Journal of Avian Biology最新文献

Resource acquisition and allocation are central to life history theory, explaining the diversity of strategies among species as well as the distribution of events over the annual cycle. Moult is a major phase in the annual cycle of birds, but explanations for moult scheduling are heavily biased towards temperate systems with seasonal breeding patterns. Our research on a year-round breeding tropical bird, the common bulbul Pycnonotus barbatus (bulbul), tests whether moult depends primarily on stored body reserves (capital) or on resources acquired throughout the moult period (income). Making this distinction elucidates trade-offs between moult, and other annual cycle events, and responses to environmental change. We estimated moult start date and duration in captive bulbuls whose body condition we experimentally manipulated by feeding them fruits or invertebrates 6–3 and 3–0 months before moult, and fruits or a mixed diet during moult. We studied free-living bulbuls as reference group. We found that moult onset is best predicted by diet-manipulated condition just before moult, while moult duration is best predicted by diet-manipulated condition during moult. Specifically, invertebrate-fed bulbuls started moult 33 days later than fruit-fed bulbuls. In addition, once invertebrate-fed bulbuls were switched to a mixed diet, they moulted 52 days quicker than fruit-fed bulbuls, albeit still 36 days slower than free-living bulbuls on average. Males started moult 15 days earlier and had a more variable start of 20 days, but did not moult quicker than females on average. Our findings indicate that moult in bulbuls is both income- and capital-dependent, with moult initiation determined by individual body reserves and feather growth still occurring on a fruit-only-diet, but is significantly improved by dietary proteins from a mixed diet and in field bulbuls. In this year-round breeding bird, moult seasonality is maintained in the absence of breeding, but heavily influenced by foraging conditions.

Global climate change has intensified extreme weather events, including short-duration but intense rainfall, which can significantly impact avian reproduction. This study examines the effects of extreme rainfall on the reproductive phenology and performance of the European roller Coracias garrulus, a single-brooded migratory bird, in a semiarid ecosystem. We analysed breeding parameters during 2023, characterised by exceptionally intense and concentrated rainfall, and compared them with long-term data (2006–2024). Our results indicate that breeding phenology in 2023 exhibited significant shifts. Rollers initiated egg-laying approximately five days earlier than the long-term average in the early breeding season, while late-season laying was delayed by approximately 11 days. The prolonged breeding season was marked by a high incidence of breeding failures, particularly during incubation. Moreover, we observed an unprecedented surge in apparent replacement breeding attempts, a rare phenomenon in this single-brooded species. Despite these challenges, overall population productivity remained consistent with long-term averages because successful breeding was heavily concentrated in resilient nest substrates, nest boxes and sandstone burrows, which buffered the weather's impact and compensated for catastrophic failures in stone cavities. Our findings underscore the European roller's capacity for reproductive flexibility, but also suggest the significant fitness costs and constraints associated with these adaptive responses. These results highlight the need for conservation strategies that consider climate-induced phenological shifts and the importance of nest box designs optimised to mitigate the interacting adverse effects of extreme rainfall and subsequent heatwaves.

The timing of avian reproduction might be affected by the costs of egg production in early spring. To study these costs, supplemental feeding experiments have been conducted where birds are provided with food prior to egg laying. We discuss the opportunities and pitfalls of such experiments. We stress that supplemental feeding experiments alter the conditions for the birds and hence can be used to measure the response in laying date, but not to determine whether this response is adaptive. We present two experiments, one on pied flycatchers and one on great tits, both showing no effect of supplemental feeding on laying date, but in great tits laying was affected in an additional treatment where night temperatures in the nest boxes were manipulated. For both studies we present ample data on the ecological conditions of the years in which the experiments were carried out. We furthermore present a mini-review of 31 studies on food supplementation in passerines that examined effects on laying date, focussing on between year variation in the response to supplemental feeding within the same studies. For 11 out of 19 species at least two estimates were available, and effects of food supplementation clearly varied between species. In (the three) studies done in the tropics there was a large effect of supplemental feeding while in temperate zone populations this effect was about one week for resident species, and small and non-significant for migrant species. We argue that it is crucial that studies report on the ecological conditions during the year(s) of the study to aid interpretation of the results. We conclude that in general there is an effect of supplemental feeding on timing of reproduction in most resident birds, but that the variation in the response needs to be explored further.

Many forest specialist birds are in widespread decline across Europe. In Britain, marsh tits are an indicator species of mature native woodlands, but have suffered an 81% population decline since the 1960s. We assessed whether habitat degradation during the mid 20th century, through the widespread conversion of native deciduous woodland to conifer plantations, could have impacted marsh tit populations. We surveyed the recent number of occupied marsh tit territories in 74 discrete woodland patches (‘woods') of 1–296 ha in England, comprising purely native deciduous woodland or with varying coverages of conifer plantations (0–89%). We found that the number of marsh tit territories increased with the woods' size, but this increase was significantly greater for deciduous woods, and lower for woods with a greater proportion of conifer plantation. The area of woodland in the local landscape, reflecting a wood's isolation, had no significant effect on marsh tit abundance in a focal wood. The results indicated that the historical conversion of native deciduous woodland to conifer plantation likely degraded a substantial proportion of formerly high-quality habitat for marsh tits, affecting up to 37.3% of potentially suitable woods and possibly one-fifth of the former marsh tit population directly, likely contributing to the species' national decline. Many of the larger coniferized woodlands are in public/state ownership, which could facilitate habitat restoration for the conservation of woodland specialists, like marsh tits, via centralized policies, with additional incentives targeted at woodlands in private ownership. We cautiously estimated that restoration of native woodland could re-establish a median of 24 610 marsh tit territories in Britain, equivalent to an additional 86% of the current national population.

Here, I discuss the utility of the concept of frailty, which is widely used in human gerontology, for studies of ageing in wild animals and birds in particular. Frailty indices generally aim to capture the age-specific health status of individuals via relatively simple measures of whole-organism performance. I discuss why we might find frailty indices useful in studies of animal life histories, environmental factors and ageing, and how frailty might be usefully measured in birds by different methods. I also discuss limitations and caveats.

Senescence, the decline in reproductive value with age, is well documented in natural systems, but the underlying mechanisms remain an enigma. A decline in parental effort with age potentially explains reproductive senescence, but age effects on parental effort have been little investigated. I measured daily energy expenditure (DEE) of great tits raising offspring using doubly labelled water. Independent of sex, DEE declined with age. This correlation would also arise when individuals with high energy expenditure are more likely to die, but we have previously shown that survival was independent of DEE. The observed decline in DEE with age can therefore be attributed to changes within individuals over time. Although DEE declined with age, provisioning rate was independent of age in the same dataset, and neither did age significantly predict number and growth of offspring. I discuss potential explanations, and stress that variation in reproductive success, insofar as it reflects provisioning offspring, is an emergent property of parental effort and foraging efficiency. Depending on the relative strength of age effects on effort and efficiency, reproductive success can either decrease, increase or remain unchanged with increasing age. In our study, a negative effect of age on the capacity to work may have been offset by an increase in efficiency, yielding on balance no change in reproductive output.

Avian haemosporidian parasites are globally widespread with a broad repertoire of hosts. When infected, the host can either reduce (resistance) and/or limit the severity of parasitaemia (tolerance). Oxidative stress plays a pivotal role in the host's resistance and tolerance, as well as its detrimental endpoints. The rationale behind this paradox lies in the dual role of reactive oxygen species (ROS): they are both beneficial and detrimental for the host, while being harmful to the parasite. Thus, it is in the parasite's interest to maintain a reduced environment within the host's cell, whereas the host needs a fine-tuned balance between generating ROS to eliminate the parasites and maintaining sufficient antioxidant levels to protect itself. This dynamic we refer to as the host–parasite oxidative arms race. Here, Eurasian siskins Spinus spinus were experimentally infected with Plasmodium ashfordi to investigate how the fundamental antioxidant system – the glutathione system – responds to infection over time compared to control birds. By combining physiological and gene expression data from both the parasite and the host at different time points, we provide evidence for this oxidative arms race. The gene expression data show that the parasite aims to eliminate ROS through its high expression of superoxide dismutase (SOD), glutathione reductase (GR), and glutathione synthetase. In contrast, the host upregulates glutathione S-transferases (GSTs) and glutathione peroxidases (GPX), which may result in the reduced physiological levels of glutathione seen at the end of the experiment. Although the parasite seems to win the race in terms of the oxidative state of the cell, the marked decrease in parasitaemia from day 21 (44%) to day 31 (15%) suggests that the host's strategy is sufficient to defeat the parasite. Future studies should include measures of oxidative damage to reveal whether there are any long-term costs related to the host's strategy at different time points of infection.

Ecological specialization shapes biodiversity patterns, yet the relative importance of historical, climatic, and biotic drivers of specialization remains debated. We investigated how time since colonization (TSC), climatic stability (decomposed in temperature and precipitation stability), and competition influence ecological specialization across diet, foraging behavior, and habitat dimensions in Emberizoidea bird assemblages globally and across three bioregions (Nearctic, Neotropics, Palearctic). Using phylogenetic data, species distributions, and trait databases, we quantified specialization with the Gini Index and modeled its predictors using spatial autoregressive models. Globally, diet and habitat specialization increased with TSC and competition, while foraging specialization decreased with the same predictors. Regional patterns diverged markedly: in the Nearctic, competition increased foraging specialization, and climatic stability decreased diet and habitat specialization; in the Neotropics, TSC and temperature stability promoted diet specialization, competition enhanced habitat specialization but decreased diet and foraging specialization, while all three factors reduced foraging specialization; in the Palearctic, competition and temperature stability increased diet specialization, while TSC promoted habitat but decreased diet and foraging specialization. These contrasting patterns reveal that ecological specialization emerges from contingent interactions between evolutionary history, climatic stability, and biotic factors rather than universal rules, with different specialization axes responding distinctly to the same drivers across biogeographic contexts.

Portable devices, or biologgers, attached to animals are designed to record data on unobservable aspects of animal behaviour, physiology and ecology. However, wearing such devices is expected to be costly, and researchers must ascertain how such devices influence the behaviour, reproduction and survival of tagged individuals. GPS tracking devices with solar-powered batteries are now available in the 1.6–2.0 g mass range, allowing for long-term, high-resolution spatial data collection in species as small as 32–40 g (assuming a relative device mass ≤ 5% of body mass). In this study, we investigate the impact of wearing GPS devices on adult Kentish plovers Anarhynchus alexandrinus weighing 34–46 g. To assess any potential adverse effects of tagging, we quantified 1) general behaviour with behavioural time-budget assays, 2) detailed incubation behaviour with nest cameras, 3) reproductive outcomes, and 4) apparent survival from non-breeding season resighting rates 4–5 months after initial deployment. In each case, tagged birds were compared with untagged, colour-ringed control group birds. Despite wearing devices of up to 5% of body mass, we found no clear effects of tagging in any aspects of behaviour or life-history that we assessed. Our results collectively support the conclusion that any impacts of tagging on individuals in this study system are negligible and appear unlikely to manifest demographic consequences.