Avian demography in a changing world: a report on the BOU's Annual Conference held at the University of Leicester, 1–3 April 2013


The 2013 BOU annual conference took place at the University of Leicester and brought together professional and amateur ornithologists to present the most recent work and discuss ideas under the theme From populations to policy impact: avian demography in a changing world. Demographic rates provide a measure of how many individuals within a population are born (productivity), mature (recruitment), survive and ultimately die, and are therefore closely linked to the annual and life-cycles of those same individuals. Here I summarize the most recent work presented by the conference speakers.

Ornithologists have long been interested in investigating the drivers of population change and in doing so have pushed this field of biology to the forefront of demographic studies (Jenny Gill, BOU President, University of East Anglia). But why does birth and death matter for bird conservation? or, in other words, ‘why bother?’ (Rhys Green, RSPB & University of Cambridge). Green's overview of avian demographic studies, with special attention to conservation, illustrated the vast evolution that this field has undergone over the last 150 years. The study of avian demography has moved from correlative studies between variation in population numbers and plausible external drivers (Green & Stowe 1993), to measuring demographic rates in order to identify their drivers (Peach et al. 1999) and subsequently linking variation in demography and its causes (Niles et al. 2009), to attaining demographic information across different areas in order to investigate factors that vary spatially (Etheridge et al. 1997), to modelling potential drivers of demographic variation to predict changes in population (Green et al. 1997) and using these results to influence policy makers and produce change (Oaks et al. 2004, Prakash et al. 2012).

Recent studies of productivity have used modelling approaches to estimate re-nesting probabilities, showing the relevance of incorporating individual (Patrick White, Game & Wildlife Conservation Trust) and habitat variation (Rosemary Setchfield, RSPB) in these models. At the same time, field studies on productivity have indicated differences between techniques, with the monitoring of individual nesting attempts (nest records) estimating lower declines than ringing (e.g. Constant Efforts Sites, CES). Although the reasons for such differences are unclear, these might be due to the fact that CES represent a sub-sample of overall data collected at a given site (David Leech, BTO).

The dispersal and recruitment of individuals is, in some cases, determined by their state, with individuals in poorer condition generally dispersing larger distances, as is the case of the territorial Eagle Owls Bubo bubo (Maria Delgado, University of Helsinki), creating spatially structured populations. There is evidence for a lack of genetic structure in island populations, as is the case for Starlings Sternus vulgaris on Fair Isle, despite showing spatially explicit differences in productivity at such small scales (Jessica Walkup, University of Aberdeen). But if migratory species, such as the Shag Phalacrocorax aristotelis, can show very high levels of site fidelity within the same and consecutive years (Hannah Grist, University of Aberdeen), then it is the naive juveniles that make the important decision about where to settle and by doing so they are likely to determine their life-long fitness, as seems to be the case for Icelandic Black-tailed Godwits Limosa limosa islandica (Tómas Gunnarsson, University of Iceland).

During the non-breeding season, the key demographic parameter is survival. But this season presents great challenges to researchers with respect to the detection of individuals. New technologies might be used to aid research in this area, such as the Song Meter. This automated sound recording system records birds' calls, which can later be recognized using specific software (song scope, Wildlife Acoustics, Concord, MA, USA) thus allowing remote identification of species presence (John Quinn, Furman University). However, to obtain survival estimates, individual- (or cohort-) level data are required, and the use of ringing and recoveries datasets to estimate season-specific survival is becoming more common. For some species, such as the Yellowhammer Emberiza citrinella and the Chaffinch Fringilla coelebs in East Anglia, survival seems to be lower during the non-breeding season (Gavin Siriwardena, BTO).

The effects of environmental conditions on survival can be hard to detect and recent research indicates that sub-lethal effects, such as changes in body mass of adults in response to predation risk (Ross Macleod, University of Glasgow), may also be important. Similar sub-lethal and indirect effects on demographic rates can also affect productivity, as is the case in the Shag populations of the East of Scotland. After undergoing treatment, the survival of the offspring of Shags with higher burdens of parasites increased, demonstrating an indirect effect of parasites on demographic rates (Francis Daunt, CEH). The most striking example of such effects is perhaps that of urban cats, which are becoming increasingly abundant in the UK. When presented with a model of a cat, breeding Blackbirds Turdus merula responded more acutely than when presented with squirrel or rabbit models. Although alarming and mobbing behaviour towards cats could reduce direct nest predation in the short term, those nests had lower provisioning rates and a significantly higher risk of being abandoned or predated (even by other species). This suggests that non-lethal and indirect effects of non-native predators might impact avian demography at a higher level than previously considered (Karl Evans, University of Sheffield).

Several types of interaction occur in demographic parameters between seasons (and between areas for migratory birds). Studies of seasonal interactions mediated by carry-over effects show that conditions experienced in one season can have effects on demographic parameters in the subsequent season. This is the case for trans-Saharan migrants breeding in the UK, whose variation in breeding phenology is explained by the climatic conditions in the Mediterranean (i.e. conditions during migration), as is breeding performance, although climate conditions during the breeding season also explain some of this variation (Tom Finch, University of East Anglia). Many UK migrants have stable or increasing populations in Scotland, whereas decreasing populations occur in England, which shows the relevance of seasonal interactions on demography. One such species is the Willow Warbler Phylloscopus trochilus, for which estimations of sex-specific survival revealed that females have lower survival in both countries. As male-skewed sex ratios are likely to restrict the number of breeding attempts, this could be particularly detrimental as per-nest productivity is also low (Catriona Morrison, BTO). For resident birds (e.g. Long-tailed Tits Aegithalos caudatus), seasonal interactions are also apparent, as survival is mostly affected by spring and autumn temperatures and not by temperatures in winter and summer (Philippa Gullett, University of Sheffield). Another modelling exercise showed that winter temperatures have opposite effects in two demographic parameters of the Eurasian Oystercatcher Haematopus ostralegus, with a positive relationship with survival but a negative relationship with reproduction (Yngvild Vindenes, University of Oslo). For both these species, future climate change might positively affect survival (but not reproduction for Oystercatchers), given the prediction of higher temperatures in forthcoming years. Indeed, a recent review of the impacts of climate change on bird demography highlighted that populations at high latitudes (> 30°N) tended to be positively influenced by higher temperatures (with higher productivity, survival and growth). While populations at lower latitudes (< 30°N) tended to be positively correlated with variation in precipitation, which is a stronger driver of bird demography for these populations than temperature (James Pearce-Higgins, BTO).

Advances in the study of demography over the last decades have been possible partly due to improvements in the modelling capacity at the disposal of scientists. The work developed by statisticians allows biologists to estimate demographic parameters, such as survival and productivity, which are extremely difficult to measure directly and to link these to variation in external factors. Recent modelling advances involve the use of analysis of deviance (Grosbois et al. 2008) in capture-mark-recapture models (Morten Frederiksen, Aarhus University), the inclusion of transients between age-classes in structured demographic models (Thomas Ezard, University of Southampton) and the capacity to use landscape simulation models in combination with spatially explicit population models in order to investigate the effects of land-use change on distribution and population structure (Matthew Geary, University of Southampton). The recent use of integrated population models (IPM) allows researchers to formally combine information from difference sources (e.g. capture-mark-recapture, monitoring of reproductive success and counts of population size) to estimate immigration (Res Altwegg, South African National Biodiversity Institute). Similarly, combining demographic rates generated by several different volunteer monitoring schemes (e.g. CES, nest records) is currently being developed using state-space modelling, creating large-scale species-specific population models (Rob Robinson, BTO). By constructing population models across species that vary, for example, in their use of habitats or resources, the contrast between species' demographic rates can be used to identify factors affecting these populations.

In view of these advances in avian demography, ornithologists can now use this knowledge towards the conservation of species. Demographic models highlighted that the population decline of the Wandering Albatross Diomedia exulans breeding in South Georgia was not due to productivity, which has increased, or to survival, which has decreased only slightly, but rather to juvenile recruitment, which has crashed since the 1980s (Richard Phillips, British Antarctic Survey). The effect of fisheries (including by-catch) was tested using the population model under several scenarios of intensity, demonstrating that the human exploitation of marine resources has a detrimental effect on the population growth of this species. A similar case of population decline was reported for Red Knots Calidris canutus, migrating to the Arctic via Delaware Bay, where they stop over to refuel by feeding on Horseshoe Crab Limulus polyphemus eggs, a species that is also harvested by humans (Conor McGrowan, US Geological Survey). Applying demographic models to species of conservation priority can result in valuable insights that can feed back into conservation programmes. In Mauritius, the populations of the Pink Pigeon Nesoenas mayeri and the Mauritius Kestrel Falco punctatus have been intensively monitored since their respective population bottlenecks (1990s and 1970s). While for the Pink Pigeon the population growth seems to be limited by survival (Lianne Concannon, University of Reading), the Mauritius Kestrel population appears to be limited by productivity (Malcolm Nicoll, University of Reading), with breeding success being affected by timing of breeding, weather and habitat as well as by the interference between juveniles, which aggregate in good quality habitats (Nevoux et al. 2011, 2013). In the UK, the re-introduction of the Red Kite Milvus milvus has been a success in many but not all populations. Investigating the differences in demographic parameters between populations has identified illegal persecution as the cause of slower population growth in North Scotland (Jennifer Smart, RSPB). When the effect of this additional mortality was removed, the estimated population growth rate of this population became similar to others where this kind of persecution is absent (Smart et al. 2010).

Bringing science closer to the general public increases awareness and promotes conservation. The ring-reading networks for both the White-fronted Goose Anser albifrons (Tony Fox, Aarhus University) and the Oystercatcher (Bruno Ens, SOVON) are striking examples of such citizen science in action. This involvement, which also includes survey counts (White-fronted Geese) and colour ringing (Oystercatchers), allows scientists to produce meta-population models to explore variation in demographic parameters between populations. Such large-scale studies are only possible with the contribution of members of the public. Ultimately, avian demography studies and their conclusions serve to deliver science-based evidence to decision makers and politicians. One of the biggest challenges in this final step is the different backgrounds and different forms of knowledge that scientists and stakeholders can have, which often leads to science being easily dismissed or ignored. The best solution for this issue is for conservationists to engage early on with these processes (Steve Redpath, University of Aberdeen), as this can have very good outcomes (Young et al. 2013). One such example is the English Nature Improvement Areas, which was a recommendation arising from the Making Space for Nature review and that was established by the UK Government in 2012 with supplementary funding and assistance from elsewhere since (Sir John Lawton). Understanding why some proposals are accepted and others are dismissed is crucial and, in many cases, political interests respond more to other parameters (e.g. timing of events) than to scientific information. The lack of understanding of science by politicians can be a considerable problem, but it is also the conservationists' responsibility to recognize other forms of thinking and work towards translating science into action.

Accompanying the main talks, 15 poster displays were presented, with authors briefly promoting their work in the main lecture room. Rob Fuller (BTO) was presented with the BOU's Godman-Salvin Award (BOU 2013a) for his significant contribution to ornithology, and Neil Bucknell (former Honorary Secretary of the BOU) was awarded the BOU's Union Medal (BOU 2013b) for his significant contribution to the BOU and to local and national ornithology in Britain. Student awards for best talk and best poster presentations went to Philippa Gullett (University of Sheffield) and Catharine Horswill (British Antarctic Survey), respectively. During the conference the BOU proactively engaged with early-career ornithologists in the now annual BOU Young Researcher Workshop.

Summaries of the above presentations are now available free of charge via the BOU's proceedings portal website at www.bouproc.net.

This conference would not have been possible without the dedication of the principal programme organizers Rob Robinson (BTO) and Karl Evans (University of Sheffield), assisted by Mel Kershaw (Defra) and Jane Reid (University of Aberdeen), and the BOU's Steve Dudley and Angela Langford.

The BOU is grateful to the conference programme organizers for delivering such a strong programme, to all who presented their work as a talk or poster, and to the ornithologists, young and old, who enthusiastically participated in the Young Researcher Workshop, and those who voluntarily assisted with the many conference jobs.