Conservation management to protect coastal ecosystems sometimes overlooks site-specific fine-scale heterogeneity. For example, while habitat loss is a known key driver of population declines in many shorebirds, these birds are also dependent on high-quality habitats to maximise energy stores. Here we describe the microhabitats provided by beach-cast wrack (washed up macroalgae and seagrasses), a resource threatened by harvesting and beach cleaning, and how shorebirds utilise these.

We measured the temperature and absolute humidity at 10 cm above three substrates (fresh wrack, aged wrack, sand) and then related bird behaviour (roosting versus foraging) to climatic and environmental data.

Freshly beach-cast wrack mostly provided cooler and less humid habitats, but warmer temperatures than aged wrack or sand in the early mornings. Microtopography created by shelter from prevailing winds and wrack depth modified these general trends.

Generally, temperature predicted where shorebirds overall and the two most common species, the double-banded plover (Charadrius bicinctus) and red-necked stint (Calidris ruficollis), were observed. During most of the day, foraging and roosting were more likely to occur on the warmer aged wrack. In the early morning, when fresh wrack provided the warmer temperatures, birds tended to roost and forage on fresh wrack.

Synthesis and applications: Beach-cast wrack created a complex mosaic of unique microclimates varying in space and time, which seemingly allowed shorebirds to minimize energy expenditure by selecting the thermally most favourable habitats for roosting and foraging. Removal of beach-cast wrack reduces habitat quality and increases energy expenditure in shorebirds and hence may contribute to declines in shorebird populations. Management of coastal ecosystems and shorebirds therefore needs to consider and maintain fine-scale environmental heterogeneity at local scales.

Microclimate Data:

A total of 24 stainless steel iButton Hygrochron temperature/humidity loggers (Maxim Integrated Products Inc.) were calibrated in an ice bath and programmed to record temperature (°C) and relative humidity (%) measurements at 10-minute intervals. Loggers were mounted to the ceiling of pre-fabricated white practice golf balls (Maxfli) to allow ventilation of the logger and to prevent impacts from direct solar radiation. The top quarter of the golf ball was covered with white duct tape to prevent sunrays from reaching the loggers and to mimic a Stevenson screen on a microscale. Because we were interested in the temperatures experienced by shorebirds and to measure air temperature rather than surface temperature, the loggers were suspended 10 cm above the substrate surface by attaching them to bamboo stakes. Loggers were randomly placed in two bands of up to twelve sensors along the sandy beach over a distance of about 150 m on three substrates (sand, fresh wrack and aged wrack) and deployed up to 6 hours on thirteen days (see Table 1 for details) between February-April 2016 (days n=13). This resulted in up to 24 samples of microclimate on various substrates. Because loggers were placed approximately equally spaced along 150 m transects, sampling of habitats was not equal because aged wrack was more common than fresh wrack.

Substrate Type and Exposure Variables:

Three substrate types were identified; aged wrack, fresh wrack and (uncovered) sand. Fresh wrack was defined as beach-cast macroalgae with no visual signs of aging, while aged wrack was defined as decomposing macroalgae that was visibly dehydrated. The depth of the wrack was measured directly below the sensor and the position of the logger classified as sheltered or exposed. Exposed positions had no wrack or sand formation that was greater than the height of the logger within a 2 m radius, while sheltered positions were within 20 cm of a wrack or sand formation that was greater than the height of the logger. Hence, each microclimate sample was designated as either sheltered or exposed. The position of the wrack or sand formation relative to that of the logger was recorded as a compass bearing (°) with a handheld compass (Silva). Hourly wind directions were recorded as a bearing from a fixed point on the beach based on visual observation of a 50 cm length of plastic flag tape attached to a bamboo stake, set 20 cm from the ground surface. Loggers classified as sheltered were cross-validated with the wind direction, and loggers that did not have protection from the wind were reclassified as exposed.

Bird Behaviour Data:

Instantaneous scan sampling was used to classify migratory shorebird behaviour for the entire Danger Point study area (i.e., the area over which the microsensors were placed) at 15 min intervals from 7.30-11.00 am on four days during May. Observations were made from a permanent bird hide located in the fore-dune, using a 20-60x spotting scope (Nikon Prostaff 5 Fieldscope 82A) to determine for each species the number of individuals roosting and foraging on each of the three substrate types. Roosting was defined as any bird participating in loafing, sleeping or preening.

Note that the time is that in South Australia and that time switched from daylight saving time to 'regular' time on Sunday, April 3, 3:00 am, 2016.