The benefit of the information accrued when capturing wild animals for study needs to outweigh the potential risk to individuals that are caught. New BTO research, just published, assesses the potential effects of capturing wild birds. Wild birds have been marked with metal rings as part of scientific studies throughout the world for over a century. The data generated by these studies have provided much needed information on movements, demographic processes and individual life histories, as well as helping to guide conservation actions. Any effect of capturing wild birds on their individual welfare, or that of their wider populations, is an important ethical consideration; it also has significant implications for the integrity of the data collected, as biases may be introduced if capture and handling bring about changes in behaviour or survival. Ultimately, the benefit of the information accrued when capturing wild animals for study needs to outweigh the potential risk to individuals that are caught. The potential effect of fittings rings or other devices to birds has been assessed previously, through a series of reviews and studies. The direct effects of the capture methods themselves have received less attention, however, with few published estimates of capture-related injury or mortality. A new study, examining 1.5 million recapture records of 166 passerine species, has investigated this issue for those birds caught using mist-nets by individuals operating under the British and Irish Ringing Scheme. The training process is both structured and intensive, with volunteers individually mentored and then formally appraised before they are able to operate unaccompanied, a process that typically takes at least a couple of years and involves handling a significant number of birds. Mist-netting is considered to be safe and effective when carried out by experienced individuals trained in this way, and following published guidelines; despite this there are occasional incidents of capture-related mortality or injury. Defining any threshold of ‘acceptable’ mortality in the context of research into wild bird populations is a difficult and sensitive issue but it is important that we understand the level of risk, and its implications for welfare and data quality, at the level of both the individual and the wider species population. This study quantified the reported mortality rate among common passerines recaptured using mist-nets, using data submitted to the BTO. Factors which may influence the likelihood of mortality were also investigated, facilitating improved guidance for those operating mist-nets to capture wild birds. Overall mortality rates were low – the average mortality rate was 0.0011 – with most fatalities reported to have occurred before individuals had been extracted from the nets. Juvenile birds appeared to be at greater risk than adults, and the incidence of predation – the single largest identified cause of mortality, accounting for over 70% of deaths – was seasonal, with increased risk during the winter. Mortality rates, although very low, varied between species; the apparent risk was greatest for Bullfinch and Chiffchaff. Erica Spotswood and colleagues carried out an assessment of mortalities associated with mist-netting using data from several North American stations, reporting a slightly higher mortality estimate (0.0029) than that found here. Whilst it is important to understand and to minimise the scale of any effects on capture-related mortality or injury, it is unlikely that a zero mortality rate will ever be possible when trapping and handling wild animals for scientific purposes. Several previous studies have highlighted the importance of appropriate training, something that is central to the British and Irish Ringing Scheme where highly-valued ringing trainers help to maintain the highest possible standards. The study also makes a number of recommendations; some of these will help to refine guidelines for those trapping and handling wild birds, while others suggest additional opportunities to secure information that could be used to better understand risk factors and associated mortality. Clewley, G.D., Robinson, R.A. & Clark, J.A. (2018). Estimating mortality rates among passerines caught for ringing with mist-nets using data from previously-ringed birds. Ecology & Evolution.

Our understanding of the impact of feeding wild birds is far from complete, but we are beginning to unravel the effects of providing foods at garden feeding stations. An important area of research has been to examine how supplementary foods shape populations through its impacts in individuals. Feeding wild birds is a popular pastime and many of us provide seed and other foods to help our feathered friends. But what impact does all this food have? It is a huge resource and one that can increase overwinter survival and bring forward the timing of breeding, but we also know that the feeding of wild birds has been linked to the transmission of disease. Research by Kate Plummer and colleagues provides new insight into one particular aspect of food provision – how it shapes bird populations. Kate’s research has examined the extent to which the provision of supplementary food during the winter months influences the physiological condition of individuals and populations the following breeding season. By using woodland populations of Blue Tits, Kate and fellow researchers have been able to compare the effects of providing fat, and fat plus vitamin E (an antioxidant), against a control population of unfed birds. The feeding carried out during the winter months ended at least a month before the tits began egg-laying. Provisioning with fat and vitamin E was found to alter the composition of Blue Tit populations, such that they included birds that had been in significantly poorer condition prior to feeding. Because those individuals were found to have lower levels of carotenoids in their breast feathers than unfed birds, Kate was able to conclude that supplementing with vitamin E and fat in winter had altered the survival and recruitment prospects of these lower quality individuals; lower levels of carotenoids are indicative of poorer physiological condition. However, provisioning with fat alone was found to have a detrimental impact on breeding birds. It appears that the provision of supplementary foods during the winter months can alter both the structure of the breeding population the following season, and the condition of individual breeding birds. Such effects may have consequences even longer term; through this work, for example, it was found that individuals with higher blood plasma concentrations of malondialdehyde (which is indicative of oxidative damage) produced offspring that were structurally smaller and which suffered from reduced fledging success. The importance of antioxidants, like vitamin E, can also be seen from Kate Plummer’s earlier work on yolk mass. While Plummer et al. (2013) found that winter provisioning with fat subsequently impaired an individual’s ability to acquire, assimilate and/or mobilise key resources for yolk formation, this was not the case where vitamin E was also included in the food presented. A high fat diet, such as that potentially obtained from the food provided at garden feeding stations, may well increase the requirement for antioxidants in order to combat the greater levels of oxidative damage associated with a diet rich in fats. Clearly, there is still much to learn about how the provision of supplementary food affects wild birds and their populations.

The increasing temperatures associated with a changing climate may disrupt ecological systems, including by affecting the timing of key events. If events within different trophic levels are affected in different ways then this can lead to what is known as phenological mismatch. But what is the evidence for trophic mismatch, and are there spatial or temporal patterns within the UK that might point to mismatch as a driver of regional declines in key insect-eating birds? A changing climate is leading to changes in the timing of key ecological events, including the timing of bud burst, the spring peak in leaf-eating caterpillar biomass and the timing of egg-laying in many bird species. If the timings of these different events shift at different rates then there is a danger that they may get out of synch with one another, something that is referred to as phenological mismatch. This may be a particular problem for birds like Blue Tit, Great Tit and Pied Flycatcher, which time their breeding attempts to exploit the spring peak in caterpillar abundance. Much of the recent work on mismatch and its impacts on the fitness and population trends of caterpillar-eating birds has looked at changes over time. However, it is also possible for mismatch to vary in space if species respond differently in different areas, perhaps because of local adaptation to geographic variation in the cues that they use. This paper looks at mismatch in both space and time, using information from three trophic levels, namely trees, caterpillars and caterpillar-eating birds. While information on bud burst came from 10,000 observations of oak first leafing for the period 1998-2016, that for caterpillar biomass was inferred from frass traps set beneath oak trees at sites across the UK for the period 2008-2016. Bird phenology data came from the ‘first egg date’ values calculated from 85,000 nest records of Blue Tit, Great Tit and Pied Flycatcher. The focus of the work was on the relationship between the phenologies of these interacting species; where timing changes more in one species than the other, this is indicative of spatial or temporal variation in the magnitude of mismatch. The results reveal that, for the average latitude (52.63°N) and year, there is a 27.6 day interval between the timing of oak first leaf and peak caterpillar biomass. With increasing latitude, the delay in oak leafing is significantly steeper than that of the caterpillar peak. At 56°N the predicted interval between these two trophic levels drops to 22 days. In the average year and at the average latitude, the first egg dates of Blue Tits and Great Tits were roughly a month earlier than peak caterpillar biomass, meaning that peak demand for hungry chicks occurred soon after the peak in resource availability. Interestingly, peak demand in Pied Flycatchers occurred nearly two weeks later than peak caterpillar availability, suggesting a substantial trophic mismatch between demand and availability for this species within the UK. However, it is worth noting that Pied Flycatchers provision their nestlings with fewer caterpillars and more winged invertebrates compared to the tit species studied, so they may be less dependent on the caterpillar peaks. The work also revealed that the timing of first egg date between years varied by less than the variation seen in timing of the caterpillar resource peak, which gave rise to year-to-year variation in the degree of mismatch. For every 10 day advance in the caterpillar peak, the corresponding advance in the three bird species is 5.0 days (Blue Tit), 5.3 days (Great Tit) and 3.4 days (Pied Flycatcher). In late springs, peak demand from the tits is expected to coincide with the peak resource availability, with flycatcher demand occurring shortly after. In early springs, the peak demand of nestlings of all three species falls substantially later than the peak, leaving the three mismatched. Warmer conditions also shortened the duration of caterpillar peaks. One of the key findings of the work is that in the average year there is little latitudinal variation in the degree of caterpillar-bird mismatch. This means that more negative declines in population trends of certain insectivorous birds in the southern UK, driven by productivity, are unlikely to have been driven by greater mismatch in the south than the north. The lack of evidence for latitudinal variation in mismatch between these bird species and their caterpillar prey suggests that mismatch is unlikely to be the driver of the spatially varying population trends found in these and related species within the UK.

Cuckoos have been documented using a range of host species but most of their eggs are laid in the nests of just a small number of species. Why do they select these species and to what extent might other species be suitable hosts, as yet unused to any great degree? The Cuckoo is a generalist avian brood parasite, known to have utilized at least 125 different bird species as a host within Europe. Despite this, individual female Cuckoos are thought to be host-specific, preferentially laying their eggs in one – or a few – host nests. This has led to individual female Cuckoos being classified into host races, termed ‘gentes’, some 20 of which have been described within Europe. Given the large number of songbird species breeding in Europe, and the relatively few female Cuckoo gentes, it raises the question of what makes a good host. By examining data from three different sources, including nest record data from the BTO, Bård Stokke and colleagues sought to establish which characteristics determined the suitability of a songbird species as a potential Cuckoo host. Some of the characteristics they investigated included nest placement, chick diet, habitat, abundance and the timing of the breeding period. The results of analysing two independent data sets, one from the UK and one from Germany, produced similar results. The analyses revealed that species breeding in forest and rocky areas were used less than species breeding in other habitats; that those feeding their nestlings with plant material were used less often than those feeding their nestlings with invertebrates (though not significantly so in the UK) and that hosts tended to be of intermediate body size and have larger populations. The analyses also showed that, perhaps unsurprisingly, species nesting in cavities are used less frequently than those nesting elsewhere. Nest height, nest depth and overlap in the breeding period did not appear to affect parasitism in any of the data sets examined. Through the work the researchers were able to calculate a ‘host suitability index’ for each of the European species in the study. Interestingly, there was no evidence of a clear separation between suitable and unsuitable host species’ instead, there appeared to be a continuum from low to high suitability. However, all of the species with a recognized corresponding Cuckoo gens are ranked towards the high suitability end of the continuum. The host suitability index could indicate species that might become the focus of Cuckoo nest parasitism in future if, for example, the host population increases because of climate- or habitat- change.