Mass-dependent Predation Risk Research Articles

Early life adversity increases foraging and information gathering in European starlings, Sturnus vulgaris.

Animals can insure themselves against the risk of starvation associated with unpredictable food availability by storing energy reserves or gathering information about alternative food sources. The former strategy carries costs in terms of mass-dependent predation risk, while the latter trades off against foraging for food; both trade-offs may be influenced by an individual's developmental history. Here, we consider a possible role of early developmental experience in inducing different mass regulation and foraging strategies in European starlings. We measured the body mass, body condition, foraging effort, food consumption and contrafreeloading (foraging for food hidden in sand when equivalent food is freely available) of adult birds (≥10 months old) that had previously undergone a subtle early life manipulation of food competition (cross-fostering into the highest or lowest ranks in the brood size hierarchy when 2–12 days of age). We found that developmentally disadvantaged birds were fatter in adulthood and differed in foraging behaviour compared with their advantaged siblings. Disadvantaged birds were hyperphagic compared with advantaged birds, but only following a period of food deprivation, and also spent more time contrafreeloading. Advantaged birds experienced a trade-off between foraging success and time spent contrafreeloading, whereas disadvantaged birds faced no such trade-off, owing to their greater foraging efficiency. Thus, developmentally disadvantaged birds appeared to retain a phenotypic memory of increased nestling food competition, employing both energy storage and information-gathering insurance strategies to a greater extent than their advantaged siblings. Our results suggest that subtle early life disadvantage in the form of psychosocial stress and/or food insecurity can leave a lasting legacy on foraging behaviour and mass regulation even in the absence of food insufficiency during development or adulthood.

Resident but not transient Eurasian Siskins reduce body mass in response to increasing predation risk: a natural experiment

The body mass of birds is the result of a trade-off between predation and starvation risks. According to the mass-dependent predation hypothesis, birds reduce their body mass when predation risk increases. Many studies have supported this hypothesis, but it has rarely been possible to investigate the mass-dependent predation risk response by comparing individuals of the same species foraging simultaneously at the same location, in the field and facing a real (not simulated) risk of predation. Wintering Siskins (Carduelis spinus) are divided in two subpopulations: residents, which stay in a foraging area for several weeks or months, and transients, which only remain a few hours or days. In this paper, we present a natural experiment by comparing the body mass variation of resident and transient Siskins between a period of the wintering season without avian predators and another period with a Sparrowhawk (Accipiter nisus) that was hunting regularly. We found that resident Siskins reduced their body mass with the presence of the hawk, while transients did not. Since in a previous study with no avian predators at the study area it was found that the difference in body condition between residents and transients did not vary throughout the wintering season, we can confidently say that the reduction in body mass observed in this study was directly linked to the presence of the hawk. We additionally found that this reduction was not associated with the dominance of residents over transients. We discuss the advantages of residence related to the knowledge of actual predation risk. Our results strongly support the mass-dependent predation hypothesis.

Many routes lead to Rome: potential causes for the multi‐route migration system of Red Knots, Calidris canutus islandica

Migrants, such as birds or representatives of other taxa, usually make use of several stopover sites to cover the distance between their site of origin and destination. Potentially, multiple routes exist, but often little is known about the causes and consequences of alternative migration routes. Apart from their geographical distribution, the suitability of potential sites might play an important role in the animals' decisions for a particular itinerary. We used an optimal-migration model to test three nonmutually exclusive hypotheses leading to variations in the spring migration routes of a subspecies of Red Knot, Calidris canutus islandica, which migrates from wintering grounds in Western Europe to breeding grounds in Greenland and the Canadian Arctic: the breeding location hypothesis, the energy budget hypothesis, and the predation risk hypothesis. Varying only breeding location, the model predicted that birds breeding in the Canadian Arctic and on West Greenland stop over on Iceland, whereas birds breeding in East and Northeast Greenland migrate via northern Norway, a prediction that is supported by empirical findings. Energy budgets on stopover sites had a strong influence on the choice of route and staging times. Varying foraging-intensity and mass-dependent predation risk prompted the birds to use less risky sites, if possible. The effect of simultaneous changes in the energy budget and predation risk strongly depended on the site where these occurred. Our findings provide potential explanations for the observations that C. canutus islandica uses a diverse array of migration routes. Scrutinizing the three alternative driving forces for the choice of migratory routes awaits further, specific data collection in rapidly developing fields of research (e.g., predation risk assessment, GPS tracking). Generally, the type of modeling presented here may not only highlight alternative explanations, but also direct follow-up empirical research.

The Young, the Weak and the Sick: Evidence of Natural Selection by Predation

It is assumed that predators mainly prey on substandard individuals, but even though some studies partially support this idea, evidence with large sample sizes, exhaustive analysis of prey and robust analysis is lacking. We gathered data from a culling program of yellow-legged gulls killed by two methods: by the use of raptors or by shooting at random. We compared both data sets to assess whether birds of prey killed randomly or by relying on specific individual features of the prey. We carried out a meticulous post-mortem examination of individuals, and analysing multiple prey characteristics simultaneously we show that raptors did not hunt randomly, but rather preferentially predate on juveniles, sick gulls, and individuals with poor muscle condition. Strikingly, gulls with an unusually good muscle condition were also predated more than expected, supporting the mass-dependent predation risk theory. This article provides a reliable example of how natural selection may operate in the wild and proves that predators mainly prey on substandard individuals.

Diurnal foraging routines in a tropical bird, the rock finch Lagonosticta sanguinodorsalis: how important is predation risk?

An animal's foraging decisions are the outcome of the relative importance of the risk of starvation and predation. Fat deposition insures against periods of food shortage but it also carries a cost in terms of mass dependent predation risk due to reduced escape probability and extended exposure time. Accordingly, birds have been observed to show a unimodal foraging pattern with foraging concentrated at the end of the day under conditions of predictable food resources and high predation risk. We tested this hypothesis in a tropical granivorous finch, the rock firefinch Lagonosticta sanguinodorsalis, in an outdoor aviary experiment during which food was provided ad lib and the risk of predation was varied by providing food either adjacent to, or 5 m away from cover. Rock firefinches showed a bimodal foraging pattern regardless of the risk of predation at which they fed. The results suggest that predation is relatively unimportant in shaping their daily feeding pattern despite mass gain during the day being similar to temperate birds. Foraging patterns closely follow diurnal temperature variation and this is suggested to be the main determinant of the observed bimodal pattern.

To run or to fly: low cost versus low risk escape strategies in blackbirds

When escaping from predators, ground-foraging birds could choose between two escape strategies with different costs: running or flying. We simulated predator attacks on blackbirds Turdus merula to investigate the factors influencing blackbirds to select between these strategies. The probability that blackbirds would fly increased as flight initiation distance decreased, suggesting that they may trade-off benefits of delaying escape with a costly escape strategy. The probability of flying also increased as the number of potential predators increased, indicating an increase in perceived predation risk with the number of non-attacking predators. Running was more likely to be used in the mornings and flying in the afternoons, suggesting a possible mass-dependent predation risk effect or restrictions in the use of costly escape strategies when energetic reserves are lower. Juvenile blackbirds tended to fly away more often than adults, probably to compensate for their less reliable risk assessments due to lack of experience. We conclude that different factors act independently on choosing an optimal escape strategy, and that the decision about when to escape (flight initiation distance) is associated with the decision about how to escape (escape strategy), which in turn can affect the decision about how far to escape (distance fled).

Mass-dependent predation risk and lethal dolphin–porpoise interactions

In small birds, mass-dependent predation risk (MDPR) is known to make the trade-off between avoiding starvation and avoiding predation dependent on individual mass. This occurs because carrying increased fat reserves not only reduces starvation risk but also results in a higher predation risk due to reduced escape flight performance and/or the increased foraging exposure needed to maintain a higher body mass. In principle, the theory of MDPR could also apply to any animal capable of storing energy reserves to reduce starvation and whose escape performance decreases with increasing mass. We used a unique situation along certain parts of coastal Britain, where harbour porpoises (Phocoena phocoena) are pursued and killed but crucially not eaten by bottlenose dolphins (Tursiops truncatus), to investigate whether a MDPR effect can occur in non-avian species. We show that where high levels of dolphin 'predation' occur, porpoises carry significantly less energy reserves than would otherwise be expected and this equates to reducing by approximately 37% the length of time that a porpoise could survive without feeding. These results provide the first evidence that a mass-dependent starvation-predation risk trade-off may be a general ecological principle that can apply to widely different animal types rather than, as is currently thought, only to birds.

Mass-dependent predation risk as a mechanism for house sparrow declines?

House sparrow (Passer domesticus) numbers have declined rapidly in both rural and urban habitats across Western Europe over the last 30 years, leading to their inclusion on the UK conservation red list. The decline in farmland has been linked to a reduction in winter survival caused by reduced food supply. This reduction in food supply is associated with agricultural intensification that has led to the loss of seed-rich winter stubble and access to spilt grain. However, urban house sparrows have also declined, suggesting that reduced food supply in farmland is not the sole reason for the decline. Here, we show that changes in house sparrow mass and thus fat reserves are not regulated to minimize starvation risk, as would be expected if limited winter food were the only cause of population decline. Instead, the species appears to be responding to mass-dependent predation risk, with starvation risk and predation risk traded-off such that house sparrows may be particularly vulnerable to environmental change that reduces the predictability of the food supply.

Diurnal mass gain strategies and perceived predation risk in the great tit Parus major

1 Individual animals optimize their body mass to enhance fitness and mass is expected to be optimized over both short and long time-scales. On the short time-scale small birds increase mass as they build up energy reserves over each day to avoid starving at night. Theory predicts that starvation risk should be minimized by gaining mass at the start of the day thus insuring against unpredictable food supply later. However, if predation risk is mass-dependent due to reduced escape flight performance, birds should delay mass gain until later in the day to minimize predation risk. 2 Regulation of mass change over longer time-scales has been well documented and a number of studies have been able to show that over weeks, months and years mass is regulated consistent with the starvation–predation risk trade-off being mass-dependent. However, it is crucial to mass-dependent predation theory that it functions over shorter time-scales as well and that birds are able to regulate their diurnal mass gain strategies in response to predation risk. This has not yet been investigated and recent studies of flight performance have been unable to show that small-scale mass change (< 10%) over short time-scales (e.g. diurnal mass change) affects escape flight performance as predicted. 3 We used a unique design of automated identification and weighing system that for the first time allowed the diurnal mass gain of individual great tits Parus major to be monitored remotely in the wild. While automatically tracking diurnal body mass changes of individuals we manipulated perceived predation risk by using model sparrowhawks. We were thus able to investigate experimentally the effect of increased perceived predation risk on diurnal mass gain patterns. 4 We show, for the first time, that birds are capable of manipulating their diurnal body mass gain strategy in response to heightened predation risk and that they do so by delaying mass gain until later in the day, as predicted by mass-dependent predation risk theory. Furthermore, the results are consistent with changing flight performance rather than changing exposure time to predators being the driving force for mass-dependent predation risk.

What determines probability of surviving predator attacks in bird migration?: the relative importance of vigilance and fuel load

Migrating birds must accumulate fuel during their journeys and this fuel load should incur an increased risk of predation. Migratory fuelling should increase individual mass-dependent predation risk for two reasons. First, acquisition costs are connected to the increased time a bird must spend foraging to accumulate the fuel loads and the reduced predator detection that accompanies foraging. Second, birds with large fuel loads have been shown to suffer from impaired predator evasion which makes them more vulnerable when actually attacked. Here, I investigate the relative importance of these two aspects of mass-dependent predation risk and I have used published data and a hypothetical situation for a foraging bird to investigate how much migratory fuelling in terms of escape performance and natural variation in predator detection contribute to individual risk during foraging. Results suggest that for birds foraging close to protective cover the negative impact of fuel load on flight performance is very small, whereas variation in time to predator detection is of great importance for a bird's survival. However, the importance of flight performance for predation risk increases as the distance to cover increases. Hence, variation in predator detection (and vigilance) probably influences individual survival much more than migratory fuel load and consequently, to understand risk management during migration studies that focus on vigilance and predator detection during fuelling are much needed.

Testing the mass-dependent predation hypothesis: in European blackbirds poor foragers have higher overwinter body reserves

As foraging becomes more unpredictable animals should increase their body reserves to reduce the risk of starvation. However, any increases in reserves may increase the risk of predation because extra mass probably compromises escape ability. Because of differences in foraging ability not all individuals will be affected in the same way by changes in foraging conditions. Relatively poor foragers will have more unpredictable foraging success for any given availability of food and therefore should carry larger body reserves. The mass-dependent predation hypothesis then predicts a negative correlation between levels of body reserves and foraging ability, although this may be modified by state-dependent compensation. I measured foraging rates and body masses of wintering European blackbirds, Turdus merula. Individuals with the lowest foraging rates had the largest gain in mass for the winter and had relatively high mass overall, independently of age and sex. That foraging rate determined mass rather than the reverse was demonstrated because foraging rate was independent of daily and seasonal mass change. Foraging rate within the experimental system was also independent of predation risk (as measured by distance from protective cover) and so the relation between mass and foraging rate was unlikely to have been confounded by any changes in vigilance to compensate for increased mass-dependent predation risk. The results suggest that blackbirds with high relative foraging rates have lower body reserves during the winter. Therefore there is probably a direct link between overwinter condition and fitness at least in blackbirds. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.

Fat reserves and perceived predation risk in the great tit, Parus major.

The fat reserves of small birds are built up daily as insurance against starvation. They are believed to reflect a trade-off between the risks of starvation and predation such that in situations of high predation risk birds are expected either to reduce their fat reserves in response to mass-dependent predation risk or to increase them in response to foraging interruptions. We assessed the effect on fat reserves of experimentally altering the perceived (but not the actual) risk of predation of wild great tits at a winter feeding site. The perceived predation risk was alternated between 'safe' and 'risky'. Increasing the perceived risk of predation involved 'swooping' a model sparrowhawk over the feeder at four unpredictable times each day using a remote mechanism We produce evidence that the experiment was suceessfull in altering the perceived risk of predation. As predicted from the hypothesis of mass-dependent predation risk, great tits (Parus major) carried significantly reduced fat reserves during the 'risky' treatment. Furthermore, dominant individuals were able to reduce their reserves more than subordinates. As birds returned to feeders within seconds after a predator 'attack', the reduction in fat reserves cannot be attributed to an interruption in feeding.

A dynamic model of short-term energy management in small food-caching and non-caching birds

The survival of small birds in winter is critically dependent on the birds’ ability to accumulate and maintain safe levels of energy reserves. In some species, food caching facilitates energy regulation by providing an energy source complementary to body fat. We present a dynamic optimization model of short-term, diurnal energy management for both food-caching and non-caching birds in which only short-day, winter conditions are considered. We assumed that birds can either rest, forage and eat, forage and cache, or retrieve existing caches (the two latter options are available only to caching birds). The model predicted that when there is variability in foraging success (here modeled strictly as within-day variability), both caching and non-caching birds should increase their fat reserves almost linearly in the morning slowing down toward late afternoon, a result consistent with field data but different than the result of a previous dynamic program. Non-cachers were predicted to carry higher fat levels than cachers especially when the variability in foraging success is high. Probability of death for non-caching birds was predicted to be higher than that for cachers, especially at higher levels of variability in foraging success. Among caching birds, an increase in number of caches and fat reserves was also predicted if: (1) mean foraging success was decreased, (2) variability in foraging success was increased, and (3) energy expenditure at night was increased over our baseline conditions. Under the conditions simulated in our model, birds were predicted to cache only if cache half-life (i.e., time interval over which 50% of the caches are forgotten or lost to pilferage) exceeded 2.5 days, indicating that low pilferage rate and long memory favor more caching. Finally, we showed that such daily patterns of energy management do not necessarily require relaxing assumptions about massdependent predation risk. Key words: caching, dynamic model, foraging, predation risk, pilferage. [Behav Ecol 12:207–218 (2001)]

Escape flights of yellowhammers and greenfinches: more than just physics

Wintering birds increase their fat reserves throughout the day, and impaired escape performance is often considered to be an important cost of fat reserves. Since lifting a larger mass requires more energy, if birds escape at maximum power output, an increase in mass will impair the escape flight. In this study we did not find support for mass-dependent escape performance for yellowhammers, Emberiza citrinella, and greenfinches, Carduelis chloris, with natural daily mass increases of 7–8%. This suggests either that the birds were not performing at maximum output at dawn, when light, or that maximum power output was higher at dusk, when heavy. Either way, the birds seemed to be able to put more effort into their escape flight when heavier. In both species, when alarmed, birds took off significantly faster and at a steeper angle than when not alarmed. Yellowhammers escaped at a higher speed and angle than greenfinches, and reacted faster to the predator model. This suggests that predator escape is more than just Newtonian physics, and may be influenced by behavioural, as well as morphological, adjustments. Different species may have evolved different responses to predation risk. Our results seem to be in disagreement with recent ideas about mass-dependent predation risk. However, to build up reserves, birds have to increase exposure time, which increases predation risk. This cost may be more important than impaired escape performance when relatively small, daily, changes in body mass are considered.

High Migratory Fuel Loads Impair Predator Evasion in Sedge Warblers

During migration, many species of birds rely on stored fat for fuel. The extra mass taken on for migration entails costs (Witter and Cuthill 1993). Time and energy must be devoted to foraging to build up fat loads, and increased feeding may increase the risk of being attacked by predators. An additional cost of increased fuel loads may be higher predation risk owing to reduced ability to take off, maneuver, and climb. Mass-dependent predation risk has been the focus of several recent theoretical studies (McNamara and Houston 1990, Hedenstrdm 1992, Witter and Cuthill 1993, Brodin 2000). In species that depend on flight to escape from predators, takeoff ability is crucial because once the prey are airborne, the success rate of predators diminishes (e.g. Rudebeck 1950, Kenward 1978, Lindstrdm 1989, Cresswell 1993). Within the natural range of body mass of nonmigratory birds (ca. 10% diurnal increase in mass), mass seems to have no measurable effect on takeoff ability (Kullberg 1998, Kullberg et al. 1998, Veasey et al. 1998, van der Veen and Lindstrdm 2000). In migratory birds, fuel loads of 20 to 30% of lean mass are common (Alerstam and Lindstrbm 1990), and fuel loads may exceed 100% of lean mass when passerines are about to cross wide barriers (e.g. Fry et al. 1970, Finlayson 1981). Although fat storage is the most common explanation for mass changes in birds, mass may change because of other reversible processes, e.g. by increases or decreases in muscle mass and in various internal organs (Piersma and Lindstrdm 1997). To date, only two species of migrants have been studied with respect to takeoff ability in a predatorescape situation. Kullberg et al. (1996) calculated that Blackcaps (Sylvia atricapilla) carrying 60% of lean body mass as fuel would have an angle of ascent that was 32% lower and a velocity that was 17% lower

Versatility from a Single Song: The Case of the Nightingale Wren

LIND, J., T. FRANSSON, S. JAKOBSSON, AND C. KULLBERG. 1999. Reduced take-off ability in Robins (Erithacus rubecula) due to migratory fuel load. Behavioral Ecology and Sociobiology 46:65-70. LINDSTROM, A. 1989. Finch flock size and risk of hawk predation at a migratory stopover site. Auk 106:225-232. MCNAMARA, J. M., AND A. I. HOUSTON. 1990. The value of fat reserves and the trade-off between starvation and predation. Acta Biotheoretica 38: 37-61. MOREAU, R. E. 1972. The Palaearctic-African bird migration systems. Academic Press, London. PENNYCUICK, C. J. 1989. Bird flight performance. A practical calculation manual. Oxford University Press, Oxford. PETTERSSON, J., AND D. HASSELQUIST. 1985. Fat deposition and migration capacity of Robins Erithacus rubecula and Goldcrests Regulus regulus at Ottenby, Sweden. Ringing and Migration 6:6676. PIERSMA, T., AND A. LINDSTROM. 1997. Rapid reversible changes in organ size as a component of adaptive behaviour. Trends in Ecology and Evolution 12:134-138. PULLIAM, H. R., AND G. . MILLS. 1977. The use of space by wintering sparrows. Ecology 58:13931399. RUDEBECK, G. 1950. The choice of prey and modes of hunting of predatory birds with special references to their selective effect. Oikos 2:65-88. VAN DER VEEN, I. T., AND K. M. LINDSTROM. 2000. Escape flights of Yellowhammers and Greenfinches: More than just physics. Animal Behaviour 59: 593-601. VEASEY, J. S., N. B. METCALFE, AND D. C. HOUSTON. 1998. A reassessment of the effect of body mass upon flight speed and predation risk in birds. Animal Behaviour 56:883-889. WITTER, M. S., AND I. C. CUTHILL. 1993. The ecological costs of avian fat storage. Philosophical Transactions of the Royal Society of London Series B 340:73-92. WITTER, M. S., I. C. CUTHILL, AND R. H. C. BONSER. 1994. Experimental investigation of mass-dependent predation risk in the European Starling, Sturnus vulgaris. Animal Behaviour 48:201-222.

Why do hoarding birds gain fat in winter in the wrong way? Suggestions from a dynamic model

In winter, small birds should be fat to avoid starvation and lean and agile to escape predators. This means that they face a tradeoff between the costs and benefits of carrying fat reserves. Every day they must gain enough fat to survive the coming night. Food-hoarding species can afford to carry less fat than nonhoarders because they can store energy outside the body. Furthermore, hoarders should avoid carrying excessive fat during the day because they can gain fat fast by retrieving food late in the afternoon. With no stored supplies, nonhoarders face more unpredictable access to food, and they should start gaining fat earlier in the day. The predicted pattern is then that nonhoarders gain fat early and that hoarders gain fat late in the day. Recent field data show the opposite pattern: hoarders gain relatively more fat reserves in the morning than nonhoarders do. Using a dynamic model that mimics the conditions in a boreal winter forest, I investigated under which conditions this pattern will arise. The only assumption of those investigated that produced this pattern was to relax the effect of mass-dependent predation risk. I did this by introducing a limit under which fat reserves did not affect predation risk. Hoarders then started the day by gaining fat in the morning. Later; when they had reached a safer (but still not risky) level, they switched to hoarding. The pattern I searched would only occur if either. not all food was possible to store, or if retrieval gave less energy than foraging in good weather conditions. If I assumed that low levels of body fat also increased predation risk, hoarders would cache in the morning when they carried least fat. I discuss empirical evidence for how body fat affects predation risk. In summary the factors that produced the pattern I searched were a change in the predation-mortality function combined with restrictions on hoarding.

Does diurnal variation in body mass affect take-off ability in wintering willow tits?

Recently, some empirical studies have shown an effect of body mass on take-off ability in birds, supporting the theory that birds with increased fat loads face a greater predation risk. Small wintering birds accumulate fat reserves of about 10% of their fat-free body mass during the day to survive long, cold, winter nights. Theoretically, mass-dependent predation risk could be an important factor affecting their daily foraging routines. I studied the take-off ability of wintering willow tits,Parus montanus, in relation to their fat load. Willow tits were on average 7.7% heavier at dusk than at dawn but there was no measurable effect of body mass on take-off ability. The results indicate that the relatively small fat loads accumulated during a day by willow tits do not increase the risk of predation as a consequence of reduced take-off ability.

Fat Storage in Male Willow Warblers in Spring: Do Residents Arrive Lean or Fat?

Early arrival on the breeding grounds confers important advantages for migratory birds during competition for resources and breeding opportunities (von Haartman 1968, Klomp 1970, M0ller 1994, Wiggins et al. 1994). Arriving in good condition also can be important for arctic-breeding birds (Davidson and Evans 1988) because clutch size and timing of breeding may be constrained by parental condition (Rowe et al. 1994). Long-distance passerine migrants arrive at near-arctic breeding sites with larger fat stores than do mediumand short-distance migrants (Sandberg 1996). Recently, Sandberg and Moore (1996) advanced a number of hypothesized benefits to passerine migrants that arrive with extra energy on their breeding grounds, including improved breeding performance, relaxed handling-time constraints, and insurance against adverse conditions. Fat accumulation is not without cost, however. Migrants accumulate fat at stopover sites along migration routes, and time spent foraging at these sites strongly affects overall migration speed (Alerstam and Lindstrom 1990). Other potential costs that should select against excess fat accumulation include: (1) diminishing return of increased fat depots on potential flight distance (e.g. Lindstrom and Alerstam 1992) and (2) mass-dependent predation risk due to impaired flight performance and higher exposure to predators during intense foraging (e.g. Hedenstrbm 1992, Houston and McNamara 1993, Witter and Cuthill 1993, Metcalfe and Ure 1995, Kullberg et al. 1996). Despite the importance of fat, detailed information about fat content in migrant passerines when they arrive at breeding sites and during the transition from migration to breeding is rare (but see Ojanen 1984, Sandberg 1996). In this study, we investigated the amount of visible fat stores in male Willow Warblers (Phylloscopus trochilus) on the day they arrived at a breeding site in southern Sweden. The Willow Warbler is a long-distance migrant; birds that breed in northern Europe spend the winter in tropical Africa, some of them as far south as South Africa (Hedenstrom and Pettersson 1987). We compared visible

Impaired predator evasion in fat blackcaps (Sylvia atricapilla)

When birds are attacked by predators, take-off ability is crucial for the chance of survival. Recently, theoretical studies have predicted that predation risk in terms of reduced flight performance increases with body mass. However, empirical data are scarce. Because migratory birds sometimes double their body mass, mass dependent predation risk may be especially important during migratory fattening. Here we present the first study of take-off ability in relation to migratory fat load. Alarmed take-off flights of caged blackcaps (Sylvia atricapilla) induced by a simulated predator attack were analysed in terms of velocity and angle of ascent. Fat loads (percentage of fat-free body mass) of the birds ranged from 1% to 59%. An increase in fat load was found to influence both velocity and angle of ascent. From our results we calculated that blackcaps carrying 60% fat loads would have 32% lower angle of ascent and 17% lower velocity than blackcaps carrying no fat load. Even though the effect of fat load on the blackcaps was less than indicated in previous experimental studies of other species, our results suggest that the large fat loads needed for migration probably place them at increased risk of predation.