Tit taxonomy is complex, with several species and subspecies reclassified many times since they were first formally described. This paper uses data collected during ringing to examine subspecies in Marsh Tits, with conservation implications for this declining species. The taxonomy of tits has been reviewed several times since they were first classified, with species such as Willow Tit and Marsh Tit, once considered one and the same, now classed as separate. Debate continues about subspecies, despite advances in molecular techniques and other tools. Marsh Tit, which has variation in coloration, wing and tail length across its range, is one species for which classification at this level remains unresolved. Marsh Tits in Britain underwent a 73% decline between 1966 and 2013. Two subspecies, Poecile palustris palustris and Poecile palustris dresseri, are designated in government biodiversity action plans, but it is not clear whether this distinction actually exists in British birds. BTO is part of a new collaborative study, led by the Centre for Ecology and Hydrology and published in the BTO journal Bird Study, which examines the question of Marsh Tit subspecies using measurements of wing and tail length collected during bird ringing at 14 sites across Europe, including eight in Britain. The results indicate that all British Marsh Tits in the study belonged to the subspecies dresseri, whereas those from elsewhere in Europe were from the palustris subspecies. This has implications for Marsh Tit conservation, as this species has declined in northern England and Scotland faster than in other regions. Previously it was thought the northern populations might be from the palustris subspecies, but evidence from this study suggests there is no ecological or taxonomic reason why birds from southern Britain could not be used for reintroductions further north. This therefore opens a possible new avenue for conservation of this declining species.

Data collected by volunteer citizen scientists have been used to show how the timing of bird migration to and from the UK has changed since the 1960s. The spring arrival dates for 11 of 14 common migrants have got significantly earlier, with six species, including Swallow, House Martin and Chiffchaff, coming back to breed more than 10 days earlier than they used to. Species that advanced their timing of arrival also showed the most positive trends in abundance over this period. BTO ecologists, led by Stuart Newson, compared data from the Inland observation points summary survey, which ran from 1962–1966, with information collected from 2002 to 2011 via Migration Watch and BirdTrack. The change in spring arrival across species equated to an advance of 0.22 days per year on average. In addition to the early arrival, four species were found to depart for their wintering grounds significantly later than previously. As a consequence of earlier arrival and for some species later departure, nine of 14 species spent significantly longer in the UK in the 2000s than in the 1960s. These changes in bird migration are likely to be a response to warming on the birds’ UK breeding grounds. The extended stays in the UK may have benefitted some species in allowing birds to lay more than one clutch, when they previously would not have had time to attempt to raise an extra brood because conditions were not suitable. Studies such as this are important in understanding how birds can adapt to climate change, and the effect of their response to their population dynamics at a time when many species are in decline and effective conservation measures are being sought.

Non-uniform monitoring BTO is fortunate to hold unique large, long-term datasets on bird abundance and distribution collected by dedicated volunteers. Our scientists translate these data into trends describing populations, which can be used in turn by policymakers and land managers for decision-making. Not all taxa are so well understood, as for many species, little information is collected. One way of accounting for this is using a so-called 'surrogacy approach', where patterns of variation in less-well monitored species are assumed to be similar to those in better monitored species. Recent BTO research has set out to test this assumption, asking whether the population trends of highly mobile vertebrates, like birds, really be used to infer those of much smaller invertebrates, like butterflies. Cross-taxa variation at a fine scale Members of the BTO’s Monitoring and Population Ecology and Modelling Teams worked with Butterfly Conservation and the Centre for Ecology and Hydrology to analyse data collected by volunteers taking part in the UK’s BTO/RSPB/JNCC Breeding Bird Survey (BBS). Since 2009, BBS volunteers have also been asked to survey butterflies on their randomly-generated BBS squares, as part of the Wider Countryside Butterfly Survey (WCBS). This provides a contrast between these small, specialised insects and birds. The size of BBS squares (1 km2) is critical to this work, as the fine spatial resolution allowed the study’s authors to assess patterns in a way that has not been possible in previous studies of cross-taxa relationships. Data were analysed to examine spatial variation in bird and butterfly distribution and community structure over a range of habitats and latitudes. Specifically, the researchers considered the total number of species recorded in each visit to the square (species richness), the diversity of these species and their “evenness”, which is a measure of how the abundance of species within a community differs (evenness is lowest when a community is dominated by a few, highly abundant species). Calculations were also made to assess the degree to which communities were composed of generalist or specialist species (the 'community specialisation index', CSI). Support for surrogacy This study found significant similarities in the way UK birds and butterflies varied in species richness and diversity. These relationships remained when latitude and habitat-type was included in the analysis, showing that they were not simply a result of the common effects of temperature and land-cover on both groups. Instead they are likely to have resulted from common responses to variation in habitat-type, such as vegetation structure and heterogeneity within habitat-types. However, associations with evenness and CSI were much weaker, suggesting that the most specialised bird and butterfly species did not associate at relatively fine spatial scales. Specialist butterflies tended to occur in semi-natural habitats, whilst the most specialised bird communities were found in non-fragmented landscapes. Our results suggest that surrogacy approaches may be used where management decisions are based upon species richness and diversity. However, if conservation decisions prioritise particular rare and specialised species, then they will only benefit other taxa if those taxa also have similar responses to environmental change and management.