Characterising demographic contributions to observed population change in a declining migrant bird

Abstract

Populations of Afro‐Palearctic migrant birds have shown severe declines in recent decades. To identify the causes of these declines, accurate measures of both demographic rates (seasonal productivity, apparent survival, immigration) and environmental parameters will allow conservation and research actions to be targeted effectively. We used detailed observations of marked breeding birds from a ‘stronghold’ population of whinchats Saxicola rubetra in England (stable against the declining European trend) to reveal both on‐site and external mechanisms that contribute to population change. From field data, a population model was developed based on demographic rates from 2011 to 2014. Observed population trends were compared to the predicted population trends to assess model‐accuracy and the influence of outside factors, such as immigration. The sensitivity of the projected population growth rate to relative change in each demographic rate was also explored. Against expectations of high productivity, we identified low seasonal breeding success due to nocturnal predation and low apparent first‐year survival, which led to a projected population growth rate (λ) of 0.818, indicating a declining trend. However, this trend was not reflected in the census counts, suggesting that high immigration was probably responsible for buffering against this decline. Elasticity analysis indicated λ was most sensitive to changes in adult survival but with covariance between demographic rates accounted for, most temporal variation in λ was due to variation in productivity. Our study demonstrates that high quality breeding habitat can buffer against population decline but high immigration and low productivity will expose even such stronghold populations to potential decline or abandonment if either factor is unsustainable. First‐year survival also appeared low, however this result is potentially confounded by high natal dispersal. First‐year survival and/or dispersal remains a significant knowledge gap that potentially undermines local solutions aimed at counteracting low productivity.

Early life conditions influence fledging success and subsequent local recruitment rates in a declining migratory songbird, the Whinchat Saxicola rubetra

“…Earlier fledging can lower rates of post-fledging predation (Naef-Daenzer & Grüebler, 2016;Verhulst & Nilsson, 2007), and pre-fledging conditions can influence post-fledging survival; with fledglings from earlier broods seemingly less susceptible to mortality in subsequent stages (Naef-Daenzer & Grüebler, 2016). Furthermore, higher success for larger broods is unlikely to be due to buffering against complete failure due to cumulative events of mortality, as Whinchat experience very low rates of partial brood failure (7.3% in our study; 3% in Border, Henderson, Ash, et al, 2017), meaning that the number of fledglings was highly correlated with brood size (r s = .827).…”

“…However, the quadratic nature of the relationship is interesting, with fledging success declining above 50% Bracken cover. Whinchats preferentially nest in areas with high structural vegetation diversity (Border, Henderson, Redhead, et al, 2017; Douglas et al, 2017; Fischer et al, 2013), which are associated with greater quality, diversity and abundance of invertebrate prey (Britschgi et al, 2006; Evans et al, 2015) but avoid foraging in areas with high Bracken cover (Murray et al, 2016). Therefore, a trade‐off may exist between nest concealment offered by high Bracken cover and the low foraging efficiency of Bracken monocultures.…”

Whinchat survival estimates across Europe: can excessive adult mortality explain population declines?

“…However, the available information about predation in our study populations does not support this interpretation. Indeed, although females from the UK population suffering from strong nest predation (Border et al ., 2017), they have only slightly lower apparent survival probabilities than males.…”

Opportunities and challenges for big data ornithology

“…Avian demographic research has played an important role in advancing the field of avian population ecology (Saether andBakke 2000, Sillett andHolmes 2002) and has played a critical role in supporting bird conservation (Green 1999). These efforts have taken on greater relevance for migratory bird species as researchers explore the demographic drivers underlying current population declines (Morrison et al 2016, Border et al 2017. Several large datasets with highly structured protocols provide a unique platform for estimating demographic parameters and population trends across broad spatial and temporal extents.…”