Thermoregulatory behaviour and microhabitat use by Dune Larks in the Namib Sand Sea

Abstract

The thermoregulation of small birds that inhabit desert environments, can provide insights into the use of behaviour patterns and microhabitats for daily survival. The only bird species endemic to Namibia, the Dune Lark (Calendulauda erythrochlamys), is non-drinking, ground foraging, and range- restricted to the Namib Sand Sea, one of the most hyper-arid areas on earth.

This study aimed to identify the air and sand surface temperature (Tair and Tsand) thresholds associated with Dune Lark daily behaviour and microhabitat use. Copper operative temperature models recorded temperature in the sun and shade throughout the study period and continuous ±20 min behavioural observations of wild, free-ranging Dune Larks were recorded in the summer of 2019–2020. The relationship between maximum temperature and mass change was investigated using Dune Larks habituated to stand on scales for a food reward. Operative temperatures were assessed in sun and shade microhabitats and used to calculate expected evaporative water loss. Dune Lark future persistence in the area was evaluated using these estimates, local historical weather records, and the current and future dune vegetation distributions of MaxEnt models.

All investigated Dune Lark behaviours were strongly linked to Tsand. As Tsand increased, so did the observed proportions of time spent occupying shade [Shade50sand = 41 °C (the Tsand for 50% shade occupation)], perching off the ground (OffG10sand = 49.9 °C), wing drooping (Wing50sand = 58.9 °C), and resting (Rest50sand = 58.8 °C). The time spent foraging decreased with increasing Tsand (For50sand = 24.3 °C). Surprisingly, Tair was only significantly related to wing drooping behaviour (Wing50air = 35.3 °C). In addition, the absolute morning body mass of the Dune Larks was significantly negatively related to the maximum air temperature of the previous day.

Dune Larks were found to rely on shaded microhabitats provided by dune vegetation (mainly Stipagrostis sabulicola) for more than 75% of the time between 13:00–16:00. From 10:00–15:00, operative temperatures in the sun were 8.6 °C higher than those in the shade on average days and they were 10.7 °C higher on days with maximum air temperatures >35 °C (hot days). If Dune Larks remained in the sun, they would exceed estimated lethal dehydration tolerance limits, even on average days. Combined, these results confirm that Dune Larks are critically reliant on shade from dune vegetation under current temperatures.

Air temperatures in southwestern Africa are expected to increase by 2–4 °C by 2100. This study indicates that Dune Larks are likely to tolerate a 2 °C increase on average days with current habitat- use patterns. However, on hot days, even in full shade, they would be exposed to lethal levels of dehydration. Under a 4 °C increase, Dune Larks in full shade would reach near-lethal dehydration levels even on average days. Therefore, despite the mitigating effects of shaded microhabitats (which are expected to decrease), Dune Larks are unlikely to persist in the central northern Namib Sand Sea if temperatures rise, without resorting to distribution shifts, local migration, or local extinction.