7 April, 2021
Desert at sunrise in Egypt’s Sinai mountains. Credit: Nico Arcilla
High mountain bird communities tend to be poorly studied due to their relative inaccessibility, but research shows they are vulnerable to declines and extirpations driven by global warming and habitat change, even in protected areas. Recent studies also show that deserts are warming and drying rapidly in response to climate change, which is accelerated at higher elevations, increasing the pressure on birds and other wildlife already living near the edge of their physiological limits.
In light of these novel threats to birds in montane deserts, providing accurate information about their populations is essential to understand their responses to environmental change and develop science-based conservation plans. But what is the best way to use limited resources to get this critical information? To answer this question, we compared two sampling methods to estimate densities for resident desert bird species inhabiting Egypt’s Sinai mountains.
Map showing our study sites in vegetated (green points) and unvegetated (black points) desert wadis where surveys were carried out within the high mountains of South Sinai, Egypt. Credit: Alaaeldin Soultan
Birds of a feather?
We conducted surveys in the Sinai desert for birds including the desert lark (Ammomanes deserti) and white-crowned wheatear (Oenanthe leucopyga). Like many kinds of desert birds, particularly in the Middle East, these two species remain relatively poorly studied with few studies focused on their unique ecology. While both species are year-round residents found in the same habitats, they vary in their detectability. Survey methods must therefore address differences in species’ plumage and behavior as well as habitat, all of which may influence their detectability.
Among many other adaptations to desert living, desert larks appear to be able to survive far from water, and their cryptic plumage matches desert landscapes so well that they can be difficult to detect even at close range. White-crowned wheatears, on the other hand, have striking black plumage that contrasts with the surrounding landscape. In addition, some display curiosity and tameness, even insofar as approaching human observers, traits that may be related to their being revered in Bedouin traditions.
A desert lark, whose cryptic plumage blends perfectly with desert landscapes. Credit: Alaaeldin Soultan
Comparing survey methods for different species in different habitats
Line transects using the distance sampling method and fixed-width strip transects are both standard bird survey methods used in a variety of habitats. We used both approaches to conduct bird surveys in wadis, or dry river valleys that receive ephemeral water flow during the rainy season, in both vegetated and unvegetated desert. We defined vegetated desert as having either patchy wild or cultivated herbaceous and woody plants, and unvegetated desert as devoid of wild or cultivated vegetation.
We conducted surveys after dawn between 06:00 to 09:00 and again before dusk between 16:00 to 18:00. At each site, we conducted two transects at a fixed length of 3000m, for a total of 22 transects, including 10 in unvegetated and 12 in vegetated habitats. We calculated the differences in detectability using distance software and incorporated this calculation to estimate species densities. We then used several statistical approaches to test for any significant differences between the means of the two field methods and compared both methods in terms of their reliability, precision, effort, and costs.
A white-crowned wheatear, whose striking plumage contrasts strongly with desert landscapes. Credit: Alaaeldin Soultan
The results are in!
While both methods provided reliable density estimates given sufficient detections of target species, strip transects exhibited more flexibility overall for estimating cryptic and rare species, which comprise a large proportion of this and other montane desert bird communities. Arid regions and desert habitats host many species with small population sizes, which limits the application of distance sampling in these regions.
Strip transects also entail lower effort and costs, an important consideration given research funding constraints. We therefore recommend strip transects over distance sampling for estimating bird densities in this and other arid montane regions.
A Bedouin guide leads a bird survey in a vegetated wadi (dry river valley) in Egypt’s Sinai mountains. Credit: Nico Arcilla
We suggest that future studies comparing bird sampling and population estimation approaches for montane desert birds take into account additional species, habitats, food resources, and seasonality to continue to advance our knowledge of quantitative methods for studying the ecology and population responses of these unique species.
More information: Mohamed Kadry, Nico Arcilla, Sandra Goded, Alaaeldin Soultan. 2021. Estimating bird densities in montane deserts: A methodological comparison in South Sinai, Egypt. Journal of Arid Environments189, June 2021, 104477