Both the past and the present affect risk-sensitive decisions of foraging rufous hummingbirds

Behavioral Ecology doi:10.1093/beheco/arq031 Advance Access publication 22 March 2010 Both the past and the present affect risk-sensitive decisions of foraging rufous hummingbirds Ida E. Bacon,a T. Andrew Hurly,b and Susan Healyc Institute of Evolutionary Biology, University of Edinburgh, Kings Buildings, Edinburgh EH9 3JT, UK, b Department of Biological Sciences, University of Lethbridge, 4401 University Drive, Lethbridge, Alberta, T1K 3M4, Canada, and cSchool of Psychology, University of St Andrews, Saint Andrews, Fife KY16 9JP, UK a There is substantial evidence that an animal’s current energy budget affects its preference for food patches that provide a constant reward relative to patches that provide a variable reward, when both patches have the same mean reward. Animals currently on a positive energy budget are expected to choose the constant option, whereas animals on a negative budget are expected to use the variable option. There is increasing evidence that prior experience can affect an animal’s current decisions. We investigated choices made by rufous hummingbirds when they were tested with strong or weak sucrose solutions after several days of foraging on those strong or weak solutions. Foraging from weak concentrations prior to and during testing led to a higher preference for the variable option, whereas foraging from strong concentrations led to an increased preference for the constant option. We suggest that the energetic conditions experienced by animals prior to testing had a significant impact on the animals’ risk-sensitive decisions, and their memories of those prior conditions may have played an additional role. This implies that the conditions animals are maintained under prior to testing may significantly affect the outcome of risk-sensitivity experiments. Key words: decisions, foraging, hummingbirds, past, risk-sensitive. [Behav Ecol 21:626–632 (2010)] iven 2 food options where reward quality and mean reward amount over time are equal for both options but where the variability in reward amount differs, animals often prefer one option over the other (Logan 1965; Young 1981; Real et al. 1982; Kacelnik and Bateson 1996; Drezner-Levy and Shafir 2007; Schuck-Paim and Kacelnik 2007; Heilbronner et al. 2008). For example, blue jays (Cyanocitta cristata) given the option of foraging in a variable patch that provided either 2- or 4-half mealworms (a mean of 3 halves) or a constant patch that provided 3-half mealworms preferred to feed from the constant patch (ca. 70% of feeds, Clements 1990). This sensitivity to variation in reward can be explained by the energy-budget rule (Stephens 1981), which stipulates that when an animal is on a positive energy budget and is not in danger of starvation, it should choose the constant option in order to minimize the chance that it will encounter numerous small rewards and risk starvation. However, an animal in danger of starvation should choose the variable option as it increases the chance that the animal will encounter sufficiently large rewards to avoid starvation. The energy-budget rule does not fully explain risk-sensitive choice as it predicts exclusive preference for either the constant or variable option, whereas partial preferences are observed in the vast majority of cases (Kacelnik and Bateson 1996; Hurly 2003). One possible explanation for partial preferences is that animals continue sampling all options in order to enable resource tracking (Krebs et al. 1978; Schuck-Paim and Kacelnik 2007). Another possible reason for partial preferences is that animals forage G Address correspondence to I.E. Bacon. E-mail: .uk. Received 30 July 2009; revised 18 February 2010; accepted 20 February 2010.
The Author 2010. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved. For permissions, please e-mail: among rewards in proportion to how rewarding they find them rather than entirely avoiding less preferred options (Shapiro 2000; Schuck-Paim and Kacelnik 2007). Manipulations of energy budgets either before or during choice trials show that an animal’s sensitivity to reward variability is, indeed, state-dependant (Kacelnik and Bateson 1996). For example, dark-eyed juncos (Junco hyemalis) preferred the constant reward when on a positive energy budget, were indifferent when on a balanced energy budget, and preferred the variable reward when on a negative energy budget (Caraco 1981). Likewise, yellow-eyed juncos (Junco phaeonotus) preferred the variable option under conditions of low ambient temperature and the constant option under warmer, less energetically demanding, conditions (Caraco et al. 1990). Typically, in this kind of experiment, manipulations are designed so that animals experience a particular energy budget, which is then expected to be the main influence on their risk sensitivity. However, the duration of the energy-budget manipulations, which varies considerably among experiments, may play a significant role in the outcome of the experiments as previous foraging experiences often affect current foraging decisions. For example, young canaries fed for 8 weeks on a single seed type (hemp, niger, millet, or linseed) and then on a mixed seed diet for 15 weeks, preferred the seed type on which they were reared. Birds reared on a mix of all 4 seeds, on the other hand, preferred hemp seed (Doherty and Cowie 1994). If the context animals experienced weeks ago (such as the canaries’ previous diet) can affect current preferences, then it seems likely that previous experiences may also affect an animal’s sensitivity to variability. Prior knowledge of patch types and their frequency in the environment can be used by animals along side current sampling information to form an estimate/expectation of patch quality and inform, for example, patch-leaving decisions (e.g., McNamara 1982). This is Bacon et al. • Past experience affects hummingbirds’ risk sensitivity referred to as Bayesian or Bayesian-like decision making and has been described in many species including: the Arizona pocket mouse, Perognathus amplus; Merriam’s kangaroo rat, Dipodomys merriami; and the round-tail ground squirrel, Spermophilus tereticaudus (Valone and Brown 1989); Inca doves, Columbina inca (Valone 1991); Black-chinned hummingbirds, Archilochus alexandri (Valone 1992), and Bumblebees, Bombus impatiens Cresson (Biernaskie et al. 2009). Expectations can be affected by experiences from throughout the animal’s life (Simitzis, Bizelis, et al. 2008; Simitzis, Deligeorgis, et al. 2008). Effects of expectations about the quality of specific foraging locations on foraging behavior have been demonstrated both in vertebrates and invertebrates (Lima 1983; Schilman and Roces 2003; Gil et al. 2007): animals return more often to and invest more effort in investigating locations or food types associated with higher rewards, but if those reward values are decreased, animals continue (...truncated)