To what extent is the foraging behaviour of aquatic birds constrained by their physiology?

Aquatic birds have access to limited amounts of usable oxygen when they forage (dive) underwater, so the major physiological constraint to their behaviour is the need to periodically visit the water surface to replenish these stores and remove accumulated carbon dioxide. The size of the oxygen stores and the rate at which they are used (V dot o2) or carbon dioxide accumulates are the ultimate determinants of the duration that aquatic birds can remain feeding underwater. However, the assumption that the decision to terminate a dive is governed solely by the level of the respiratory stores is not always valid. Quantification of an optimal diving model for tufted ducks (Aythya fuligula) shows that while they dive efficiently by spending a minimum amount of time on the surface to replenish the oxygen used during a dive, they dive with nearly full oxygen stores and surface well before these stores are exhausted. The rates of carbon dioxide production during dives and removal during surface intervals are likely to be at least as important a constraint as oxygen; thus, further developments of optimal diving models should account for their effects. In the field, diving birds will adapt to changing environmental conditions and often maximise the time spent submerged during diving bouts. However, other factors influence the diving depths and durations of aquatic birds, and in some circumstances they are unable to forage sufficiently well to provide food for their offspring. The latest developments in telemetry have demonstrated how diving birds can make physiological decisions based on complex environmental factors. Diving penguins can control their inhaled air volume to match the expected depth, likely prey encounter rate, and buoyancy challenges of the following dive.     

Do heterospecific size and demeanour influence visitation behaviour of birds at urban foraging patches?

We compared the feeding behaviour of garden birds in the presence of four heterospecific decoy species at suburban feeding stations to assess the relative importance of body size (large vs. small) and demeanour (high vs. low potential for inter-specific aggression) as cues for threat recognition. We conducted separate trials with live, caged Australian magpie, Cracticus tibicen, and common myna, Acridotheres tristis, large and small species known to chase and harass heterospecifics, and comparably sized species not known for inter-species aggression (i.e. rock dove, Columba livia, and ringneck dove, Streptopelia risoria). Birds of all species arrived at feeding stations sooner and displayed higher-risk activities in the presence of small decoys. The two most common species that visited the feeding stations, silvereye, Zosterops lateralis, and house sparrow, Passer domesticus, were most likely to perform high-risk behaviours (i.e. landing on the stimulus cage and feeding from the station) in the presence of common mynas. The activities were performed least frequently in the presence of Australian magpies. As garden birds likely had the most experience interacting with common mynas, the results suggest that a two-step decision rule, with size and familiarity as important cues, may be used to recognize potential threats.     

Threshold foraging behaviour of basking sharks on zooplankton: life on an energetic knife-edge?

The minimum threshold foraging response of basking sharks has not been determined despite the widely held view that has been perpetuated in the literature for the past 45 years that this species cannot use low prey densities for net energy gain and so lives on an energetic ''knife-edge''. An early theoretical estimate suggested basking sharks would expend more energy collecting zooplankton at concentrations less than 1.36 g m^-3 than could be obtained from it. This led to the claim that basking sharks will feed at an energetic loss for much of the annual cycle as zooplankton abundance outside summer months is too low for net energy gain to occur. Here I show from theoretical calculations and behavioural studies on individual and group-feeding sharks in the English Channel that basking sharks have a theoretical threshold prey density of between 0.55 and 0.74 g m^-3 and an observed foraging threshold of between 0.48 and 0.70 g m^-3 (mean = 0.62 g m^-3). The close agreement between theoretical and empirical threshold values suggests basking sharks can achieve net energy gain in much lower zooplankton densities than previously thought. The findings imply that this species may not be reliant upon the ''migration–hibernation'' energy conservation strategy it is purported to exhibit when seasonal zooplankton abundance decreases below 1.36 g m^-3.     

Sex‐specific foraging behaviour in tropical boobies: does size matter?

Sex differences in the foraging behaviour of adults have been observed in a number of sexually size-dimorphic birds, and the usual inference has been that these sex-specific differences are driven primarily by differences in body size. An alternative explanation is that foraging differences result from sex differences unrelated to size, such as sex-specific nutritional requirements. To examine these alternative hypotheses, the foraging behaviour of parents was compared between two sympatric and congeneric species of seabird, the Brown Booby Sula leucogaster , which is highly sexually size-dimorphic (females 38% larger) and the Red-footed Booby S. sula , in which sex differences in body size are less marked (females 15% larger). Using temperature and depth loggers, we found that there were highly significant differences in the foraging trip durations and diving behaviour of male and female Brown Boobies. These sex differences were less marked in Red-footed Boobies. Thus, our interspecies comparison revealed that the magnitude of the difference between the sexes matched the sexual size dimorphism of the species, providing support for the size hypothesis.     

Breeding birds on small islands: island biogeography or optimal foraging?

1. We test MacArthur and Wilson's theory about the biogeography of communities on isolated habitat patches using bird breeding records from 16 small islands off the coasts of Britain and Ireland. 2. A traditional examination of patterns of species richness on these islands suggests that area and habitat diversity are important predictors, but that isolation and latitude have a negligible impact in this system. 3. Unlike traditional studies, we directly examine the fundamental processes of colonization and local extinction (cessation of breeding), rather than higher-order phenomena such as species richness. 4. We find that many of MacArthur and Wilson's predictions hold: colonization probability is lower on more isolated islands, and extinction probability is lower on larger islands and those with a greater diversity of habitats. 5. We also find an unexpected pattern: extinction probability is much lower on more isolated islands. This is the strongest relationship in these data, and isolation is the best single predictor of colonization and extinction. 6. Our results show that examination of species richness alone is misleading. Isolation has a strong effect on both of the dynamic processes that underlie richness, and in this system, the reductions in both colonization and extinction probability seen on more distant islands have opposing influences on species richness, and largely cancel each other out. 7. We suggest that an appropriate model for this system might be optimal foraging theory, which predicts that organisms will stay longer in a resource patch if the distance to a neighbouring patch is large. If nest sites and food are the resources in this system, then optimal foraging theory predicts the pattern we observe. 8. We advance the hypothesis that there is a class of spatial systems, defined by their scale and by the taxon under consideration, at which decision-making processes are a key driver of the spatiotemporal dynamics. The appropriate theory for such systems will be a hybrid of concepts from biogeography/metapopulation theory and behavioural ecology.     

Hypsodonty in ungulates: an adaptation for grass consumption or for foraging in open habitat?

Hypsodont (i.e. high-crowned) teeth have been interpreted as an indicator of feeding preferences and habitat selection in ungulates. For this reason, the degree of hypsodonty has been used for estimating the diet of ancient taxa and in palaeoenvironmental reconstructions. The goal of this study is to elucidate the relative importance of grass consumption and open habitat foraging in the development of hypsodont teeth, using novel computer techniques of knowledge discovery applied to a dataset of 134 species of artiodactyls and perissodactyls distributed among thirteen families. The results obtained suggest that high-crowned teeth represent an adaptation for feeding in an open habitat, although the minimum threshold of hypsodonty seems to increase with the relative length of the anterior part of the jaw. On the contrary, there is no direct relationship between the degree of hypsodonty and the percentage of grass consumed, except for the correspondence between grazing and dwelling in open habitats. A relatively wide muzzle evidences an adaptation for grass foraging in open and mixed habitats, but there are some non-grazing species from a closed habitat that also show wide muzzles. Thus, the hypsodonty index, combined with the length of the anterior part of the jaw and the width of the muzzle, allows accurate inferences on the ecological preferences of extinct ungulates.     

Plasticity in body composition in breeding birds: what drives the metabolic costs of egg production?

Body composition in vertebrates is known to show phenotypic plasticity, and changes in organ masses are usually rapid and reversible. One of the most rapid and reversible changes is the transformation of the female avian reproductive organs before breeding. This provides an excellent system to investigate the effects of plasticity in organ size on basal metabolic rate (BMR) through relationships between organ masses and BMR. We compared body composition of female European starlings (Sturnus vulgaris) during various reproductive stages over 3 yr and investigated the pattern of changes in reproductive and nonreproductive organ mass during follicular development and ovulation. Furthermore, we analyzed the relationship between organ mass and resting metabolic rate (RMR) in nonbreeding, laying, and chick-rearing females. Our analysis revealed marked variation in organ masses between breeding stages but no consistent pattern among years except for kidney and pectoralis muscle. Furthermore, changes in nonreproductive organs did not parallel the cycle of growth and regression of the reproductive organs. The oviduct gained 62% of its 22-fold increase in mass in only 3 d, and oviduct regression was just as rapid and began even before the final egg of the clutch was laid, with 42% of the oviduct mass lost before laying of the final egg. In laying females, 18% of variation in mass-corrected RMR was explained by the mass of the oviduct (r2=0.18, n=80, P<0.0005), while pectoralis muscle mass in nonbreeding individuals and liver and gizzard mass in chick-rearing females were the only organs significantly related to RMR (r2=0.31-0.44). We suggest that the nonreproductive organs are affected more by changes in local ecological conditions than the reproductive state itself and that the activity and maintenance cost of the oviduct is high enough that selection has led to a very tight size-function relationship for this organ.     

Pheromones in birds: myth or reality?

Birds are anosmic or at best microsmatic… This misbelief persisted until very recently and has strongly influenced the outcome of communication studies in birds, with olfaction remaining neglected as compared to acoustic and visual channels. However, there is now clear empirical evidence showing that olfaction is perfectly functional in birds and birds use olfactory information in a variety of ethological contexts. Although the existence of pheromones has never been formally demonstrated in this vertebrate class, different groups of birds, such as petrels, auklets and ducks have been shown to produce specific scents that could play a significant role in within-species social interactions. Behavioral experiments have indeed demonstrated that these odors influence the behavior of conspecifics. Additionally, in quail, deprivation of olfactory inputs decreases neuronal activation induced by sexual interactions with a female. It seems therefore well established that birds enjoy a functional sense of smell and a fast growing body of experimental evidence suggests that they use this channel of olfactory communication to control their social life. The unequivocal identification of an avian pheromone is, however, still ahead of us but there are now many exciting opportunities to unravel the behavioral and physiological particularities of chemical communication in birds.     

Hormone-mediated maternal effects in birds: mechanisms matter but what do we know of them?

Over the past decade, birds have proven to be excellent models to study hormone-mediated maternal effects in an evolutionary framework. Almost all these studies focus on the function of maternal steroid hormones for offspring development, but lack of knowledge about the underlying mechanisms hampers further progress. We discuss several hypotheses concerning these mechanisms, point out their relevance for ecological and evolutionary interpretations, and review the relevant data. We first examine whether maternal hormones can accumulate in the egg independently of changes in hormone concentrations in the maternal circulation. This is important for Darwinian selection and female physiological trade-offs, and possible mechanisms for hormone accumulation in the egg, which may differ among hormones, are reviewed. Although independent regulation of plasma and yolk concentrations of hormones is conceivable, the data are as yet inconclusive for ovarian hormones. Next, we discuss embryonic utilization of maternal steroids, since enzyme and receptor systems in the embryo may have coevolved with maternal effect mechanisms in the mother. We consider dose-response relationships and action pathways of androgens and argue that these considerations may help to explain the apparent lack of interference of maternal steroids with sexual differentiation. Finally, we discuss mechanisms underlying the pleiotropic actions of maternal steroids, since linked effects may influence the coevolution of parent and offspring traits, owing to their role in the mediation of physiological trade-offs. Possible mechanisms here are interactions with other hormonal systems in the embryo. We urge endocrinologists to embark on suggested mechanistic studies and behavioural ecologists to adjust their interpretations to accommodate the current knowledge of mechanisms.     

Blown away? Wind speed and foraging success in an acoustic predator

Mammal Research - Foraging animals must contend with fluctuating environmental variables that affect foraging success, including conditions like wind noise, which could diminish the usefulness of...     

Animal behaviour: emotion in invertebrates?

Bees exposed to vigorous shaking designed to simulate a dangerous event judge ambiguous stimuli as predicting a negative outcome - a 'pessimistic' cognitive bias that is characteristic of anxious or depressed humans and other vertebrates in putative negative emotional states.     

Do house mice modify their foraging behaviour in response to predator odours and habitat?

Predator odours and habitat structure are thought to influence the behaviour of small mammalian prey, which use them as cues to reduce risks of predation. We tested this general hypothesis for house mice, Mus domesticus, by manipulating fox odour density via addition of fox scats and habitat via patchy mowing of vegetation, for populations in 15 × 15-m field enclosures. Using giving-up densities (GUDs), the density of food remaining when an animal quits harvesting a patch, we measured foraging behaviours in response to these treatments. Mice consistently avoided open areas, leaving GUDs two to four times greater in these areas than in densely vegetated patches. However, mouse GUDs did not change in response to the addition of fox scats, even immediately after fresh scats were added. There was no interaction between fox odour and habitat use. We then tested whether habituation to fox odours had occurred, by comparing the individual responses to scats of eight mice born into enclosures with fox scats to those of eight mice born into scat-free enclosures and five wild mice. In smaller enclosures, GUDs of trays with scats did not differ from GUDs of trays without scats for any treatment. We conclude that exposure to high levels of fox odours did not alter the foraging behaviour of mice, but that mice did reduce foraging in areas where habitat was removed, perceiving predation risk to be greater in these areas than controls. We suggest further that studies using the ‘scat-at-trap’ technique, which have shown avoidance of predator odours by mice and other small mammals, may overestimate the general avoidance of predator odours by free-living prey, which must forage with a constant background of predator odours.     

What influences aggression and foraging activity in social birds? Measuring individual,group and environmental characteristics

For specialised feeders, accessing food resources may impact on the performance of appetitive foraging and social behaviours at individual and population levels. Flamingos are excellent examples of social species with complex, species-specific feeding strategies. As attainment of coloured plumage depends upon intake of dietary carotenoids, and as study of free-ranging flamingos shows that foraging is disrupted by aggression from other birds, we investigated the effect of four feeding styles on foraging and aggression in captive lesser flamingos. We evaluated individual and group differences in foraging and aggression when birds consumed bespoke “flamingo pellet” from a bowl, an indoor feeding pool and an outdoor feeding section of their pool. Natural foraging (when birds were feeding irrespective of the presence of pellet) was recorded for comparison with artificial feeding styles. One-minute long video footage of the birds' activities in these different locations, recorded between 2013 and 2016, was used to evaluate behaviour. Total number of seconds engaged in feeding and in aggression was recorded by continuous sampling. The colour of individual birds was scored from 1 (mainly white) to 4 (mainly pink). For natural filter feeding in the outdoor pool, maximum foraging was twice as much as bowl feeding, whilst aggression was less than half as much as other feeding methods. Overall, a more restricted feeding style significantly predicted aggression, along with increasing group size. Plumage colour significantly influenced aggression (brightest flamingos were more aggressive) and showed a non-significant trend with foraging (brighter birds fed less than paler birds). No sex effect on feeding or aggression was found. This study enhances our understanding of husbandry and species' biology impacts on captive behaviour and provides data-based evidence to improve food presentation. For flamingos, implementation of spacious outdoor feeding areas can encourage natural foraging patterns by reducing excess aggression and enhances welfare by improving flock social stability.