The effects of food supply on reproductive hormones and timing of reproduction in an income-breeding seabird

Corticosterone release in very young siblicidal seabird chicks (Rissa tridactyla) is sensitive to environmental variability and responds rapidly and robustly to external challenges

In birds, patterns of development of the adrenocortical response to stressors vary among individuals, types of stressors, and species. Since there are benefits and costs of exposure to elevated glucocorticoids, this variation is presumably a product of selection such that animals modulate glucocorticoid secretion in contexts where doing so increases their fitness. In this study, we evaluated hypothalamo-pituitary-adrenal (HPA) activity in first-hatched free-living seabird nestlings that engage in intense sibling competition and facultative siblicide (black-legged kittiwakes, Rissa tridactyla). We sampled 5 day old chicks (of the ∼45 day nestling period), a critical early age when food availability drives establishment of important parent-offspring and intra-brood dynamics. We experimentally supplemented parents with food ("supplemented") and measured chick baseline corticosterone secretion and capacity to rapidly increase corticosterone in response to an acute challenge (handling and 15 min of restraint in a bag). We also used topical administration of corticosterone to evaluate the ability of chicks to downregulate physiologically relevant corticosterone levels on a short time scale (minutes). We found that 5 day old chicks are not hypo-responsive but release corticosterone in proportion to the magnitude of the challenge, showing differences in baseline between parental feeding treatments (supplemented vs non-supplemented), moderate increases in response to handling, and a larger response to restraint (comparable to adults) that also differed between chicks from supplemented and control nests. Topical application of exogenous corticosterone increased circulating levels nearly to restraint-induced levels and induced downregulation of HPA responsiveness to the acute challenge of handling. Parental supplemental feeding did not affect absorbance/clearance or negative feedback. Thus, while endogenous secretion of corticosterone in young chicks is sensitive to environmental context, other aspects of the HPA function, such as rapid negative feedback and/or the ability to clear acute elevations in corticosterone, are not. We conclude that 5 day old kittiwake chicks are capable of robust adrenocortical responses to novel challenges, and are sensitive to parental food availability, which may be transduced behaviorally, nutritionally, or via maternal effects. Questions remain about the function of such rapid, large acute stress-induced increases in corticosterone in very young chicks.

Beyond body condition: Experimental evidence that plasma metabolites improve nutritional state measurements in a free-living seabird

The ability to efficiently measure the health and nutritional status of wild populations in situ is a valuable tool, as many methods of evaluating animal physiology do not occur in real-time, limiting the possibilities for direct intervention. This study investigates the use of blood plasma metabolite concentrations, measured via point-of-care devices or a simple plate reader assay, as indicators of nutritional state in free-living seabirds. We experimentally manipulated the energy expenditure of wild black-legged kittiwakes on Middleton Island, Alaska, and measured the plasma concentrations of glucose, cholesterol, B-hydroxybutyrate, and triglycerides throughout the breeding season, along with measures of body condition (size-corrected mass [SCM] and muscle depth). Supplemental feeding improved the nutritional state of kittiwakes by increasing feeding rate (higher glucose and triglycerides, lower cholesterol), and flight-handicapping caused a slight nutritional decline (lower glucose and triglycerides, higher cholesterol and B-hydroxybutyrate). Glucose and triglycerides were the best indicators of nutritional state when used alongside SCM, and improved upon commonly used metrics for measuring individual condition (i.e. SCM or mass alone). Metabolite concentrations varied across the breeding period, suggesting that the pre-laying stage, when feeding rates tend to be lower, was the most nutritionally challenging period for kittiwakes (low glucose, high cholesterol). Muscle depth also varied by treatment and breeding stage, but differed from other nutritional indices, suggesting that muscle depth is an indicator of exercise and activity level rather than nutrition. Here we demonstrate potential for the use of blood plasma metabolites measured via point-of-care devices as proxies for evaluating individual health, population health, and environmental food availability.

Sex-specific responses to GnRH challenge, but not food supply, in kittiwakes: Evidence for the "sensitivity to information" hypothesis

Seasonal timing of breeding is usually considered to be triggered by endogenous responses linked to predictive cues (e.g., photoperiod) and supplementary cues that vary annually (e.g., food supply), but social cues are also important. Females may be more sensitive to supplementary cues because of their greater role in reproductive timing decisions, while males may only require predictive cues. We tested this hypothesis by food-supplementing female and male colonial seabirds (black-legged kittiwakes, Rissa tridactyla) during the pre-breeding season. We measured colony attendance via GPS devices, quantified pituitary and gonadal responses to gonadotropin releasing hormone (GnRH) challenge, and observed subsequent laying phenology. Food supplementation advanced laying phenology and increased colony attendance. While female pituitary responses to GnRH were consistent across the pre-breeding season, males showed a peak in pituitary sensitivity at approximately the same time that most females were initiating follicle development. The late peak in male pituitary response to GnRH questions a common assumption that males primarily rely on predictive cues (e.g., photoperiod) while females also rely on supplementary cues (e.g., food availability). Instead, male kittiwakes may integrate synchronising cues from their social environment to adjust their reproductive timing to coincide with female timing.

Female and male Leach's Storm Petrels (Hydrobates leucorhous) pursue different foraging strategies during the incubation period

Reproduction in procellariiform birds is characterized by a single egg clutch, slow development, a long breeding season and obligate biparental care. Female Leach's Storm Petrels Hydrobates leucorhous, nearly monomorphic members of this order, produce eggs that are between 20 and 25% of adult body weight. We tested whether female foraging behaviour differs from male foraging behaviour during the ~ 44-day incubation period across seven breeding colonies in the Northwest Atlantic. Over six breeding seasons, we used a combination of Global Positioning System and Global Location Sensor devices to measure characteristics of individual foraging trips during the incubation period. Females travelled significantly greater distances and went farther from the breeding colony than did males on individual foraging trips. For both sexes, the longer the foraging trip, the greater the distance. Independent of trip duration, females travelled farther, and spent a greater proportion of their foraging trips prospecting widely as defined by behavioural categories derived from a Hidden Markov Model. For both sexes, trip duration decreased with date. Sex differences in these foraging metrics were apparently not a consequence of morphological differences or spatial segregation. Our data are consistent with the idea that female foraging strategies differed from male foraging strategies during incubation in ways that would be expected if females were still compensating for egg formation.

Telomere length correlates with physiological and behavioural responses of a long-lived seabird to an ecologically relevant challenge

Determinants of individual variation in reallocation of limited resources towards self-maintenance versus reproduction are not well known. We tested the hypothesis that individual heterogeneity in long-term 'somatic state' (i) explains variation in endocrine and behavioural responses to environmental challenges, and (ii) is associated with variation in strategies for allocating to self-maintenance versus reproduction. We used relative telomere length as an indicator of somatic state and experimentally generated an abrupt short-term reduction of food availability (withdrawal of food supplementation) for free-living seabirds (black-legged kittiwakes, Rissa tridactyla). Incubating male kittiwakes responded to withdrawal by increasing circulating corticosterone and losing more weight compared to continuously supplemented controls. Males with longer telomeres increased time in directed travel regardless of treatment, while experiencing smaller increases in corticosterone. Males with longer telomeres fledged more chicks in the control group and tended to be more likely to return regardless of treatment. This study supports the hypothesis that somatic state can explain variation in short-term physiological and behavioural responses to challenges, and longer-term consequences for fitness. Male kittiwakes with longer telomeres appear to have prioritized investment in self over investment in offspring under challenging conditions.

Does mast seeding shape mating time in wild boar? A comparative study

In seasonal environments, the timing of reproduction often matches with the peak of food resources. One well-known effect of global warming is an earlier phenology of resources, leading to a possible mismatch between the timing of reproduction for consumers and food peak. However, global warming may also change the dynamics of food resources, such as the intensity and frequency of pulsed mast seeding. How quantitative changes in mast seeding influence the timing of reproduction of seed consumers remains unexplored. Here, we assess how yearly variation in mast seeding influences mating time in wild boar (Sus scrofa), a widespread seed consumer species. We took advantage of the intensive monitoring of both female reproduction (1636 females) and acorn production over 6 consecutive years across 15 populations of wild boar in the wild. We found that mating time occurs earlier when acorn production increases in most but not all populations. In two out of 15 populations, heavy females mated earlier than light ones. Our findings demonstrate that mast seeding advances the mating time in some populations, which could perhaps impact how boars respond to climate change.

Corticosterone in three species of free-ranging watersnakes: Testing for reproductive suppression and an association with body condition

The potentially suppressive effects of the hypothalamic-pituitary-adrenal (HPA) axis on the hypothalamic-pituitary-gonadal (HPG) axis revolve around the central role that glucocorticoids play in mobilizing energy. As an individual's energy balance becomes negative, the HPA axis helps mobilize energy and shifts energy expenditure away from reproduction toward maintenance and survival. While there is evidence in support of these relationships, substantial species variability exists. Studies in a greater diversity of species promise to enhance our understanding of the interactions between these axes. In this field study we tested for relationships among body condition, corticosterone, and sex steroid concentrations in three species of closely related watersnakes: the common watersnake (Nerodia sipedon), the diamondback watersnake (Nerodia rhombifer) and the queen snake (Regina septemvittata). Snakes were sampled before and after a 30 min acute confinement stressor and body condition was estimated using the scaled mass index. All three species exhibited robust elevations of corticosterone in response to acute stress, but only plasma progesterone was elevated by the acute stressor in queen snakes. There was no evidence for a suppression of sex steroid concentrations in any of the species. Body condition was negatively associated with baseline corticosterone in queen snakes and with post-stressor corticosterone in both queen and common watersnakes. Overall we found fairly strong support for the proposed link between corticosterone and energetics in two of the three watersnake species, but no support for the hypothesis that acute stressors are associated with reproductive suppression, at least as measured by steroid concentrations.

Ecological inference using data from accelerometers needs careful protocols

Accelerometers in animal‐attached tags are powerful tools in behavioural ecology, they can be used to determine behaviour and provide proxies for movement‐based energy expenditure. Researchers are collecting and archiving data across systems, seasons and device types. However, using data repositories to draw ecological inference requires a good understanding of the error introduced according to sensor type and position on the study animal and protocols for error assessment and minimisation.

Using laboratory trials, we examine the absolute accuracy of tri‐axial accelerometers and determine how inaccuracies impact measurements of dynamic body acceleration (DBA), a proxy for energy expenditure, in human participants. We then examine how tag type and placement affect the acceleration signal in birds, using pigeons Columba livia flying in a wind tunnel, with tags mounted simultaneously in two positions, and back‐ and tail‐mounted tags deployed on wild kittiwakes Rissa tridactyla. Finally, we present a case study where two generations of tag were deployed using different attachment procedures on red‐tailed tropicbirds Phaethon rubricauda foraging in different seasons.

Bench tests showed that individual acceleration axes required a two‐level correction to eliminate measurement error. This resulted in DBA differences of up to 5% between calibrated and uncalibrated tags for humans walking at a range of speeds. Device position was associated with greater variation in DBA, with upper and lower back‐mounted tags varying by 9% in pigeons, and tail‐ and back‐mounted tags varying by 13% in kittiwakes. The tropicbird study highlighted the difficulties of attributing changes in signal amplitude to a single factor when confounding influences tend to covary, as DBA varied by 25% between seasons.

Accelerometer accuracy, tag placement and attachment critically affect the signal amplitude and thereby the ability of the system to detect biologically meaningful phenomena. We propose a simple method to calibrate accelerometers that can be executed under field conditions. This should be used prior to deployments and archived with resulting data. We also suggest a way that researchers can assess accuracy in previously collected data, and caution that variable tag placement and attachment can increase sensor noise and even generate trends that have no biological meaning.

Resting costs too: the relative importance of active and resting energy expenditure in a sub-arctic seabird

Breeding is costly for many animals, including birds that must deliver food to a central place (i.e. nest). Measuring energy expenditure throughout the breeding season can provide valuable insight into physiological limitations by highlighting periods of high demand, and ultimately allows improvement of conservation strategies. However, quantifying energy expenditure in wildlife can be challenging, as existing methods do not measure both active (e.g. foraging) and resting energy costs across short and long time scales. Here, we developed a novel method for comparing active and resting costs in 66 pre-breeding and breeding seabirds (black-legged kittiwakes, Rissa tridactyla) by combining accelerometry and triiodothyronine (T3) as proxies for active and resting costs, respectively. Active energy costs were higher during incubation (P=0.0004) and chick rearing (P<0.0001) than during pre-laying, because of an increase in the time spent in flight of 11% (P=0.0005) and 15% (P<0.0001), respectively. Levels of T3, reflecting resting costs, peaked marginally during incubation with a mean (±s.d.) concentration of 4.71±1.97 pg ml⁻¹ in comparison to 2.66±1.30 pg ml⁻¹ during pre-laying (P=0.05) and 3.16±2.85 pg ml⁻¹ during chick rearing (P=0.11). Thus, although chick rearing is often assumed to be the costliest breeding stage by multiple studies, our results suggest that incubation could be more costly as a result of high resting costs. We highlight the importance of accounting for both active and resting costs when assessing energy expenditure.

Summary: Measurements of both active and resting energy expenditure in breeding black-legged kittiwakes suggest that chick rearing may not be the costliest breeding stage, highlighting the need to measure both active and resting cost when assessing energy expenditure.