Cecilia Kullberg

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.

Escalated fighting behaviour incurs increased predation risk

Fights over limited resources have been studied in a wide range of animal species (Huntingford & Turner 1987). Realized costs of fighting behaviour are inflicted injuries, and loss of time and energy. A neglected additional cost of intense fighting behaviour in prey species could be an increased risk of being killed by a predator (Jakobsson 1987; Hammerstein & Reichert 1988). We have studied male fighting behaviour in two species vulnerable to predation, Nannacara anomala, a South

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. Copyright 1998 The Association for the Study of Animal Behaviour.

Predator–induced take–off strategy in great tits (Parus major)

When birds are attacked by predators the initial take–off is crucial for survival. The strategy in the initial phase of predator evasion is probably affected by factors such as body mass and presence of cover and conspecifics, but it may also be a response to the character of the predators attack. In choosing an angle of flight, birds face a trade–off between climbing from the ground and accelerating across the ground. This is, to our knowledge, the first study investigating whether the attack trajectory of a raptor affects the take–off strategy of the prey bird. First–year male great tits (Parus major) adjusted take–off angle to a model predators angle of attack. Birds attacked from a steep angle took off at a lower angle than birds attacked from a low angle. We also compared take–offs at dawn and dusk but could not find any measurable effect of the diurnal body mass gain (on average 7.9%) in the great tits on either flight velocity or angle of ascent.

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

Linking social complexity and vocal complexity: a parid perspective

The Paridae family (chickadees, tits and titmice) is an interesting avian group in that species vary in important aspects of their social structure and many species have large and complex vocal repertoires. For this reason, parids represent an important set of species for testing the social complexity hypothesis for vocal communication—the notion that as groups increase in social complexity, there is a need for increased vocal complexity. Here, we describe the hypothesis and some of the early evidence that supported the hypothesis. Next, we review literature on social complexity and on vocal complexity in parids, and describe some of the studies that have made explicit tests of the social complexity hypothesis in one parid—Carolina chickadees, Poecile carolinensis. We conclude with a discussion, primarily from a parid perspective, of the benefits and costs of grouping and of physiological factors that might mediate the relationship between social complexity and changes in signalling behaviour.

Magnetic cues and time of season affect fuel deposition in migratory thrush nightingales (Luscinia luscinia)

Bird migration requires high energy expenditure, and long–distance migrants accumulate fat for use as fuel during stopovers throughout their journey. Recent studies have shown that long–distance migratory birds, besides accumulating fat for use as fuel, also show adaptive phenotypic flexibility in several organs during migration. The migratory routes of many songbirds include stretches of sea and desert where fuelling is not possible. Large fuel loads increase flight costs and predation risk, therefore extensive fuelling should occur only immediately prior to crossing inhospitable zones. However, despite their crucial importance for the survival of migratory birds, both strategic refuelling decisions and variation in phenotypic flexibility during migration are not well understood. First–year thrush nightingales (Luscinia luscinia) caught in the early phase of the onset of autumn migration in southeast Sweden and exposed to a magnetic treatment simulating a migratory flight to northern Egypt increased more in fuel load than control birds. By contrast, birds trapped during the late phase of the onset of autumn migration accumulated a high fuel load irrespective of magnetic treatment. Furthermore, early birds increased less in flight–muscle size than birds trapped later in autumn. We suggest that the relative importance of endogenous and environmental factors in individual birds is affected by the time of season and by geographical area. When approaching a barrier, environmental cues may act irrespective of the endogenous time programme.

Geolocators reveal three consecutive wintering areas in the thrush nightingale

The winter distribution of many migratory birds wintering in tropical Africa is poorly known. After the crossing of the Sahara Desert, some long-distance migrants typically stay in the Sahel zone for an extended period before continuing migration to their main wintering areas south of the equator. Here we show how two thrush nightingales (Luscinia luscinia) fitted with light-level geolocators, after a six to seven week long stay in the Sahel zone of Sudan, moved to an intermediate area in northern Kenya for a month-long stay before continuing to their final wintering areas in southern Africa. These data indicate that thrush nightingales may use three consecutive wintering sites during their stay in Africa. The migratory movements in Africa between wintering sites are well-coordinated with high precipitation in these areas, suggesting that thrush nightingales track peaks of insect abundance occurring after rains. This three-stage wintering strategy has, to our knowledge, previously not been described, and shows that long-distance migrants can have complex wintering behaviour.

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.

Geomagnetic field affects spring migratory direction in a long distance migrant

Night-migrating song birds travel to and from their wintering and breeding areas often separated thousands of kilometers apart and are clearly capable of finding intended goal areas from a distant location. Displacement experiments provide a useful way to highlight orientation and navigational skills in migrants. To investigate which cues birds actually use to compensate for displacement and the exact mechanism of each cue, experiments with manipulation of single cues are required. We conducted a simulated displacement of lesser whitethroats Sylvia curruca on spring migration. Birds were displaced not geographically but in geomagnetic space only, north and south of their breeding area to test whether they incorporate information from the geomagnetic field to find their breeding area. Lesser whitethroats held in southeast Sweden but experiencing a simulated displacement north of their breeding area (Norway) failed to show a consistent direction of orientation, whereas birds displaced south of their breeding area (Czech Republic) exhibited consistent northerly orientation, close to the expected seasonally appropriate direction, after displacement toward the trapping location. The absence of a clear compensatory direction in birds displaced north might be due to unfamiliar magnetic information or lack of sufficient information such as a magnetic gradient when moving around. By isolating one orientation cue, the geomagnetic field, we have been able to show that lesser whitethroats might incorporate geomagnetic field information to determine latitude during spring migration.