Verónica Méndez explores the drivers of worldwide variation in shorebird survival rates
Check out the full paper here
Why study shorebirds demographic rates?
Migratory waders (also known as shorebirds) include some of the most northerly breeding terrestrial vertebrates on the planet, with many populations breeding in the arctic and subarctic zones. During the non-breeding season, migratory waders occupy temperate and tropical coastal areas and inland wetlands. At both ends of their migratory routes, the habitats on which waders depend are highly vulnerable to environmental change, through processes such as global warming, sea level rise and land claim. These changes can greatly limit their ability to find the food needed to survive the winter months, fuel their migratory journeys and successfully reproduce. It is therefore perhaps not surprising that the majority of wader populations are currently in decline. For example, the numbers of Black-tailed Godwits breeding in continental Europe have decreased by nearly 75% and the charismatic Spoonbill sandpiper is threatened with global extinction, with the current global population estimated at only ~700 individuals. Wader population declines are apparent across every shorebird flyway (the main migration routes used by populations moving between breeding and wintering areas, see below in figure 1).
Figure 1. The migration routes used by wader populations moving between breeding areas in the north and their southerly wintering grounds. From left to right: American (yellow), African Eurasian (red), Central Asian (grey) and East Asian Australasian routes (blue).
In order to address these declines, we need to know both how and why the underlying demographic parameters, such as recruitment and survival, of wader populations are changing. Unfortunately, measuring these parameters can often be very challenging. For example, measures of annual survival can be challenging to obtain because large-scale, long-term tracking of individuals (information within and between years and across their distribution range is required) is difficult and the resulting data may contain inherent biases. Luckily for us the number of studies on wader demography has greatly increased over the last few decades, meaning that we had a ready-made library from which we could collect information on wader demographic rates and explore the variation in waders’ survival rates across the world.
We had lots of questions that needed answering: Do bigger waders live longer? Do species in Europe have similar survival than species in another part of the world? Is the variation a result of different analytical methods used to derive the estimates? Is annual survival different between females and males?
For which species are adult survival rates available?
It was time to get to work, we rolled up our sleeves, opened our laptops and searched for published estimates of annual adult survival rates. It took us 6 months! We extracted information for 56 species from 5 families, totalling 126 studies and 295 survival estimates. The first thing we thought was “Great, this is a lot of information!”. However, the 56 species with estimated survival rates accounted for only 25% of all waders and represented only a small number of families (there are currently 215 species distributed among 14 families). While for most species we were able to find just 1 or 2 estimates of survival, some species have received more attention, as in the case of the Oystercatcher, Redshank and Black-tailed Godwit in Europe and Piping Plover in America that have at least 10 published survival estimates each. These species have been the focus of comprehensive long-term monitoring programmes (mostly for conservation purposes), generating enough data to allow estimation of demographic rates in different parts of their ranges, annual cycles and using different analytical methods.
We also found that the majority of published studies are from species using the African-Eurasian and North American migratory routes, with very few studies for species in the East-Asian flyway and none in the Central-Asian flyway (given the rapid environmental changes occurring on these flyways, we really need more studies from this part of the world). In Europe and North America, bird ringing schemes were established long ago (early 1900s), and thus they have been running for long enough to produce the long-term data-sets needed to allow the calculation of robust survival rates.
How are annual survival rates estimated?
The methods used to calculate survival rates in birds have changed over time, as different approaches have been developed and more sophisticated statistical models have become available. For all methods, the fate of individual birds needs to be known, so we have to be able to identify individuals within populations and we can do this by marking individuals with leg rings. Usually, waders are marked on their legs either with a numbered metal ring, that can be read when the bird is recaptured alive or recovered dead, or with unique combinations of coloured leg rings that can be read from a distance. Survival rates can then be estimated from these data using one of three methods: recovery models which use information generated from the recovery of dead birds, mark-recapture models which require data on recapture or resighting of marked individuals (termed ‘live-encounter’) or through simple calculation of return rates (proportion of marked individuals returning to the site of marking) from live-encounter data. We also collected some survival estimates that had been generated using more complex modelling techniques, involving the combination of two or more types of ringing data.
Methods that use resighting/recapture data can underestimate true survival if birds leave the study area and never return, as no distinction can be made between this permanent emigration and mortality, whereas methods that use recovery data alone, or in conjunction with live-encounter data, tend to yield less-biased estimates of survival. This mean that the variation in our survival estimates may not just be due differences in species survival rates, but also variation in the sources of error arising from different estimation methods. In order to compare survival rates between species, we therefore firstly needed to explore the extent of this problem.
Do survival rate estimates vary among analytical methods?
We found that most of the published estimates have been generated using live encounter data (i.e. by recapturing or resighting marked individuals). When we compared estimates of survival among species, we found no differences between those estimates generated using recovery (dead birds) data alone or combined with live-encounter and those generated using live-encounter data. This is likely to reflect the fact that most shorebirds are highly site faithful, returning to the same breeding and non-breeding sites throughout their life, decreasing the problem of underestimating survival rates by mistaking permanent emigration for mortality. However, we did find that when live-encounter data were used to measure return rates, the estimates were significantly lower than the estimates derived from live-encounter data using mark-recapture models. We were expecting this result because, unlike estimates from mark-recapture models, return rates do not account for the imperfect detection of individuals. This can often be a problem when observing wild populations and, in waders, which often stand on one leg to roost, spotting all colour-ringed individuals in a flock can be very tricky. In our paper, we discuss why return rates might be a good starting point to measure survival when no other information is known and when resources or capacity for complex modelling are not available.
Which ecological and biological factors influence variation in survival rates?
Now that we had a better understanding of how the different analytical methods can influence survival rates, our next step was to explore the possible phylogenetic, geographic, seasonal and sex-based drivers of variation in our survival rates. In Table 1, I provide a summary of the main effects of each variable on survival rates and the potential drivers of these relationships. Interestingly, we found that published survival estimates from smaller waders are likely to be underestimates. This was most likely due to lower detectability (it’s harder to see smaller species) and/or lower levels of site fidelity in small waders. As most of the species studied on the American flyway are small waders, the published survival rates from this flyway were also often underestimates.
Table 1. Summary of the effects of biological and ecological factors on annual survival rates in shorebirds and the potential causes of variation.
In addition, we provide method-corrected species- and genus-specific adult annual survival estimates for all the species included in our study (52 species of 15 genera), by accounting for the analytical method used in the original study to generate survival estimates. Our corrected estimates can potentially aid the rapid identification of locations in which species may be experiencing lower than expected survival rates, and may therefore be places where efforts should be focused to identify and address the causes. We hope this is useful to other researchers in the field to assess their own estimates and particularly to explore those that deviate from the expected values. Check them out!
If you have questions about the paper (or other wader related question), you can easily find me on twitter @VMendezAragon
Verónica Méndez is a post-doctoral researcher at University of East Anglia and spends part of the year in Iceland, where she carries out her field work. Her current research investigates individual level responses to environmental changes, and particularly changes in migratory behaviour and associated fitness outcomes in a population of Eurasian Oystercatcher breeding in Iceland.