Conservation in a changing world: assessing the conservation status of an agriculturally adapted species, the blue crane: Chapter 4: Trackers

<p dir="ltr">Chapter 4 from: Craig CA. 2024. <i>Conservation in a Changing World: Assessing the Conservation Status of an Agriculturally Adapted Species, the Blue Crane</i>. PhD. University of Cape Town.</p><p dir="ltr">Tracking data provides insights into bird movements and survival. One of the primary threats to Blue Cranes Anthropoides paradiseus is collision with powerlines, which is intrinsically linked to crane movement patterns. It is thus important to understand how and when Blue Cranes move. Between 2016 and 2021, 31 Blue Crane adults, 25 fledglings and 3 juveniles were fitted with GPS-GSM and Iridium satellite trackers in the Overberg, Swartland and Karoo. Of the fully-grown birds (juveniles and adults), fitted in the Western Cape only, 29 had sufficient data for analysis i.e., more than 5 months data (Swartland = 8, Overberg = 21). Mean daily distance travelled did not differ between regions: Swartland: 9.5 km (SD 11.7), Overberg: 7.5 km (SD 9.8); but non-breeding cranes moved significantly farther per day, 9.5 km (SD 13.3) than breeding cranes, 5.8 km (SD 5.4). Using autocorrelated kernel density home range analysis, I demonstrate that Swartland cranes have home ranges 2.7 times larger than Overberg cranes. Home ranges of non-breeding cranes averaged 13 times larger than breeding cranes. Four different movement types were displayed: 28% resident birds (semi-variance < 20 km2), 28% regional residents (semi-variance 20–100 km2), 34% intra-regional migrants that remained within the general region they were captured (semi-variance 100–1500 km2) and 10% inter-regional migrants (semi-variance > 4000 km2). All three inter-regional migrants were captured in the Overberg; two moved to the Eastern Cape and one to the Swartland. Non-breeding cranes intersect powerlines at a significantly higher rate, 1.3 intersections per day (SE 1.4), than breeding cranes 0.64 intersections per day (SE 1.3). Using a Known Fate Mark Recapture Model, I estimate annual survival for non-breeding birds to be 0.85 (95% CI 0.61–0.95, n = 13), and 0.95 (95% CI 0.71–0.99, n = 12) for breeding birds. Tracker failure rates were very high for fledglings; only 11 sent a limited number of fixes over a short duration (less than a year), so I just describe the movements for these individuals, rather than doing detailed analyses. From these limited data, in the year post fledgling, most fledglings stay within 30 km of their natal site in the Karoo and 15 km in the Western Cape, but one Overberg chick moved 80 km east two months after fledging.</p>