England’s House Sparrow population fell by 70% between 1977 and 2016, and this once ubiquitous species is now absent from many urban areas. New research involving the BTO has found evidence that malarial parasites may be linked to this species’ decline. During a three-year study led by the ZSL Institute of Zoology, in collaboration with the RSPB and later BTO, almost 400 individual House Sparrows were colour-ringed to allow their survival to be tracked through the winter. Blood and faecal samples were also collected from each individual, which were then screened for parasites and bacteria. The birds trapped and tested all originated from private gardens across 11 colonies scattered across London, where the House Sparrow population has fallen by 70% since 1995. Seven of these colonies had a negative population trend whilst four had a stable or increasing population. The results showed that the parasite Plasmodium relictum, which causes avian malaria, was found in 74% of House Sparrows, the highest prevalence recorded in populations of wild birds in Northern Europe. It was also found that the intensity of the infection (the number of parasites per individual bird) of avian malaria was correlated with negative population trends, especially in juvenile sparrows. Survival analyses on individual House Sparrows showed that birds with higher infection of malaria, whether juveniles or adults, had a lower probability of surviving the winter than did individuals with less severe infections. It was therefore concluded that avian malaria was linked to population decline through reduced overwinter survival of juveniles. It is unclear why avian malaria, which was likely native to the UK before the onset of the population decline of House Sparrows, may be affecting this species so significantly in London. House Sparrows are sedentary and BTO ringing data suggest that dispersal distances of this species have decreased from 1970 to 2010. It is possible that loss of habitat within cities might have led to isolated populations which are not very genetically diverse. This can result in a less-effective immune system and higher mortality, which, combined with low recruitment of new birds into the colonies, can lead to population decline. Whilst the underlying causes of House Sparrow declines remain unknown, and are likely due to a combination of factors, this study suggests that avian malaria may be implicated. Improved disease surveillance and continuing monitoring of the species at national level are essential for effective conservation management.

BTO research reveals that the release of Pheasants and Red-legged Partridges for commercial shoots may be boosting numbers of the avian predators and scavengers. Every year, 41-50 million non-native gamebirds (Pheasant and Red-legged Partridge) are released in the UK. Fewer than half these birds are shot, meaning there is potentially a large food resource available to predators and scavengers, sustaining their populations above the levels they would otherwise reach. If it occurs, this inflation of predator numbers might alter predator-prey dynamics, increasing predation pressure on some vulnerable species, including declining breeding waders like Curlew. This study used data from the Breeding Bird Survey and Bird Atlas 2007-11 to identify associations between the occurrence patterns of gamebirds and the abundance and population growth rates of several generalist predators, including Buzzard, Jay, Raven, Magpie and crows (Carrion and Hooded combined). While many other factors influence predator abundance, such as fine-scale habitat variation, availability of other food sources, and game management activities, the results suggest that large-scale variation in avian predator populations is predominantly positively affected by gamebird releases. The potential implications of this finding need to be thoroughly tested. Such tests could include regulation of releases on a trial basis, to determine effects on ground nesting birds, for example. The compulsory recording of releases and the number of predators controlled would also be valuable for a better understanding of the impacts, positive or negative, of gamebird releases on the wider environment. Read a blog post by publication author Henrietta Pringle discussing the work behind this paper.

The ability to predict spatial variation in biodiversity is a long-standing but elusive objective of landscape ecology. It depends on a detailed understanding of relationships between landscape and patch structure and taxonomic richness, and accurate spatial modelling. Complex heterogeneous environments such as cities pose particular challenges, as well as heightened relevance, given the increasing rate of urbanisation globally. Here we use a GIS-linked Bayesian Belief Network approach to test whether landscape and patch structural characteristics (including vegetation height, green-space patch size and their connectivity) drive measured taxonomic richness of numerous invertebrate, plant, and avian groups. We find that modelled richness is typically higher in larger and better-connected green-spaces with taller vegetation, indicative of more complex vegetation structure and consistent with the principle of ‘bigger, better, and more joined up’. Assessing the relative importance of these variables indicates that vegetation height is the most influential in determining richness for a majority of taxa. There is variation, however, between taxonomic groups in the relationships between richness and landscape structural characteristics, and the sensitivity of these relationships to particular predictors. Consequently, despite some broad commonalities, there will be trade-offs between different taxonomic groups when designing urban landscapes to maximise biodiversity. This research demonstrates the feasibility of using a GIS-coupled Bayesian Belief Network approach to model biodiversity at fine spatial scales in complex landscapes where current data and appropriate modelling approaches are lacking, and our findings have important implications for ecologists, conservationists and planners.