Current distribution and population size of the Blue Swallow Hirundo atrocaerulea in the southern Tanzanian highlands

Abstract

Two surveys of Blue Swallows were conducted in the southern Tanzanian highland grasslands in order to determine the habitat preferences and estimate the size of this subpopulation. During the 2008/09 and 2012 surveys, a total distance of 3 635 km was travelled in search of Blue Swallows (at an altitude of above 1 400 m above sea level). Blue Swallows showed no preference for any altitude range above 1 400 m. The Blue Swallows showed a preference for natural pastures and for rural villages interspersed with crops and natural pasture. The buildings in the rural villages provide the nesting sites and, in close proximity, the crops, natural pastures, livestock and their dung possibly provide the food source for the aerial arthropods on which the Blue Swallows feed. In total, 151 Blue Swallows were recorded consisting of 62 males, 68 females and 21 unsexed individuals at 62 localities. These data along with environmental variables were used to construct an ecological niche model for the Blue Swallow. It was estimated that a mean of 12 791 km² of suitable habitat was available for Blue Swallows. The 151 Blue Swallows recorded within the maximum perpendicular distance of 262 m on either side of the survey route resulted in a subpopulation estimate of 1 014 (338 to 507 pairs) Blue Swallows in the southern Tanzanian highland grasslands (12 791 km²). Mean densities were 0.023 to 0.035 pairs km⁻². A density of 0.13 pairs km⁻² recorded on a cattle farm consisting primarily of grasslands and wetlands was similar to the density of pairs found on sites in South Africa and Swaziland of comparable size and with matching characteristics to this site in Tanzania. Based on this new information the global Blue Swallow population estimate is updated to between 1 169 and 1 338 pairs.

Keywords: altitude, Blue Swallow, distribution, habitat, population, swallows