metabolically and behaviorally active corticosteroid hormone, corticosterone, were studied in garden warblers in the laboratory and in the field during the autumnal migratory phase. Garden warblers showing nocturnal migratory activity in the laboratory had elevated levels of corticosterone at the end of the dark phase and low levels during daytime. When nocturnal migratory activity was experimentally disrupted by food deprivation and subsequent refeeding or after spontaneous termination of migratory activity this rhythm was absent. Garden warblers stopping over in the Sahara desert during autumnal migration had low levels of corticosterone. Levels were negatively correlated with fat stores and body mass in birds sampled throughout the day. These levels were generally lower than those associated with stress in response to repeated handling and blood sampling. The results suggest (1) the existence of diel changes in adrenocortical hormonal activity that could be involved in regulation of migration, and (2) that garden warblers carrying large fat depots are not stressed by prolonged flight or lack of appropiate feeding areas during migration over the desert.

Obligatory barrier crossing and adaptive fuel management in migratory birds: the case of the Atlantic crossing in Northern Wheatears (Oenanthe oenanthe)

Behaviour on migration was often suggested to be selected for time-minimising strategies. Current optimality models predict that optimal fuel loads at departure from stopover sites should increase with increasing fuel deposition rates. We modified such models for the special case of the east Atlantic crossing of the Northern Wheatear (Oenanthe oenanthe). From optimality theory, we predict that optimal time-minimising behaviour in front of such a barrier should result in a positive correlation between fuel deposition rates and departure fuel loads only above a certain threshold, which is the minimum fuel load (fmin) required for the barrier crossing. Using a robust range equation, we calculated the minimum fuel loads for different barrier crossings and predict that time-minimising wheatears should deposit a minimum of 24% fuel in relation to lean body mass (m0) for the sea crossing between Iceland and Scotland. Fuel loads of departing birds in autumn in Iceland reached this value only marginally but showed positive correlation between fuel deposition rate (FDR) and departure fuel load (DFL). Birds at Fair Isle (Scotland) in spring, which were heading towards Iceland or Greenland, were significantly heavier and even showed signs of overloading with fuel loads up to 50% of lean body mass. Departure decisions of Icelandic birds correlated significantly with favourable wind situations when assuming a migration direction towards Spain; however, the low departure fuel loads contradict a direct non-stop flight.

Is there a "migratory syndrome" common to all migrant birds?

Abstract: Bird migration has been assumed, mostly implicitly, to represent a distinct class of animal behavior, with deep and strong homologies in the various phenotypic expressions of migratory behavior between different taxa. Here the evidence for the existence of what could be called a “migratory syndrome,” a tightly integrated, old group of adaptive traits that enables birds to commit themselves to highly organized seasonal migrations, is assessed. A list of problems faced by migratory birds is listed first and the traits that migratory birds have evolved to deal with these problems are discussed. The usefulness of comparative approaches to investigate which traits are unique to migrants is then discussed. A provisional conclusion that, perhaps apart from a capacity for night‐time compass orientation, there is little evidence for deeply rooted coadapted trait complexes that could make up such a migratory syndrome, is suggested. Detailed analyses of the genetic and physiological architecture of potential adaptations to migration, combined with a comparative approach to further identify the phylogenetic levels at which different adaptive traits for migration have evolved, are recommended.

The migratory strategy of the Garden Warbler: A survey of field and laboratory data

Garden Warblers appear to be equipped with a migratory strategy which matches the problems arising from migration very well. They are equipped with innate endogenous programmes which determine the seasonal events for migration (for reviews see Berthold 1984, Gwinner 1986). They exhibit strong habitat preferences in resting areas which are likely to be endogenously controlled and which allow the specimens to be well accommodated in the various migratory stopoverareas (Bairlein 1981). They possess physiological and behavioural adaptations in their nutrition which guarantee optimal fattening up prior to migratory flights. Garden Warblers apparently avoid the risk of dehydration during long nonstop flights across the Sahara by crossing the desert in stages with regular stopovers at suitable sites rather than in one long hop.

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 Montagus 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.

Morphological Shifts of the External Flight Apparatus across the Range of a Passerine (Northern Wheatear) with Diverging Migratory Behaviour

We studied morphological differentiation in the flight apparatus of the four currently recognised sub-species of Northern Wheatears, Oenanthe oenanthe. Considering all measured birds without assigning them a priori to any sub-species we found a clinal morphological shift. Relative wing length, wing pointedness, and the degree of tail forking were positively correlated with migratory distance, whereas tail length (relative to wing length) was negatively correlated. The large-sized, long-distance migrant “Greenland” Wheatear, O. o. leucorhoa, is characterized by relatively longer, broader and more pointed wings and more forked tails, similar to the smaller-sized nominate Northern Wheatear, O. o. oenanthe, from North Europe, Siberia and Russia. In contrast, the short distance migrant “Seebohms” Wheatear, O. o. seebohmi, from northwest Africa, possesses much rounder wings, and the tail is relatively longer and less forked. Sub-species with intermediate migratory habits (different populations of nominate Northern Wheatear, O. o. oenanthe, and “Mediterranean” Northern Wheatear, O. o. libanotica) show, as expected, intermediate features according to their intermediate migratory behaviour. Our results are congruent with other inter- and intraspecific studies finding similar adaptations for energy-effective flight in relation to migration distance (morphological migratory syndrome).

Endogenous Rhythms of Seasonal Migratory Body Mass Changes and Nocturnal Restlessness in Different Populations of Northern Wheatear Oenanthe oenanthe

The Northern Wheatear (Oenanthe oenanthe) is a migratory bird species that shows different strategies of migration between populations, adapted to cope with different ecological barriers. This raises the question whether and to which extent these adaptations are endogenously determined. We studied seasonal patterns of body mass change and nocturnal restlessness in wheatears from Iceland, which face an initial sea crossing of at least 800 km; from Norway, which fly a similar distance as Icelandic birds but without a long sea crossing; and from Morocco, which fly a shorter distance to reach their wintering grounds. To isolate the endogenous component of the regulation of these migratory traits, we kept the wheatears in a “common garden,” all 3 populations experiencing the same environmental conditions and a constant photoperiod during their first year of life. Icelandic birds showed a greater increase of their body mass in autumn than the other 2 populations, indicating preparation for the initial barrier crossing. The autumnal timing of nocturnal restlessness and the total activity during autumn were related to the distance to be covered, although the differences between populations were smaller than expected. In all 3 populations, body mass increased to a greater extent in autumn than in spring, whereas nocturnal activity was higher in spring than in autumn. This suggests that the endogenous program responds to specific seasonal needs, with more time invested in storing fuel for a safe journey in autumn and more time invested in flying to reach the breeding grounds early in spring. Contrary to expectations, the timing of onset of body mass increase and nocturnal restlessness in spring did not differ between populations. This might be explained by the lack of external cues, most likely photoperiod, which are responsible for the fine tuning of the expression of migratory behavior.

Migratory Fuelling and Global Climate Change

Publisher Summary Climate change affects ecosystems, habitats, and species with increasing velocity and continuity. Several migratory birds react to increased local temperatures or to large-scale climatic phenomena, such as the North Atlantic Oscillation with changes in arrival and departure phonologies. Migration is energetically costly. Thus, many migratory species accumulate large amounts of energy reserves, of which most is fat, prior to migratory flights. The mass of stored reserves may amount to more than 100% of lean body mass with maximum levels obtained by species crossing inhospitable areas such as sea and deserts with no feeding opportunities. Long-distance migrants are equipped with a sophisticated timing system comprising both endogenous and exogenous components. Consequently, these species may be the most affected ones by changing environmental conditions at their stopover sites, as they often rely on a few particular stopover sites while medium- and short-distance migrants may be more flexible in their decision regarding where to rest. In many of these long-distance migrating species, migratory fuelling happens immediately before crossing ecological barriers. Shortly after an obstacle, migrants need sites to recover from the long flights and to prepare for the continuation of migration. At all these sites, fuelling migrants largely rely on appropriate habitats and on good feeding opportunities both in terms of quantity and quality.