Mass-dependent Predation Research Articles

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.

Experimental investigations of mass-dependent predation risk in the European starling, Sturnus vulgaris

Abstract. The influence of mass on escape ability in the European starling was investigated with reference to aerial manoeuvrability and take-off ability. Experimental groups of higher body mass were less manoeuvrable. In the take-off experiments, manipulations of mass influenced predominantly trajectory rather than velocity or distance covered in a given time. The take-off strategies observed may represent a trade-off between maximizing rate of ascent and maximizing escape velocity. Under increased load, the birds appeared to 'defend' escape velocity at the expense of rate of ascent. It is argued that if these changes in escape performance are important determinants of predation risk then changes in body mass, or the effects of such changes, should be incorporated into decisions that might result in exposure to predation. Consistent with this proposition, manipulations of body mass influenced the decision to leave protective cover; alternative interpretations of this result are also considered. In a separate procedure, the hypothesis that perceived predation risk influences the amount of stored fat was investigated by varying the availability of protective cover. Where most cover was available, fat reserves, scored from visible subcutaneous fat deposits, were highest. There was no relationship, however, between wing loading (body weight/wing area) and protective cover, suggesting that other components of body mass may change in association with fat reserves. The relevance of these results for regulation of body mass and mass-allocation strategies is discussed.

Dynamic Models of Mass‐Dependent Predation, Risk‐Sensitive Foraging, and Premigratory Fattening in Birds

The effect of mass—dependent predation on risk—sensitive foraging choices of small birds in two ecological contexts was examined using stochastic dynamic models. In the first context, nonmigratory foraging, predation risk dependent on the bird's body mass leads to risk—averse behavior in situations were risk—neutral behavior would otherwise be expected and generally reduces the amount of foraging and the level of reserves carried by foragers. In the second context, premigratory foraging, mass—dependent predation can lead to risk proneness during the buildup of reserves for migration. Risk proneness is favored if birds can reduce mass and hence reduce predation after migration, thereby giving them an advantage to rapid departure. Other advantages of early departure can also lead to premigratory risk proneness but require more specific ecological assumptions. The literature on risk sensitivity in amount of food reward suggests a connection between migration and risk proneness on negative energy budgets but more tests are needed before any conclusions can be drawn.

The ecological costs of avian fat storage.

Avian fat storage is associated with both benefits and costs. Although the benefits of maintaining higher energetic reserves have long been considered, the associated costs have received far less attention. Spatial and temporal patterns of fat storage, together with experimental data, indicate that birds are capable of actively regulating their energetic reserves at levels below physiological or environmental maxima. This regulation implies that fat storage entails a cost. Evidence of potential costs are reviewed and discussed under the following headings: mass-dependent metabolism, mass-dependent predation risk, mass-dependent risk of injury, mass-dependent foraging, pathological costs and reproductive costs. Although the evidence that fat storage is costly is convincing, key empirical data are lacking. We indicate the sorts of data which need to be gathered and suggest ways in which this might be done. We go on to discuss the interaction of these costs and their relevance to between-individual patterns of fat storage and the interpretation of 'condition indices'. Because many of the purported costs of fat storage are dependent upon changes in body mass, or wing loading, our review is also relevant to other phenomena which may involve mass-dependent costs, such as gonadal hypertrophy, transport of food items and primary moult.

Studies on predation of the mosquito Culex fatigans by Rana tigrina tadpoles

Satiated predation, predation rate and prey preference of different weight groups of Rana tigrina (Daud) tadpoles on different larval and pupal stages of Culex fatigans were studied. Irrespective of the prey and predator size, the satiation time remained more or less equal. There exists a mass-dependent predation: Calculated predation rates or predatory constants (Kpr) showed that I instar prey was preyed upon at about equal rate, while other instars and pupa showed an increasing trend with increasing body weight of the predator. The prey preference assessed using the Kpr, revealed that prey size is an important parameter in predation. The R. tigrina tadpole is a more efficient pupal predator than other mosquito predators.