Recovery of hematocrit and fat deposits varies by cage size in food-restricted captive red crossbills (Loxia curvirostra)

Moderate mass loss enhances flight performance via alteration of flight kinematics and postures in a passerine bird

Many birds experience fluctuations in body mass throughout the annual life cycle. The flight efficiency hypothesis posits that adaptive mass loss can enhance avian flight ability. However, whether birds can increase additional wing loading following mass loss and how birds adjust flight kinematics and postures remain largely unexplored. We investigated physiological changes in body condition in breeding female Eurasian tree sparrows (Passer montanus) through a dietary restriction experiment and determined the changes in flight kinematics and postures. Body mass decreased significantly, but the external maximum load and mass-corrected total load increased significantly after 3 days of dietary restriction. After 6 days of dietary restriction (DR6), hematocrit, pectoralis and hepatic fat content, take-off speed, theoretical maximum range speed and maximum power speed declined significantly. Notably, the load capacity and power margin remained unchanged relative to the control group. The wing stroke amplitude and relative downstroke duration were not affected by the interaction between diet restriction and extra load. Wing stroke amplitude significantly increased after DR6 treatment, while the relative downstroke duration significantly decreased. The stroke plane angle significantly increased after DR6 treatment only in the load-free condition. In addition, the sparrows adjusted their body angle and stroke plane angle in response to the extra load, but stroke amplitude and wingbeat frequency remained unchanged. Therefore, birds can maintain and even enhance their flight performance by adjusting flight kinematics and postures after a short-term mass loss.

Elevated nest temperature has opposing effects on host species infested with parasitic nest flies

Environmental factors, such as elevated temperature, can have varying effects on hosts and their parasites, which can have consequences for the net outcome of this relationship. The individual direct effects of temperature must be disentangled to determine the net-effect in host-parasite relationships, yet few studies have determined the net-effects in a multi-host system. To address this gap, we experimentally manipulated temperature and parasite presence in the nests of two host species infested by parasitic blowflies (Protocalliphora sialia). We conducted a factorial experiment by increasing temperature (or not) and removing all parasites (or not) in the nests of eastern bluebirds (Sialia sialis) and tree swallows (Tachycineta bicolor). We then measured nestling morphometrics, blood loss, and survival and quantified parasite abundance. We predicted that if temperature had a direct effect on parasite abundance, then elevated temperature would cause similar directional effects on parasite abundance across host species. If temperature had a direct effect on hosts, and therefore an indirect effect on the parasite, parasite abundance would differ across host species. Swallow nests with elevated temperature had fewer parasites compared to nests without temperature manipulation. In contrast, bluebird nests with elevated temperatures had more parasites compared to nests without temperature manipulation. The results of our study demonstrate that elevated temperature can have differential effects on host species, which can impact infestation susceptibility. Furthermore, changing climates could have complex net-effects on parasite fitness and host health across multi-host-parasite interactions.

Advance social information allows red crossbills (Loxia curvirostra) to better conserve body mass and intestinal mass during food stress

Animals prepare for fluctuations in resources through advance storage of energy, planned reduction in energy costs or by moving elsewhere. Unpredictable fluctuations in food, however, may be particularly challenging if animals cannot avoid negative impacts on body condition. Social information may help animals to cope with unpredictable resources if cues from individuals with low foraging success give advance warning about deteriorating conditions. This study investigates the impact of social information on behaviour and physiology of food-restricted captive red crossbills (Loxia curvirostra). Birds were restricted to two short feeding periods per day to simulate a decline in resources and were given social information from food-restricted neighbours either before (i.e. predictive) or during (i.e. parallel) the food-restriction period. Focal birds better conserved body mass during food restriction if social information was predictive of the decline in resources. Crossbills with predictive information ate more food, had larger intestinal mass and better conserved pectoral muscle size at the end of the restriction period compared to those with parallel social information. These data suggest that birds can use social information to alter behavioural and physiological responses during food shortage in ways that may confer an adaptive advantage for survival.

Stress in paradise: effects of elevated corticosterone on immunity and avian malaria resilience in a Hawaiian passerine

Vertebrates confronted with challenging environments often experience an increase in circulating glucocorticoids, which result in morphological, physiological and behavioral changes that promote survival. However, chronically elevated glucocorticoids can suppress immunity, which may increase susceptibility to disease. Since the introduction of avian malaria to Hawaii a century ago, low-elevation populations of Hawaii Amakihi (Chlorodrepanis virens) have undergone strong selection by avian malaria and evolved increased resilience (the ability to recover from infection), while populations at high elevation with few vectors have not undergone selection and remain susceptible. We investigated how experimentally elevated corticosterone affects the ability of high- and low-elevation male Amakihi to cope with avian malaria by measuring innate immunity, hematocrit and malaria parasitemia. Corticosterone implants resulted in a decrease in hematocrit in high- and low-elevation birds but no changes to circulating natural antibodies or leukocytes. Overall, leukocyte count was higher in low- than in high-elevation birds. Malaria infections were detected in a subset of low-elevation birds. Infected individuals with corticosterone implants experienced a significant increase in circulating malaria parasites while untreated infected birds did not. Our results suggest that Amakihi innate immunity measured by natural antibodies and leukocytes is not sensitive to changes in corticosterone, and that high circulating corticosterone may reduce the ability of Amakihi to cope with infection via its effects on hematocrit and malaria parasite load. Understanding how glucocorticoids influence a host's ability to cope with introduced diseases provides new insight into the conservation of animals threatened by novel pathogens.

Summary: Amakihi innate immunity, as measured by natural antibodies and leukocytes, is not sensitive to changes in corticosterone, but high circulating corticosterone may reduce the ability of Amakihi to cope with avian malaria infection via its effects on hematocrit and malaria parasite load.

The effects of food limitation on behavior, corticosterone, and the use of social information in the red crossbill (Loxia curvirostra)

Meeting metabolic demands through foraging is a basic animal need that drives the evolution of foraging adaptations. The use of social information is one adaptation that could improve foraging success and fitness if it helps animals locate food when conditions are challenging. It is unknown if food limitation-or the glucocorticoid hormones that are often released when food is limited-can influence the extent to which animals use social information or their ability to learn novel foraging techniques. We explored the effects of limited access to food on activity levels, corticosterone secretion, and social information use in red crossbills, a highly social songbird species adapted to cope with high degrees of resource unpredictability. Using an observer/demonstrator paradigm, food limited or well fed observers were allowed to watch demonstrators solve a novel feeding puzzle before being allowed to attempt the puzzle themselves across repeated trials. Our findings suggest that food limitation transiently increased activity levels but did not result in long-term elevations of corticosterone and did not increase the speed at which red crossbills utilized social information to solve the novel foraging task. However, food limitation may have increased the value of using socially acquired information, as foraging technique performance improved faster in food limited birds relative to controls. Social learning was further demonstrated by the red crossbills in this study when naïve observers overwhelmingly learned a socially-demonstrated task over an undemonstrated task when tested on a two-task foraging board.