DOI: 10.5061/dryad.4xgxd256p
title: The importance of wetland margin microhabitat mosaics; the case of shorebirds and thermoregulation

description: Wetlands, and the species that rely upon them, are under significant
 threat worldwide, with wetlands often being completely removed or
 drastically altered. Successful wetland management requires an
 understanding of the interactions between wetland species and the
 microhabitats they use. The use of microhabitats for thermoregulation in
 wetland species is poorly studied, though anthropogenic influence on
 wetlands can reduce the diversity of microhabitats and thus the
 thermoregulatory options for animals. At high ambient temperatures birds
 may use the water-logged wetland margins to help with thermoregulation,
 and are often observed roosting in the sitting position within this
 microhabitat. However, whether sitting on the wet substrate helps in
 thermoregulation is unknown. In this study, we tested whether birds
 selectively use microhabitats across temperatures by conducting field
 observations of nine species of shorebirds. We use comparative analysis to
 determine whether birds roost more on wet substrate in the sitting
 posture, i.e. ‘wet-sitting’, at high ambient temperatures. We found
 substrate type across the wetland margins to be important in shorebird
 thermoregulation, with the time spent sitting being significantly mediated
 by the substrate on which the bird roosted. Individuals tended to sit on
 bare, wet ground much more under high ambient temperatures compared with
 low ambient temperatures. Vegetation on the other hand was used similarly
 across temperatures, and likely does not provide the same thermoregulatory
 benefits. By roosting on wet substrate at high ambient temperatures, birds
 may increase the potential for heat dissipation across the uninsulated
 legs, as water-logged wetland margins are known to remain cooler than the
 ambient temperature or vegetated microhabitats under hot climatic
 conditions. Synthesis and applications. Wetland creation and management
 requires an understanding of the functional significance of such
 microhabitats, not only for foraging and breeding, but also for roosting.
 We demonstrate that managing wetland margins is likely important in
 minimising heat stress in birds, with our findings emphasising the
 importance of maintaining open spaces in habitat mosaics for birds to use
 for thermoregulation. The ability of wetland species to manage heat stress
 is becoming exceedingly important as they are threatened by both decreased
 wetland availability and increasing ambient temperatures under climate
 change.
description: Observations of shorebird
 behaviour Behavioural
 observations of roosting shorebirds were conducted along the coastlines of
 Port Phillip and Westernport Bays, southern Victoria, Australia (Tables S1
 Supplementary Material), during the austral spring and summer (October –
 February, 2013-14). These sites included naturally occuring wetlands,
 artificially created/restored wetland reserves (e.g. from former
 saltworks, or military bases) and coastal regions. They included several
 different wetland types, such as coastal saltmarshes, permanent freshwater
 lakes and intertidal flats (Fig. 1 and Table S1). Tidal regimes and
 habitats were broadly similar across study locations with both tidal and
 non-tidal sites, with varying degrees of vegetation density. The wetland
 and broad habitat classifications are summarised in Table S1
 (Supplementary Material). All study sites were non-ornamental, and all had
 unformed margins, offering a mosaic of muddy substrates and low wetland
 vegetation. Across these habitats, we observed nine shorebird species from
 four families: sharp-tailed sandpiper <em>Calidris
 acuminata</em> (Horsfield 1821) (<em>n</em> = 42 individuals),
 curlew sandpiper <em>Calidris ferruginea</em>
 (Pontoppidan 1763) (<em>n</em> = 38), red-necked
 stint <em>Calidris ruficollis</em>
 (Pallas 1776) (<em>n</em> = 39), red-kneed
 dotterel <em>Erythrogonys cinctus</em>
 (Gould 1838) (<em>n</em> = 39), pied
 oystercatcher <em>Haematopus longirostris
 (</em>Vieillot 1817) (<em>n
 </em>= 43), black-winged stilt <em>Himantopus
 himantopus</em> (Linnaeus 1758) (<em>n</em> = 53), red-necked
 avocet <em>Recurvirostra
 novaehollandiae</em> (Vieillot 1816)
 (<em>n</em> = 41), common
 greenshank <em>Tringa nebularia
 </em>(Gunnerus 1767) (<em>n
 </em>= 38) and masked lapwing <em>Vanellus miles
 </em>(Boddaert 1783) (<em>n
 </em>= 42). These species have a large degree of
 overlap between in female and male size, and are not able to be sexed by
 eye. Therefore, sexes were combined in our analyses. During the austral
 summer, these species all forage and roost locally at the study sites;
 with some small scale movements between foraging areas and roosts (Rogers,
 Herrod, Menkhorst & Loyn. 2010, Nebel, Rogers Minton &
 Rogers 2013, Rogers, Loyn & Greer 2013). Variations in tidal
 height are likely captured evenly for each species across the data
 collected, due to the birds mostly roosting at high tide (Rogers et al.
 2006b) and filming being undertaken across a five month period. We used
 roosting birds only, with no birds demonstrating breeding or courtship
 behaviours. Birds were filmed from a stationary vehicle, from a bird hide
 or by approaching slowly on foot (for birds ~30m or more away), waiting
 five minutes before filming commenced. We filmed roosting
 birds with a Canon 60D DSLR camera using a Canon 100–400 mm lens (Canon
 Inc., Tokyo, Japan) or ‘digiscope’ (Swarovski ATM 80HD Spotting Scope,
 Swarovski Optik, Absam, Tyrol, Austria). These data were used to analyse
 roosting behaviour across species, with interspecific comparison of
 several postures presented in Ryeland <i>et al. </i>(2017,
 2019). For the purpose of this paper, we aimed to understand whether birds
 used the sitting posture selectively across temperatures and roosting
 mediums. As such, birds were categorised to be either roosting in the
 sitting or standing posture (Fig. 2), noting whether they roosted on bare,
 water-logged wetland margins (generally consisting of sand or mud), in the
 shallow water on the edges of the water bodies or, on low, predominately
 dense vegetation such as saltmarsh. The substrate on which the bird was
 roosting (‘wet ground’, ‘water’ or ‘vegetation’) was noted during filming.
 Microclimate measurements of different substrates, including substrate
 (upward) radiation, downward (solar) radiation or wind speed at the height
 and location of the bird roosting, were unable to be recorded to provide
 detailed quantifications of subtrate microclimate. Gaining operative or
 microclimate measurment would cause disturbance to the bird, precluding
 behavioural observations. As such, we took measurements of ambient
 temperature and wind speed at the (nearby) location of the observer (5 –
 580m). Our findings may therefore not be able to used for detection of
 exact thermal limits, but instead indicate the overall relationship
 between temperature, body posture and microhabitat. We took temperature
 and wind speed  parameters at 5-min intervals during each video bout using
 a Kestrel 3000 pocket weather meter (Nielsen-Kellerman Company, Boothwyn,
 PA, USA), and then averaging the values for each bout. Ambient temperature
 and wind speed has been shown in previous studies to effect the use of
 thermoregulatory behaviours (Anderson & Williams 2010, Javůrková
 et al. 2011, Yorzinski et al. 2018). The complete filming
 parameter details are described in Ryeland et al.<i>
 </i>(2017). Filming bouts were a maximum duration of 25 min (17.28 ±
 7.60; mean ± SD) with a minimum of six individuals of each species
 observed for each of six ambient temperature brackets (11 – 15, 16 – 20,
 21 – 25, 26 – 30, 31 – 35 and ≥ 36 C). We included any bird that was
 considered to be ‘roosting’, classified by a condition of being immobile
 in standing (on one or both legs) or sitting position for &gt; 120 s,
 regardless of what micohabitat the bird was in. The posture of the bird,
 sitting or standing, was recorded across each filming season, recording
 changes in postures throughout this period. If the bird was roosting in
 dense vegetation, or other areas where it could not easily be observed
 (e.g. behind a rock), the bird was excluded from the analyses as we could
 not determine roosting posture. The number and duration of video bouts
 across species is summarised in Table S2 Supplementary Material.
 Video footage was scored
 giving the duration and frequency of roosting postures using iObserver’
 app for iPad, Skware, 2011. In many cases, multiple birds were filmed
 within the same frame. Individuals from a video were choosen by first
 assigned individuals a number sequentially from left to right then top to
 bottom and then selecting individual numbers using a random number
 generator. Selecting individuals in this way limited the chance of
 overrepresentation of any specific posture (<i>i.e. </i>always
 choosing a bird that was mostly sitting). The total proportion of each
 video bout the bird was sitting was then calculated.
 Statistical
 analysis To evaluate the relationship
 of ambient temperature and substrate on the use of the sitting posture
 across species, we employed a Bayesian phylogenetic generalised linear
 mixed modelling approach, implemented through the package
 <i>MCMCglmm</i> (Hadfield 2010) in R. This method controls for
 the statistical non-independence of related species by including species
 identity as a random effect. We used the phylogeny derived from the Global
 Phylogeny of Birds website – birdtree.org (Jetz, Thomas, Joy, Hartmann and
 Mooers 2012). We first created a major rule consesus phylogeny from 2000
 downloaded putative phylogenies from the pseudo-posterior distribution of
 trees using the Hackett et al. (2008) ‘backbone’, constructed with
 Mesquite software (Maddison & Maddison 2010). Branch lengths were
 assigned using the Grafen (1989) algorithm whereby the depth of each node
 in the tree is equal to the number of daughter species derived from it.
 The constructed phylogeny is depicted in Table S3 Supplementary
 Material. We used the proportion of
 time per observation period spent roosting sitting as the response
 variable, and ambient temperature and substrate (factor levels:
 vegetation, wet ground and water) as predictors, with mean wind speed and
 body morphometrics as covariates. We also included an interaction term
 between substrate and temperature to investigate whether the reponse to
 temperature differed depending on the subtrate on which the bird was
 roosting. We included body mass and tarsus length as morphometric
 covariates as they have been previously shown to effect the use of
 thermoregulatory behaviours (Ryeland et al. 2017, 2019). Although these
 measurements are correlated (r = 0.7), as this is the most appropraite
 method to control for body size effects when interested in the effect of
 tarsus length relative to body size, as we are here (see Freckleton 2002).
 Mean tarsus size (length
 from the inner bend of the tibiotarsal articulation to the base of the
 toes) and body mass was derived from the literature for each
 species (Marchant & Higgins 1993, Higgins & Davies 1996)
 using weighted species averages for live birds (both sexes combined) with
 Victorian captured birds used in preference (where data were available).
 Tarsus is the only leg measurement routinely measured in shorebirds
 (Baker, Dettmann, Scotney, Hardy, & Drynan 1997) and as such,
 tarsus length was used as a proxy for leg length, having been shown
 previously to predict for roosting postures (Pavlovic et al. 2019, Ryeland
 et al. 2019). Migratory shorebirds generally increase in mass over the
 austral summer in preparation for migration (<i>i.e.</i>
 red-necked stint, common greenshank, sharp-tailed and curlew sandpipers),
 which could result in an increased propensity for these species to sit at
 the end of our study period when they are heavier (Pavlovic et al. 2019).
 However, we could not take morphometrics from individual birds at the time
 of recording and as such, we use an avergae body mass across seasons. We
 also capture a wide range of temperatures both prior to summer (October,
 just after return from migration) and in late summer (February, just prior
 to migration) (Figure S4 Supplementary Material), and as such, have
 captured extremes in temperaures when the birds were at their lightest and
 heaviest. As the time spent sitting is
 proportional to the length of the video bout (and restricted to values
 between 0 and 1), we modelled the response variable (proportion of time
 sitting) as a binomial response with logit-link function (family =
 “multinomial2” in <i>MCMCglmm</i>). This treats each minute of
 observation as a ‘presence’ (<i>i.e.</i> the bird was sitting)
 or ‘absence’ (<i>i.e.</i> the bird was standing) within a
 fixed number of Bernoulli trials (total minutes of observation). We
 standardised mean ambient temperaure and mean wind speed, and included
 ambient temperature with a polynomial term to reflect previous observed
 patterns of the use of the sitting posture in the mid-range temperatures
 (Ferns 1992). Video bout identity was included as a random effect in the
 model, as multiple birds were observed from particular filming sessions
 (see Ryeland et al. 2017) and the possibility exists that individuals in
 close proximity during video bouts may have influenced each other's
 behaviour (Beauchamp 2007, 2008). For the phylogenetic GLMM we
 ran the model with 401,000 iterations with a burn-in of 1,000 and thinning
 interval of 400, using uninformative priors. A Bayesian posterior
 distribution was generated from a subsequent effect sample size of 1,000,
 with convergence checked by visual inspection of plots of parameter
 estimates. This model produced a Bayesian posterior mean estimate, with
 95% credibility intervals for the distribution of estimates, and a
 probability estimate (pMCMC) which is two times the probability that the
 posterior mean is less than or greater than zero (whichever is the
 smaller). All R code used in analyses and the raw data are presented in
 Table S2 Supplementary Material.