Together with breeding, moult and migration are the greatest challenges in a bird’s annual cycle. Moulting birds suffer increased metabolic costs as well as impaired flight ability, insulation and camouflage/signalling. Birds vary greatly in how they manage these costs through changing the timing and intensity of moult, both within and between species. Globally, migrant birds are at greater risk of extinction than resident species due to the risks they face travelling across an increasingly transformed planet, and the need to have secure breeding and non-breeding areas. Understanding the strategies birds use to moult and migrate is crucial for their conservation.

Once formed, feathers are dead structures that start to degrade through mechanical abrasion, damage by UV light, and attack by ectoparasites, fungi and bacteria. Most feathers need to be replaced every year or so through a regular process of moult. The costs of moult are significant, so most birds schedule their moult to periods when they are not breeding or migrating (although there are numerous exceptions). New feathers grow from a ring of cells in the feather follicle, which limits their rate of growth to around 4-6 mm per day. Feathers that grow faster tend to be of poorer quality, providing less insulation and wearing faster than feathers grown more slowly. This largely invariant growth rate means that large birds take longer to replace a given feather than small birds, and so typically have more complex moult strategies than small birds, either greatly increasing the intensity of moult (e.g. replacing all flight feathers at once and becoming flightless for a few weeks while they grow new feathers) or staggering their moult over several years. The timing, intensity and symmetry of moult likely reflect individual health. And unlike breeding, moult is something that all birds must undergo. Thus monitoring how different birds moult, and how this changes over time, might be a way to track population health.

Migration is better studied than moult, but there is still much to learn regarding movement patterns among African birds. And given the less marked seasonality in the Southern Hemisphere, more species move in response to rainfall rather than temperature, especially in more arid areas. Our understanding of these nomadic movements is still in its infancy. Finally, we also contribute to an understanding of global shorebird movements and conservation through coastal bird surveys.

Activities in 2024

  • PhD student Rachel Dobson (Leeds) handed in her thesis at the end of 2024. She completed a draft paper describing Ludwig’s Bustard Neotis ludwigii movements based on Jessica Shaw’s tracking data, which has up to 11 years of data for some individuals. Some bustards undertake more or less regular migrations between the Nama and Succulent Karoo each year, following seasonal rains, whereas others are more sedentary. A second manuscript exploring the environmental triggers for their movements is nearing completion.
  • A paper on the moult of Tristan Albatrosses Diomedea dabbenena is almost complete, relating the extent of wing moult to age, sex and past breeding history. Peter Ryan obtained additional images of known-age Wandering Albatrosses D. exulans on Marion Island in April-May 2024, which will be compared with those of Tristan Albatrosses to improve our understanding of the complex plumage maturation sequences in these birds.
  • Following the synoptic survey of coastal birds in the Greater Bazaruto IBA/KBA in November 2023, in collaboration with African Parks (Evan Trotzuk) and the Vilanculos Coastal Wildlife Sanctuary (Christine Read and Dave Gilroy), a follow-up survey in August 2024 was conducted to assess winter shorebird numbers and explore for migrant Damara Sternula balaenarum and Saunders’s Terns S. saundersi in the Bazaruto National Park. No new tern populations were found, highlighting the importance of the sanctuary for these species in Mozambique, but a new major shorebird roost was discovered in the southwest of Vilanculos Bay. .

Highlights

  • The final paper from Oluwadunsin Adekola’s PhD was published. It described the moult of White-chinned Petrels based on the large numbers of birds killed on long-lines off South Africa. It also reported the absence of a clear impact of moult on time spent on the water by this species, contrary to the pattern found in most other albatrosses and petrels examined to date.
  • The discovery of a major shorebird roost site in the southwest of Vilanculos Bay indicates that counts made in November 2023 for the Greater Bazaruto IBA/KBA underestimate the importance of this region for shorebirds. Notable populations present in the roost include over 2 500 Bar-tailed Godwits Limosa lapponica, 1 000 Curlew Sandpipers Calidris ferruginea, 950 Tibetan Sandplovers Charadrius atrifrons and 40 Eurasian Curlews Numenius arquata, but the most surprising finding was over 1 100 Chestnut-banded Plovers Charadrius pallida, representing some 10% of the global population. Although nearly 500 Chestnut-banded Plovers were recorded during the synoptic survey of the IBA in 2023, they were almost all in the mangrove-lined bays south of the Save River. The breeding sites of these plovers is unknown.

Impact of the project
The timing and intensity of moult is thought to be related to stress in bird populations, and monitoring changes in these parameters might provide a useful measure of global change impacts. The intra-African migration project facilitated networking among research institutions across Africa. This programme helps to support the objectives of the UNEP/CMS African-Eurasian Migratory Land-birds Action Plan (AEMLAP) and the Migrant Landbird Study Group (MLSG), and meets the growing need for better understanding of the drivers of avian migratory patterns on the continent.

Key co-supporters
BirdLife International; Mohamed bin Zayed Species Conservation Fund; African Parks.

Research team 2024
Team leaders and collaborators:
Emer. Prof. Peter Ryan (FIAO, UCT)
A/Prof. Robert Thomson (FIAO, UCT)
Dr Gary Allport (BirdLife International)
Dr Alan Lee (BirdLife SA)
Dr Birgit Erni (SEEC, UCT)
Dr Samuel Temidayo Osinubi (FIAO, UCT)
Dr Jessica Shaw (NatureScot)
Prof. Les Underhill (BioSci, UCT)

Student:
Rachel Dobson (visiting PhD student, Leeds)