Cat Morrison uses citizen science data to work out how to target conservation actions to help declining migrant bird species.
Migratory species are declining
Across Europe, since the 1980s (which is the course of my life-time), severe declines have occurred in the group of birds which we broadly classify as humid-zone migrants. These are the species that, not only cross the Sahara twice a year, but travel further south into the humid tropics of Africa, to spend the winter months in countries such as the Ivory Coast and Ghana. It’s amazing to think of the diversity of conditions that these birds, some weighing as little as a few grams, experience in just one year. However, within Europe, for every three Nightingales that were singing when I was a baby there is now only one, for every three Wood Warblers, there are now only two, and for every four Garden Warblers there are now just three, I could go on. Interestingly, species that travel to the Sahel zone (known as arid-zone migrants), just south of the Sahara Desert, are generally not declining at present.
Correctly targeting conservation actions to help reverse population declines is not an easy task. The complex processes that make migratory systems so deeply fascinating, can also make them frequently challenging and occasionally slightly frustrating systems in which to work. For humid-zone species, there are many environmental changes that could be, and probably are, contributing to their population declines. Such changes span continents, from habitat loss on their distant African non-breeding grounds – an obvious candidate given the differences in population trends between humid and arid-zone migrants – climatic effects on timing of migration and breeding, to changing agricultural practices degrading the quality and availability of breeding sites. A further hurdle in our understanding of migratory systems is that we are only just starting to discover the variation in individual migratory journeys, revealing great differences in timings and routes. For example, tracking studies are now repeatedly showing that individuals that nest within the same patch of habitat in Europe can spend the non-breeding season separated by thousands of kilometres.
Many widespread and common species are declining
Unfortunately, humid-zone migrants aren’t the only group of bird species under threat; we are increasingly seeing declines in our previously common and widespread species, which remain in Europe year-round. This presents a real challenge; how can we target actions capable of influencing the population trends of all of these species across such large geographical scales?
If we go back to basics, reversing population declines requires increases in the number of birds entering the population (productivity) and/or reductions in the rate at which individuals leave (mortality). What we therefore need to do is identify places when either productivity or survival could potentially be boosted, through targeted conservation actions.
Citizen science to the rescue
photo credit David Tipling/BTO
One of our greatest assets in this task is people-power. For decades now, across much of the European breeding grounds, standardised annual counts and captures of breeding birds have been carried out by a dedicated network of volunteer ornithologists and bird ringers. It is thanks to their incredible efforts that we are currently able to track the ups and downs of species’ fortunes across vast geographic and temporal scales. In Europe, surveys of numbers of breeding birds are collated by Pan-European Common Bird Survey (PECBMS), while variation in productivity and survival rates can be calculated from the ringing data held by the European Constant Effort Sites Scheme (Euro-CES). In figure 1 you can see the amazing distribution of these sites across Europe, from the Iberian Peninsula to the far north of Finland. I have been lucky enough to work with these data throughout my career and it is extremely satisfying to be able to use this hard-won information to try to help declining species.
Figure 1: The location of PECBMS sites and Euro-CE sites in Europe, colours indicate the number of years sites were active for during the study period (PECBMS: 1994 to 2013 & Euro-CES: 2004 to 2014).
Are declines concentrated in sites that could be targeted for action?
During my PhD we found that while numbers of one humid-zone species, the Willow Warbler, have declined in England, they have remained stable or in some cases increased in Scotland. A further dive into the data showed this pattern was common among many migrant and, importantly, resident species. Something was going wrong on the English breeding grounds that wasn’t happening in Scotland, and it wasn’t just affecting migrant species. Several PhD chapters later we showed that, for willow warblers, a couple of years of high mortality, at the start of the 2000s, caused sharp population declines in both Scotland and England. However, after that time, survival rates subsequently recovered in both countries and, in Scotland, these years of good survival were also years in which productivity was high, allowing the population to recover, relatively quickly, within a decade. In England, however, productivity was not high enough to allow a similar recovery, and so numbers have stayed low.
It therefore seems that having productivity that is sufficiently high, across a sufficiently large number of sites may be key to allowing populations to recover from periods when mortality is high. These extremely interesting findings led us to ask, are similar patterns present elsewhere, and if so, what can they tell us about potential solutions to migrant population declines?
Can we identify sites where actions to boost productivity could be effective?
Identifying sites where actions to boost productivity could be effective will only be possible if sites are consistently ‘good’ or ‘poor’ for whole bird communities. We used PECBMS data to ask whether, across Europe, migrants and residents in the same sites tend to be showing the same populations trends (increasing or decreasing). We found that at the sites where migrants have positive population trends, so too do residents, and conversely, in the sites where migrants are declining, so too are the residents (see figure 2 a&b). This means that there are indeed sites that appear to be ‘good’ and sites that are ‘poor’ for the whole community of breeding birds. It also provides potential opportunities to work out why sites are good or bad, by comparing conditions at those sites. We also found that productivity varies in much the same way as population trends, that there are ‘good’ sites, which consistently produce lots of young, and ‘poor’ sites, which consistently produce few young. Once again, these sites correspond for residents and migrants (see figure 2 c&d).
Figure 2: Covariation between resident bird species and their co-occurring arid-zone (top row) and humid-zone (bottom row) migrant species in mean site-level (a,b) population trends (a: 12,103 sites; b: 13,267 sites), (c,d) standardised mean site-level productivity (c: 156 sites; d: 247 sites) and (e,f) standardised mean site-level annual survival rates (e: 156 sites; f: 247 sites). Lines of best fit are shown for significant associations and numbers indicate the number of sites. Horizontal bars indicate medians, boxes indicate interquartile range, whiskers indicate minimum and maximum values and circles indicate values 1.5 times higher or lower than 1st and 3rd interquartile, respectively. Taken from Morrison et al. 2021.
I really believe that these are hopeful findings; they demonstrate that there is room for improvement, and that there will likely be actions we can target at improving conditions on breeding sites across Europe, which will genuinely be able to help these species. By increasing the proportion of sites that are ‘good’ for productivity, then this could also act as a buffer against processes occurring elsewhere in the annual cycle, allowing populations to bounce back from population losses (much like our Scottish willow warblers). Our findings also suggest that actions taken on sites to aid migrant species could benefit declining resident species.
No such relationships existed with survival rates (see figure 2 e&f) but perhaps this is not too surprising. Survival rates at a single site on the breeding grounds are influenced by environmental conditions occurring at the multiple locations over which individuals spend their entire year. As we now know, these locations vary greatly and the only way we would expect to find the same patterns in survival rates as we found in productivity, would be if there was a large-scale driver affecting a high proportion of the population, perhaps occurring over a lot of Africa, Europe or at a location that many individuals migrate through, e.g. Gibraltar, one of the main crossing points from Europe to Africa.
What do these findings mean and what next?
This work has shown that there are opportunities for those of us living within European breeding grounds, to help our migrant species and importantly that this could be an effective approach. If we target local conservation actions at improving productivity, we could potentially boost numbers of our African-Eurasian migrants. While environmental protection across non-breeding ranges will also play a crucial role in influencing population trends, our findings highlight that on breeding grounds targeting conservation actions at survival may not be as effective as targeting productivity. Consequently, our next step is to identify the local environmental conditions that create good and bad breeding conditions, and to work with our partners to use this knowledge, to design and implement actions that improve breeding season conditions and boost productivity.
Thanks
At the risk of sounding like a Strictly Come Dancing contestant this paper has really been a journey. But luckily, I have not been on it alone; I would like to say a huge thanks for their guidance and shared determination to Jenny Gill, Rob Robinson and Simon Butler. Thank you to my European co-authors; your encouraging and supportive emails have frequently made me smile. Finally, a massive thank you to all the volunteers who contribute to these surveys; without you none of this work would be possible. Do I get a glitter-ball now?
Catriona Morrison is a post-doctoral researcher at the University of East Anglia. Her primary research interests lie in understanding the demographic and environmental processes influencing the population dynamics of birds. If you have any questions about the paper, you can easily find her on twitter @CatMorrison18.
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