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. Moreover, the determinants of environmental productivity cycles may differ among tropical environments. 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 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 and 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 identified peak periods of specific food availability and peak breeding periods of different species. Two clear breeding peaks occur in Choma – shortly before, and after the onset of the rains, creating three breeding clusters within the bird community assemblage. We are conducting a trait-based analyses to identify the specific traits that link each species to a cluster and determine whether this pattern is generalisable across bird communities. Thanks to Matt Lobenhofer (MSc CB 2022/3) 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. Although, multiple smaller peaks occur after the rains.

In 2024, MSc student Yinka Abayomi combined these long-term bird breeding datasets with more recent data collected by colleagues in the Fitz (See AfricanHoneyguides.com) for a select group of species identified from our previous multispecies analyses 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. In 2024, we conducted additional field work in Choma to understand the determinants of pre-rain leaf flush which coincides with large bird breeding and invertebrate abundance peaks. We now know that vegetation green-up before the rain is associated with a fourfold increase in invertebrate abundance compared to vegetation green-up after the rains.

In collaboration with Dr Felicity Newell at Louisiana State University working in the cloud forest of northern Peru, we are pooling year-round invertebrate sampling data from different tropical environments to identify the key determinants of invertebrate abundance cycles among tropical environments. Insights from these analyses will allow us to understand whether the timing of breeding in birds is determined by the main environmental driver of seasonal invertebrate abundance rather than rainfall seasonality.

We are also writing up results from analyses of data from immune assays from samples collected in Choma, Zambia and Jos, Nigeria to test how the immune function of birds varies across transitions from wet to dry season and vice versa. This will allow us to unravel the environmental components of rain-driven seasonal transitions that influence immune function and how factors other than the onset of the wet season or food availability influence breeding decisions.

These projects provide an exciting opportunity to disentangle components of seasonal environmental conditions that drive timing of bird breeding in Afrotropical ecosystems. Achieving this fundamental objective will help us detect and predict impacts of rapidly changing environmental conditions in Africa and other understudied biodiverse environments .

Activities in 2024

  • Chima Nwaogu completed work with Prof. Barbara Helm and Dr Crinan Jarrett on a tri-trophic phenology project, investigating differences in the timing of breeding, moult, insect abundance and plant fruiting along a latitudinal gradient.
  • Chima Nwaogu spent a month with Prof. Irene Tieleman at the University of Groningen. He completed analysis of immune function data from Choma, Zambia and Jos, Nigeria to test how the immune function of nestling and adult birds vary across seasonal transitions from wet to dry season and vice versa.
  • UCT and Groningen Nuffic-NRF joint PhD student, Rebecca Muller continued to work on over 100,000 nest record cards held in the Niven Library assessing the impact of climate change on breeding seasonality in Afrotropical birds. Rebecca spent three months working with Prof. Irene Tieleman in Groningen.
  • MSc student Yinka Abayomi is investigating determinants of pre-rain green-up and its association with insect abundance and bird breeding seasonality in the Afrotropics. He conducted fieldwork in Choma in August and September 2024.
  • Matthew Lobenhofer is writing up results from his MSc CB thesis, assessing the association between invertebrate abundance and bird breeding seasonality using our year-round invertebrate sampling data and long-term bird breeding data from Major John Colebrook-Robjent’s egg collection records.

Highlights

  • Chima attended the launching workshop of the Max Planck-UCT Centre for Behaviour and Co-evolution in Seewiesen, Germany in June 2024.
  • Chima continues to receive funding for his Junior Research Fellowship which was extended for two years until October 2025; we are grateful to the. Carnegie Developing Emerging Academic Leaders Programme for their support.
  • The Max Planck UCT Centre for Behaviour and Co-evolution will provide research funding and an additional two years of support for Chima beyond the Carnegie Developing Emerging Academic Leaders Programme Junior Research Fellowship.
  • Chima gave a presentation at the Trends in Tropical Ecology and Evolution Conference in Vila Do Conde Porto, Portugal.
  • A paper was published in Ecology and Evolution from collaborative work on the timing of breeding, moult, insect abundance and plant fruiting along a latitudinal gradient, with Prof. Barbara Helm and Dr Crinan Jarrett.

Key co-supporters
Carnegie Developing Emerging Academic Leaders Programme; Max Planck UCT Centre for Behaviour and Coevolution; British Ecological Society; British Ornithological Union.

Research team 2024
Team leaders and collaborators:
Dr Chima Nwaogu (FIAO, UCT)
Prof. Claire Spottiswoode (FIAO UCT / U. Cambridge)
Dr Gabriel Jamie (U. Cambridge / FIAO, UCT)
Dr Susan Cunningham (FIAO, UCT)
A/Prof. Arjun Amar (FIAO, UCT)
Prof. Irene Tieleman (U. Groningen)
Prof. Barbara Helm (Swiss Ornithological Institute)
Dr Felicity Newell (Louisiana State University)

Students:
Rebecca Muller (PhD, UCT), Yinka Abayomi (MSc, UCT).