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Moving Territories Inflated Blue Waxbill Density Estimates Across a Long Breeding-Season Survey

Blue waxbill standing on the ground
Research · Ecology

A study published in 2026 found that the way surveys are spread across the breeding season can strongly affect estimated population density in the blue waxbill (Uraeginthus angolensis). In a 400-hectare area of acacia savanna in northern Namibia, estimates from the separate survey periods remained close to 11–12 breeding pairs per 100 hectares, but the estimate rose sharply when records from several months were combined.

The study was carried out at the University of Namibia’s Ogongo campus in the Omusati Region. The area contained a mixture of natural and human-modified acacia savanna, including built-up areas, cultivated ground, water bodies, roads and fences. Surveys were conducted in 2020 using territory mapping, in which repeated observations are plotted to identify occupied breeding territories.

Separate survey periods gave stable density estimates

The researcher divided the breeding-season surveys into three periods: February–March, April and May–June. Estimated density was 12.0 pairs per 100 hectares in February–March, 11.3 in April and 11.8 in May–June. These similar estimates suggested a fairly stable number of breeding pairs within each part of the season.

Combining the periods inflated the estimate to 20.5 pairs per 100 hectares

A different picture emerged when observations from all three periods were merged. The combined map produced an estimate of 82 territories, equivalent to 20.5 pairs per 100 hectares. The study attributes this increase to shifts in territory locations during the breeding season. Birds beginning second or third nesting attempts could occupy a different area and therefore appear as an additional territory when records from a long period were treated as one survey.

Most territories were located close to features such as water reservoirs, canals, seasonally flooded depressions, sand pits and fences. The study also compared its results with previous estimates from southern Africa. Blue waxbills had been recorded at substantially higher densities in several urbanised habitats in northern Namibia and northeastern Botswana than in the Ogongo savanna.

The author concludes that territory-mapping surveys of blue waxbills should be concentrated within a shorter period, around one to two months, rather than spread across much of the breeding season. The study was published in Ecological Questions, volume 37, issue 2, in 2026.

Sources and image credit

  1. Kopij, G. (2026). The Blue Waxbill Uraeginthus angolensis, in an acacia savanna: Methodological implications in population density estimations. Ecological Questions, 37(2), 63–67. https://doi.org/10.12775/EQ.2026.019Primary peer-reviewed study published online 31 March 2026. Territory mapping in a 400-ha plot at Ogongo, northern Namibia, gave 11.3–12.0 pairs per 100 ha in separate survey periods. Table 1 reports 82 territories overall, equivalent to 20.5 pairs per 100 ha; the abstract/body prose gives 20.8, an internal inconsistency in the source.

Image: Hans Hillewaert — blue waxbill photographed near Chilanga, Zambia; Wikimedia Commons; representative image, not from the Namibian study · CC BY-SA 3.0 · Source

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