Human activities modify the availability of natural resources for other species, including birds, and may alter the relationships between them. The provision of supplementary food at garden feeding stations, for example, might favour some species over others and change the competitive balance between them. This paper investigates the behavioural responses to competition of the Marsh Tit, a species that is subordinate to both the Blue Tit and the Great Tit.

A review paper by BTO considers 203 studies of the effects of air pollution on 231 bird species. Of these studies, 82% document at least one negative effect associated with increasing levels of pollution. The review also highlights biases towards particular study species, especially Great Tit and Pied Flycatcher, and also towards particular geographical regions (Western Europe) and pollutants (heavy metals). The paper proposes research approaches that could help to provide a fuller understanding of how birds are impacted by air pollution.

Urban areas can and do hold significant populations of birds, but we know surprisingly little about how these populations are connected with those present within the wider countryside. It has been suggested that the populations using these different habitats may be linked through seasonal movements, with individuals breeding in rural areas moving into urban sites during the winter months to exploit the supplementary food provided at garden feeding stations. However, little work has been done to test this hypothesis.

Food production and wildlife conservation are often thought of as incompatible goals, and it is rare that conservation studies consider both economics and long-term changes in ecology. However, a decade-long study at a commercial arable farm in Buckinghamshire has found that agri-environment schemes significantly increased local bird and butterfly populations without damaging food production, offering hope for the UK’s farmland birds and butterflies.

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.

Work on emerging infectious diseases and garden birds in the UK has been supported by citizen science projects, most notably Garden BirdWatch, Garden Wildlife Health and the Garden Bird Health Initiative – the latter now superseded by Garden Wildlife Health. Through these schemes, researchers have been able to carry out national surveillance of emerging diseases, including finch trichomonosis, Paridae pox and passerine salmonellosis. This paper, part of a special issue of Philosophical Transactions focusing on wildlife disease issues, reviews the work that has been carried out on these diseases over the past 25 years. It also takes a look at the occurrence of mycotoxin contamination of food residues in bird feeders, which also pose a risk to the health of wild birds. A citizen science approach provides a cost-effective means to undertake large-scale and year-round disease surveillance (Garden Wildlife Health), delivered in parallel to the monitoring of wildlife populations (Garden BirdWatch). By combining large-scale surveillance and targeted post-mortem examinations we can differentiate between the multiple diseases that result in non-specific clinical signs (e.g. lethargy and a fluffed-up appearance). The sample archive collected through post-mortem examination also enables future identification of other disease agents, including those – such as environmental pollutants – associated with non-infectious disease. Avian trichomonosis, caused by the protozoan parasite Trichomonas gallinae, has long been known to affect pigeons, doves and birds of prey. Its emergence in finches in 2005 led to a major population decline in Greenfinch and Chaffinch, reported in an earlier paper. While multiple strains of Trichomonas gallinae are known to infect pigeons and doves in the UK, a single clonal strain is responsible for the epidemic seen in finches. Quite why Greenfinch is so susceptible to the disease is unclear. It is likely that the disease spilled over into finches following the increase in use of garden feeding stations by Woodpigeon, a recent change that follows an increase in wider countryside populations. Avian poxvirus has been documented in a number of garden bird species and is most often seen in House Sparrow, Starling, Woodpigeon and Dunnock. Its emergence in UK tits saw a more severe form of the disease, resulting in pronounced skin lesions, some of which were likely to have hampered the individual’s ability to feed and to avoid predators. Sequence analysis of the poxvirus strains affecting garden birds revealed that a single clade is responsible for the disease seen in UK tits. This form has been known in Scandinavia since the 1950s, with incidents seen elsewhere in mainland Europe since 2005. Because UK tits are relatively sedentary in their habits, and because of the geographical pattern of disease spread seen, it is likely that the disease reached the UK via a biting insect – such as a mosquito, crossing the English Channel in a warm plume of air. Salmonellosis has been reported in wild birds since at least the 1950s, with the bacterium responsible known to be capable of persisting in the environment for many months. Greenfinch and House Sparrow are the two species in which the disease is most often seen. Passerine salmonellosis incidents have a clear seasonality, peaking in January. Interestingly, the prevalence of the disease in UK passerines has dropped sharply over recent years. This may reflect increased immunity to the particular form (DT56v) that had been seen here; or it may be that transmission is density-dependent, with the sharp decline in Greenfinch populations resulting in much lower rates of transmission. Mycotoxins, which include the aflatoxins and ochratoxin, are secondary metabolites produced by certain fungi of the genera Aspergillus and Penicillium. Exposure to the aflatoxins and ochratoxin can exert a range of adverse effects in birds, and the fungi involved and their toxins are known to occur on foodstuffs, including peanuts. Food residues from bird feeders were screened for the toxins as part of the current study, with detectable aflatoxin residues found in all seven samples, two of which greatly exceeded the maximum permitted limits set for such residues in peanuts destined for livestock feed, which includes wild bird food. It therefore seems likely that garden birds may be exposed to these toxins at levels associated with toxic effects in captive birds. This review underlines the great deal of new information that has been generated through these citizen science projects. It also highlights future research needs, particularly around the identification of risk factors, and that we need to understand the balance of risks and opportunities that garden bird feeding provides.