Lukas Jenni

FUEL SUPPLY AND METABOLIC CONSTRAINTS IN MIGRATING BIRDS

Energy management for endurance flight critically determines the ecological options in the life history of migrant birds. Apart from the amount of energy stores, the types of fuel used and metabolic constraints determine endurance performance in long-distance migrants. The three main types of fuel (lipids, glycogen, protein) are evaluated regarding (a) costs of transport and maintenance, (b) supply to the muscles during flight, (c) adverse effects during, and (d) after, flight, and (e) refuelling. Lipids are the best fuel type with respect to three of these five criteria, but pose problems regarding the supply from adipose tissues to the muscles and regarding oxidation. Nevertheless, birds can maximize the contribution of energy derived from lipids as a proportion of the total energy expenditure to about 95% during migratory endurance flight. Mechanisms to do so may include enhancing fatty acid transport to the flight muscles, increasing initial fat stores and increasing aerobic capacity. A very low relative contribution of energy derived from protein (RPC) is attained by migrating birds (around 5%), similar to that of long-term fasting birds. RPC values of fasting birds depend on initial fat content and are lower than in fasting mammals. This suggests that in fasting birds the absolute amount of protein catabolism is roughly proportional to metabolic rate. Mechanisms to increase the relative contribution of energy derived from fat are costly. Hence, they are expected to modify the power curve relating energy expenditure to flight speed and predictions derived from it. It is also expected that migrants flying short non-stop distances or risking dehydration will not maximize lipid utilization to the same extent as birds flying long distances non-stop. Hence, the optimal amount and proportions of fat and protein stored and functional organ sizes are expected to depend on the migration strategy, particularly on the duration of non-stop flights and the risk of dehydration.

Timing of autumn bird migration under climate change: advances in long–distance migrants, delays in short–distance migrants

As a response to increasing spring temperature in temperate regions in recent years, populations of many plant and animal species, including migratory birds, have advanced the seasonal start of their reproduction or growth. However, the effects of climate changes on subsequent events of the annual cycle remain poorly understood. We investigated long–term changes in the timing of autumn migration in birds, a key event in the annual cycle limiting the reproductive period. Using data spanning a 42–year period, we analysed long–term changes in the passage of 65 species of migratory birds through Western Europe. The autumn passage of migrants wintering south of the Sahara has advanced in recent years, presumably as a result of selection pressure to cross the Sahel before its seasonal dry period. In contrast, migrants wintering north of the Sahara have delayed autumn passage. In addition, species with a variable rather than a fixed number of broods per year have delayed passage, possibly because they are free to attempt more broods. Recent climate changes seem to have a simple unidirectional effect on the seasonal onset of reproduction, but complex and opposing effects on the timing of subsequent events in the annual cycle, depending on the ecology and life history of a species. This complicates predictions of overall effects of global warming on avian communities.

MIGRATING BIRDS STOP OVER LONGER THAN USUALLY THOUGHT: AN IMPROVED CAPTURE–RECAPTURE ANALYSIS

Migrating animals often divide their journey into alternating phases of migration bouts and stopping over. For investigating many questions of migration ecology it is crucial (1) to estimate the duration of stopover phases, and (2) to test whether animals of different groups differ in their stopover behavior. Using recent advances in capture–recapture statistics, we show how total stopover duration can be estimated from capture–recapture data. The probabilities of immigration are estimated and modeled by recruitment analysis and are converted into the time the animals spent at the stopover place before capture; the probabilities of emigration are estimated and modeled by survival analysis and are converted into the time the animals spent at the stopover place after capture. The sum of the two parts is the total stopover duration. Tests for differences between groups can be addressed by an appropriate model selection procedure. Two examples of migrating passerine birds at a stopover site in Switzerland illustrate this method. Mean total stopover duration was 12.3 d for Reed Warblers and 7.1 d for Reed Buntings. This was considerably higher than values obtained by the minimum stopover duration estimation (6.0 and 4.4 d, respectively). Because of the fundamental weaknesses of the minimum stopover duration estimation, which has been widely used in migration ecology, many findings obtained by this method need to be reconsidered.

Fuel deposition of three passerine bird species along the migration route

Abstract The rate at which migrant birds replenish their energy stores at intermittent stopovers largely determines overall migration speed, the manner in which migration proceeds and success of migration. In this study, data on the fuel deposition rate (FDR) of three long-distance migrants from 17 ringing sites along their autumn migration route were used to examine: (1) effects of endogenous factors on FDR, and (2) how relationships between exogenous factors and FDR affect the organisation of migration. We developed a model to estimate FDR from retrapped birds which takes into account time of day and various other factors which might influence FDR. The two endogenous factors, moult and current energy stores, generally reduced FDR. This may result in lower departure energy loads and more stopovers than expected from optimal migration theory. Differences between species with respect to seasonal, year-to-year and geographical patterns of FDR could be related to differences in availability and predictability of food resources, and help to explain differences in the organisation of migration. A low FDR in northern and central Europe could be related to low, but predictable, food resources and an early departure during moult of the reed warbler (Acrocephalus scirpaceus); FDRs varying between years were related to large spatial and year-to-year variation in the density of the main prey of the sedge warbler (Acrocephalus schoenobaenus); and a high FDR in the garden warbler (Sylvia borin) was related to abundant food resources, due to a switch from a purely invertebrate diet to a mixed diet including fruits which are abundant over large areas of Europe and north Africa. This study demonstrated that the organisation of migration is the outcome of a complex interplay of the seasonal timing of moult, food availability and predictability and a seasonal switch in diet, and can be modified by individual birds in response to a limited amount of time in which to migrate.

Plasma Metabolite Levels Predict Individual Body-Mass Changes in a Small Long-Distance Migrant, the Garden Warbler

-Change in body mass is an important measurement in many studies addressing changes in energy stores or condition. Usually, change in body mass is measured in birds caught twice, but this has a number of drawbacks (e.g. low number of retraps, retraps not representative of all first captures, adverse effects of first capture on body-mass development). Therefore, we investigated whether plasma metabolites correlate with body-mass change, and which metabolites could be used to predict body-mass change in birds caught once. In an experiment, 20 Garden Warblers (Sylvia borin) were given different amounts of food to induce stable, increasing, and decreasing body masses. Most of the eight plasma metabolites we examined were significantly correlated with the change in body mass between early morning and midday, the time of blood sampling, but not with body mass or various measures of activity. Metabolites that are known to characterize resorption were elevated in birds gaining body mass and metabolites characteristic of fasting were elevated in birds losing body mass. Triglycerides and ,B-hydroxy-butyrate together explained 61% of the variation in body-mass change (triglycerides alone 44% and ,3-hydroxy-butyrate alone 51%). These metabolites may be used to predict body-mass change in birds caught once, provided that the reliability and sensitivity of this method are checked in field tests. Received 27 September 1993, accepted 30 January 1994. BIRDS MAY FACE DEFICITS in their energy balance during the nonactive period of the day, during periods of several days, or even over a series of weeks. They usually build up energy stores in anticipation of such periods and as an adaptive compromise to different selective pressures (e.g. Lima 1986, Lehikoinen 1987, Moreno 1989, Ekman and Hake 1990, Rogers and Smith 1993). This results in fluctuating energy stores between foraging and nonforaging periods (usually day and night) superimposed on longer periods of net energy storage and net energy expenditure (King 1972). For instance, cold and short winter days, bad weather, incubation, feeding young, and migration may all be associated with decreasing energy stores and preceded by energy storage. Body mass is a crude but widely used estimate of energy stores because it is easy and harmless to measure. If corrected for size, body mass may be used as an indication of condition (e.g. Blem 1990). Changes in body mass may give a fairly accurate picture of the fluctuations in energy stores (e.g. Rogers and Rogers 1990). The measurement of changes in body mass of free-living birds is important in a variety of contexts. For instance, it may allow the assessment of the effects of food availability on energy storage in different habitats, under different weather conditions, and under different degrees of competition or predation (e.g. Bibby et al. 1976, Bibby and Green 1983, Buchanan et al. 1985, Lima 1985, Lindstrom 1990, Moore and Yong 1991). Body-mass changes may indicate periods of constraints and periods of adaptive mass variations during the annual cycle (Moreno 1989). Evaluation of body-mass changes may allow study of the pattern and environmental influences of energy storage during stopovers by migrants (e.g. Bibby et al. 1976, Mehlum 1983a, b, Biebach 1985, Biebach et al. 1986, Alerstam and Lindstrom 1990). In free-living birds, changes in body mass generally are measured in birds caught at least twice, usually at the same place. This, however, has a number of drawbacks: (1) changes in body mass are measured only in the subsample of retraps and this may reduce the sample size drastically, especially when trap shyness is a factor; (2) a certain group of birds may not be recaught and, thus, the sample of retraps may not be representative (e.g. Bibby et al. 1976, Biebach et al. 1986, Veiga 1986, Winker et al. 1992); and (3) first capture may have an adverse

Year-Round Tracking of Small Trans-Saharan Migrants Using Light-Level Geolocators

Since 1899 ringing (or banding) remained the most important source of information about migration routes, stopover sites and wintering grounds for birds that are too small to carry satellite-based tracking systems. Despite the large quantity of migrating birds ringed in their breeding areas in Europe, the number of ring recoveries from sub-Saharan Africa is very low and therefore the whereabouts of most small bird species outside the breeding season remain a mystery. With new miniaturized light-level geolocators it is now possible to look beyond the limits of ring recovery data. Here we show for the first time year round tracks of a near passerine trans-Saharan migrant, the European Hoopoe (Upupa epops epops). Three birds wintered in the Sahel zone of Western Africa where they remained stationary for most of the time. One bird chose a south-easterly route following the Italian peninsula. Birds from the same breeding population used different migration routes and wintering sites, suggesting a low level of migratory connectivity between breeding and wintering areas. Our tracking of a near passerine bird, the European Hoopoe, with light-level geolocators opens a new chapter in the research of Palaearctic-African bird migration as this new tool revolutionizes our ability to discover migration routes, stopover sites and wintering grounds of small birds.

Metabolic responses to flight and fasting in night-migrating passerines

Summary1.Small passerine migrants achieve endurance flight while fasting, together with one of the highest mass-specific energy rates. Metabolic responses to flight and fasting were examined in three species of free-living migrants (Sylvia borin, Ficedula hypoleuca, Erithacus rubecula) by measuring plasma concentrations of glucose, uric acid, triglycerides, glycerol, free fatty acids (FFA), and β-hydroxybutyrate (β-OHB) in three main physiological situations (feeding, overnight fasting, nocturnal flight) and while changing between these situations.2.Overnight-fasted birds showed low triglyceride and uric acid levels. Contrary to mammals, FFA and glycerol levels were not increased in agreement with published data on birds. The transition from feeding to fasting (post-feeding) was distinguished by a temporary rise in FFA and a drop in glucose levels.3.Birds utilize fat during migratory flight, indicated by high levels of FFA, glycerol, and β-OHB. For the first time, high triglyceride levels were found in an exercising vertebrate. The use of protein during flight was demonstrated by high uric acid levels.4.Birds kept inactive after flight showed a more pronounced reduction of the fat and protein utilization and post-exercise ketosis than naturally landed birds.5.Differences among the three species in the metabolic pattern suggest that the garden warbler shows the greatest metabolic adaption to endurance flight, having the highest levels of fat metabolites and the highest body fat reserves.

Prevalence of Borrelia burgdorferi Sensu Lato in Ticks Collected from Migratory Birds in Switzerland

ABSTRACT The prevalence of ticks infected by Borrelia burgdorferi sensu lato on birds during their migrations was studied in Switzerland. A total of 1,270 birds captured at two sites were examined for tick infestation. Ixodes ricinus was the dominant tick species. Prevalences of tick infestation were 6% and 18.2% for birds migrating northward and southward, respectively. Borrelia valaisiana was the species detected most frequently in ticks, followed by Borrelia garinii and Borrelia lusitaniae. Among birds infested by infected ticks, 23% (6/26) were infested by B. lusitaniae-infected larvae. Migratory birds appear to be reservoir hosts for B. lusitaniae.

Behavioural and Physiological Reactions to Environmental Variation in Bird Migration: a Review

Many features of the migration of passerine birds are endogenously programmed and have a strong genetic basis. At least in inexperienced migrants, the general seasonal course of migration is endogenously controlled, such as the onset, the temporal pattern, the direction of migration and the seasonal pattern of energy stores. This leads to the conclusion that an endogenous spatiotemporal migration programme guides inexperienced migrants from their place of birth to their first winter quarters (summarized in Berthold 1996).

Corticosterone mediates the condition-dependent component of melanin-based coloration

The handicap principle of sexual selection theory states that colourful phenotypic traits signal aspects of individual quality because only individuals in prime condition can afford to produce and bear conspicuous traits. Melanin-based pigments participate in the elaboration of many secondary sexual characters and, given their role in sexual selection, melanin-based coloration may therefore honestly reflect individual quality. Although the expression of melanism is usually under genetic control, in some species it is condition dependent. However, the underlying physiological mechanism is yet unknown. Based on the negative feedback link between corticosterone and melanogenesis (melanocortins, tyrosinase) in response to stressful environmental factors, we hypothesize that corticosterone mediates the condition-dependent component of melanism. This hypothesis predicts that stressful factors induce a rise in circulating corticosterone which inhibits the secretion of melanocortins and tyrosinase and in turn melanin production. We tested this prediction by manipulating the level of corticosterone at the time of melanin production in nestling barn owls, Tyto alba, a species showing heritable variation in the degree of phaeomelanism from reddish-brown to white. The finding that corticosterone-implanted nestlings produced feathers with less phaeomelanic coloration than placebo-implanted nestlings is consistent with the hypothesis that the environment-mediated reduction in the degree of melanism is, at least in part, caused by a rise in corticosterone. In species in which the expression of melanin-based coloration is condition dependent, we now need a test showing that individuals with less corticosterone and more melanin-based signals are individuals in better condition.