Evidence of the Trade-Off between Starvation and Predation Risks in Ducks

Citation: Zimmer C, Boos M, Poulin N, Gosler A, Petit O, et al. ( Evidence of the Trade-Off between Starvation and Predation Risks in Ducks Ce dric Zimmer 0 Mathieu Boos 0 Nicolas Poulin 0 Andrew Gosler 0 Odile Petit 0 Jean-Patrice Robin 0 Sean A. Rands, University of Bristol, United Kingdom 0 1 Universite de Strasbourg, IPHC, Strasbourg, France , 2 CNRS, UMR 7178, Strasbourg , France , 3 Research Agency in Applied Ecology The theory of trade-off between starvation and predation risks predicts a decrease in body mass in order to improve flight performance when facing high predation risk. To date, this trade-off has mainly been validated in passerines, birds that store limited body reserves for short-term use. In the largest avian species in which the trade-off has been investigated (the mallard, Anas platyrhynchos), the slope of the relationship between mass and flight performance was steeper in proportion to lean body mass than in passerines. In order to verify whether the same case can be applied to other birds with large body reserves, we analyzed the response to this trade-off in two other duck species, the common teal (Anas crecca) and the tufted duck (Aythya fuligula). Predation risk was simulated by disturbing birds. Ducks within disturbed groups were compared to non-disturbed control birds. In disturbed groups, both species showed a much greater decrease in food intake and body mass during the period of simulated high risk than those observed in the control group. This loss of body mass allows reaching a more favourable wing loading and increases power for flight, hence enhancing flight performances and reducing predation risk. Moreover, body mass loss and power margin gain in both species were higher than in passerines, as observed in mallards. Our results suggest that the starvation-predation risk trade-off is one of the major life history traits underlying body mass adjustments, and these findings can be generalized to all birds facing predation. Additionally, the response magnitude seems to be influenced by the strategy of body reserve management. - Funding: This study was supported by grants from Region Alsace, National Fund for Biological Research on Game and Wildlife Species and CNRS. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. The competition between two or more processes for the allocation of limited resources generally results in a trade-off that underlies different life-history traits [1]. One important trade-off occurs for species acquiring food while avoiding predation [2,3]. Animals have to build up or maintain body fuel reserves which are an important buffer against starvation, especially when harsh winter weather conditions involve unpredictable food availability and energy requirements [4]. However, it is surprising to see that birds often maintain their level of body reserves below the maximum threshold [5]. Assuming that maintaining a high level of body fuel, i.e. a high body mass, also incurs a significant cost in terms of enhanced mortality risk due to predation vulnerability [5], the amount of body reserves that a bird carries has generally been viewed as a trade-off between the risk of starvation and the risk of predation [6,7]. Body mass adjustment is considered to be the consequence of this trade-off. In this context, the mass-dependent predation risk theory predicts that if the probability of an individual being caught by a predator depends on its body mass, its weight should be maintained at an appropriate level to balance predation risk against the risk of starvation [6,810]. Such body mass adjustment has the advantage of improving flight performance and reducing the associated metabolic demands. It also results in a lower investment in foraging time and less exposure to predation [7,10,11]. A high body mass is correlated with high wing loading and a greater cost of flight. These two factors could impair flight performance, particularly during take-off, due to a smaller angle of ascent and a lower speed [10,1218]. Conversely, birds have to maintain a level of body reserves which is high enough to limit the risk of starvation [6,10]. It has generally been assumed that this strategy would lead animals to carry greater body reserves when starvation risk is high and vice versa [6,13]. Nevertheless, empirical data on the starvation-predation risk trade-off that illustrates a decrease in body mass when individuals are under higher predation risks mainly originate from studies on small passerine birds [2,1926]. Furthermore, experimental studies have demonstrated that when predation risk was increased or when predator attacks were simulated by chasing the birds, food consumption decreased in order to adjust body mass [19,21,23]. This body mass adjustment improves take-off performance because the available power for flying increases when body mass declines [27] and this ultimately maximizes survival. To our knowledge, apart from the afore-mentioned studies of passerines the only other species studied in relation to the starvation-predation risk trade-off are the redshank (Tringa totanus) [28], a larger species, the mallard (Anas platyrhynchos) [29] and one non-bird species, the harbour porpoise (Phocoena phocoena) [2]. In the two last species, it has been shown that body mass or body reserves were linked to predation risk. In mallards, the relative body mass decrease was twice as high as in passerines [29] and it was hypothesized that this was due to a difference in body mass and the amount of body reserves of each species: whereas passerines build up body reserves during the day and use them during the following night for energetic purposes [4,30] mallards store more body reserves than required immediately in order to cope with possible future periods of cold spells [3133]. Furthermore, large birds have higher body reserves and a lower metabolism per unit body mass than small species which have a higher surface/volume ratio [34]. Thus, large birds can sustain greater body mass variations than small ones, even in proportion to lean mass, without dramatically increasing their starvation risk. Moreover, this is consistent with the idea that greater body mass loss allows a greater power margin gain in large birds than in passerines. The power margin is defined as the ratio between power available and power required for flight. It therefore appears that the magnitude of the response to increased predation risk depends on species size, with a higher body mass loss in large birds than in small ones due to the difference in the amount of body reserves stored [29]. The present study was carried out on one small and one medium-sized duck species, the common teal (Anas crecca) and the tufted duck (Aythya fuligula) respectively. Predation risk was artificially increased in ord (...truncated)