Breeding seasons are considered the most important period of the annual cycle of birds, but we still lack a full understanding of why birds breed when they do. Answering this long-standing question in life history research is crucial to understanding how climate change will impact bird populations, and how those effects can be mitigated.

Our current understanding of life-history evolution and phenology is heavily biased towards the north-temperate zone, where breeding seasonality is tightly correlated with temperature and photoperiod. This has led to the notion that breeding is prioritized over other annual cycle events and that food availability for nestlings is the main determinant of breeding seasonality in birds. However, in the tropical and south-temperate zones, the link between breeding phenology and food abundance is less clear-cut. Here, we might expect other patterns of environmental variability to be more important for the timing of birds’ annual cycles. This is because where annual reproductive output is low or unpredictable, birds should prioritise investing in processes promoting self-maintenance and survival (such as moult and immunity) rather than necessarily timing breeding to coincide with periods of peak food abundance for nestlings and juveniles. This alternative hypothesis remains untested to explain both the adaptive fine-tuning of timing of breeding according to environmental conditions within species, and the striking and unexplained differences among species.

Furthermore, rainfall is considered the key determinant of food availability in seasonally arid tropical environments, but it remains unclear how a single wet season influences food availability across the year for different breeding communities. 

In 2021 we started a research project to address these knowledge gaps at our long-term study site in Choma, Zambia. Choma is a seasonally arid environment with distinct wet and dry seasons and a species-rich bird community including species breeding within or across seasons. By combining year-round field sampling of invertebrates and grass seeds with analyses of long-term bird breeding data from the work of Major John Colebrook-Robjent from 1970–2008, we have now identified peak periods of specific food availability and peak breeding periods of different species. Thanks to the hard work of MSc. student Matt Lobenhofer and our resident colleagues in Choma who maintained our year-round invertebrate sampling, we now know that invertebrate abundance peaks before the onset of the rains rather than after. 

In 2023, a new MSc student will combine these long-term bird breeding datasets with more recent data collected by colleagues in the Fitz (see https://www.african cuckoos.com/) to test how environmental conditions before the rains such as leaf flush, and after the rains, such as increased grass seed availability, influence the timing of breeding in different breeding communities, using species identified from our previous multispecies analyses. 

We are also analysing data from immune assays to test how the immune function of birds varies across seasonal transitions from wet to dry season and vice versa. This will allow us to unravel how factors other than the onset of the wet season or food availability, influence breeding decisions. 

Additional work by PhD student Rebecca Muller used long-term breeding datasets available from the South African Nest Record Card scheme, holding breeding records from the 1910s to the 1990s, to assess the impact of climate change on breeding seasonality of southern African birds.  

This project provides an exciting opportunity to disentangle the components of seasonal environmental conditions that drive avian timing of breeding in the Afrotropical ecosystem. Achieving this fundamental objective will help us detect and predict early warning signs of rapidly changing environmental conditions in Africa and other understudied biodiverse environments. 

Activities in 2022

  • UCT and Groningen Nuffic-NRF joint PhD student, Rebecca Muller continues work on over 100,000 nest record cards held in the Niven Library for her PhD assessing the impact of climate change on breeding seasonality in Afrotropical birds. This year she spent three months in Groningen working with Professor Irene Tieleman.
  • Unfortunately, Chima is still unable to return to Cape Town in person due to further delays in issuing an appeal decision on his Critical Skills visa application, but he has continued to work remotely, conducting fieldwork in Choma and visiting collaborators in other institutions. 
  • Chima had two short field visits to Choma in 2022: a visit in mid-April to end-May, sampling blood for immune assays and supervising ongoing insect sampling by colleagues Collins Moya and Silky Hamama, and in September to get Matt Lobenhofer started on insect processing and habitat assessment for his master’s project. 
  • Matt Lobenhofer successfully carried out his CB MSc project, assessing the association between invertebrate abundance and bird breeding seasonality using our year-round invertebrate sampling data and long-term bird breeding data from the egg collection records of Major John Colebrook-Robjent.
  • Mastercard Foundation funded MSc student, Yinka Abayomi Omotayo, will register for his MSc in early 2023 to investigate determinants of pre-rain green-up and its association with bird breeding seasonality in the Afrotropics.

Highlights

  • Chima’s Junior Research Fellowship was extended to October 2025; we are grateful to the Carnegie Developing Emerging Academic Leaders Programme for their continuing support.
  • Chima visited Dr Martijn van de Pol at James Cook University, Townsville, Australia in August 2022 to set up a data analysis programme in R for our long-term bird breeding data from Zambia. He learnt how to use GitHub with R for documenting data analyses, version control, and collaborative analyses consistent with open science practices.
  • We hosted a visit from Professor Barbara and Dr Christoph Meier from the Swiss Ornithology Institute to our field site in Choma, Zambia.
  • Chima gave a plenary talk at the 15th Pan African Ornithology Congress in Victoria Falls, Zimbabwe.  
  • Chima was awarded the Schifferli Fellowship, which will allow him to work on his research in collaboration with researchers at the Swiss Ornithology Institute during 2023.

Key co-supporters
Carnegie Developing Emerging Academic Leaders Programme; DSI-NRF CoE grant; British Ecological Society; British Ornithological Union.

Research team 2022
Dr Chima Nwaogu (FIAO, UCT)
Prof. Claire Spottiswoode (FIAO, UCT/U. Cambridge)
Dr Gabriel Jamie (U. Cambridge/FIAO, UCT)
A/Prof. Susan Cunningham (FIAO, UCT)
A/Prof. Arjun Amar (FIAO, UCT)
Prof. Irene Tieleman (U. Groningen)
Prof. Barbara Helm (Swiss Ornithological Institute)

Students: Rebecca Muller (PhD, UCT), Matt Lobenhofer (CB MSc, UCT).fte Simanunki, and many others.