Bio‐energetic data show weak spatial but strong seasonal differences in wetland quality for waders in a Mediterranean migration bottleneck

Abstract

Assessing the quality of wetlands as refuelling areas for migrating waterbirds based on resource distribution patterns is challenging. Resources in wetlands can vary both horizontally and vertically and may be differentially available to different bird species at different times of the year. Therefore, the extent to which wetland quality can be generalised across seasons and for a diversity of birds remains unresolved. Spatiotemporal variation in abundance and quality of macrobenthos as food for migrating waders was studied in a set of wetland areas near a Mediterranean migration bottleneck in the Balkan peninsula, during both spring and autumn migration. Samples were subdivided into different depth layers to differentiate between parts of the sediment that are accessible to different groups of wader species. To quantify food availability and the resulting refuelling capacity in different wetland habitat types, abundance, wet weight, and lipid and protein content of invertebrate taxa were determined for each sample. Invertebrate food availability and quality were markedly higher in spring than in autumn. Given the higher abundance and protein and lipid content of prey in spring, the total energy that could be harvested in spring (3.81 ± 0.79 kJ/m2) was about 7 times higher than in autumn (0.56 ± 0.12 kJ/m2). Most prey were found in the top layer of the sediment (0–22 mm depth), but about a third of the total energy was present in the deepest soil layer that only longer‐billed species could reach (55–200 mm depth). Higher quality prey items such as larger‐bodied and heavier polychaetes and bivalves were found in the deeper sediment layers. For other taxa, there was no evident vertical pattern of increasing individual body size. Prey abundance differed between certain habitat types but, overall, food availability could not be linked to distinct habitats. In spring, redox potential tended to be higher, while pH, vegetation cover, conductivity, and temperature were lower than in autumn. Different wetland habitat classes used in a conservation framework may provide similar food resources for waders. As a result, linking food availability to habitat classifications is not straightforward. Furthermore, seasonal variation in wetland quality requires a re‐evaluation of the importance of wetland areas during spring and autumn migration. Finally, nutritional analyses are essential for determining the capacity of wetlands to support refuelling by migratory waterbirds.

Measuring habitat quality for waterbirds: A review

“… Prey animal biomass (Atiénzar et al, 2012 ; Deboelpaep et al, 2020 ; Herring & Gawlik, 2013 ; Holopainen et al, 2014 ; Hunt et al, 2017 ; Parks et al, 2016 ; Schultz et al, 2020 ) Plant‐derived food density or abundance (Arzel et al, 2015 ; Atiénzar et al, 2012 ; Dugger & Feddersen, 2009 ) …”

“…Prey animal biomass (Atiénzar et al, 2012 ; Deboelpaep et al, 2020 ; Herring & Gawlik, 2013 ; Holopainen et al, 2014 ; Hunt et al, 2017 ; Parks et al, 2016 ; Schultz et al, 2020 )…”

“…Direct habitat measures Food availability • Prey animal biomass (Atiénzar et al, 2012;Deboelpaep et al, 2020;Herring & Gawlik, 2013;Holopainen et al, 2014;Hunt et al, 2017;Parks et al, 2016;Schultz et al, 2020) • Plant-derived food density or abundance (Arzel et al, 2015;Atiénzar et al, 2012;Dugger & Feddersen, 2009) • Birds behaviourally track sites with highest prey biomass and density (Rose & Nol, 2010), and if only accessible prey can be measured then this relationship can be even stronger than if overall biomass is measured (Lourenço et al, 2005) • Prey availability has a positive influence on reproductive performance (Herring et al, 2010) • Chick condition is related to local prey abundance (Hunt et al, 2017;Le Fer et al, 2008) • Predicts occupancy but not abundance (Gillespie & Fontaine, 2017) • Sites with high food densities are not always the favored foraging sites (Hagy & Kaminski, 2015) and this can be due to accessibility (e.g., vegetation structure (Nam et al, 2015) or substrate hardness (Nolet & Klaassen, 2009))…”

Measuring habitat quality for waterbirds: a review

“…Direct habitat measures Food availability Prey animal biomass (Atiénzar et al, 2012, Deboelpaep et al, 2020, Herring and Gawlik, 2013, Holopainen et al, 2014, Hunt et al, 2017, Parks et al, 2016, Schultz et al, 2020 Plant-derived food density or abundance (Arzel et al, 2015, Atiénzar et al, 2012, Dugger and Feddersen, 2009 Birds and Gawlik, 2013, Townsend et al, 2006; Water level variability (Collazo et al, 2002) Availability of shallow water (Collazo et al, 2002, Gawlik and Crozier, 2007, Lantz et al, 2011 Landscape depth heterogeneity (Beerens et al, 2015) Wading birds preferentially selected ponds that had been experimentally manipulated to have shallow rather than deep water (Gawlik and Crozier, 2007) and waterbird species richness and density correlates with the availability of shallow water habitats (Wang and So, 2003) Water level recession rate was a key influence on physiological condition of two species of waterbirds (Herring and Gawlik, 2013) Water level variability did not influence habitat selection of wading birds (Gawlik and Crozier, 2007) Site/region -Instantaneous/within season/annual…”

Horizontal prey distribution determines the foraging performance of short‐ and long‐billed waders in virtual resource landscapes

“…The spatial distribution of these prey items can be highly variable depending on the local environmental conditions, tidal cycle, geographical location and season (Anderson & Smith 1998, Duijns et al 2015, Zhang et al 2016. Also, the relative abundance, size and energetic content of prey vary throughout different depth layers of the sediment (Zwarts & Blomert 1992, Zwarts & Wanink 1993, Deboelpaep et al 2020. The largest numbers of macroinvertebrates are found at the shallowest depths, but larger and/or higher-quality prey individuals generally reside in deeper sediment layers , Zwarts & Wanink 1991, Piersma et al 1993a.…”