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 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 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 assemblage. We are currently 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 CB MSc student Matt Lobenhofer and our resident colleagues in Choma who maintained our year-round invertebrate sampling, we now know that invertebrate abundance also peaks before the onset of the rains rather than after. Although, multiple smaller peaks occur after the rains.

In 2023, MSc student Yinka Abayomi combined these long-term bird breeding datasets with more recent data collected by colleagues in the Fitz (see https://www.african cuckoos.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 collaboration with Dr Felicity Newell (U.Bern and U.Florida), and working in the cloud forest of northern Peru, we are pooling together 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 rainfall seasonality.

We are also analysing data from immune assays from samples collected in Choma, Zambia and Jos, Nigeria 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 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 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 2023

  • Unfortunately, Chima is still unable to return to Cape Town due to delays in issuing an appeal decision on his Critical Skills visa application and further delays in submitting his Research Visitor’s visa application, but he has continued to work remotely, conducting fieldwork in Choma and visiting collaborators in other institutions.
  • Chima spent six months as a Schifferli Fellow working at the Swiss Ornithology Institute in Sempach, Switzerland. He continued working on the Choma egg collection data, while collaborating with Prof. Barbara Helm on a tri-trophic phenology project, investigating differences in the timing of breeding, moult, insect abundance and plant fruiting along a latitudinal gradient.
  • Chima spent three months in the Netherlands visiting Prof. Irene Tieleman at the University of Groningen. He analysed 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.
  • Matt Lobenhofer successfully completed 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 Major John Colebrook-Robjent’s egg collection records.
  • Matt returned to Cape Town in Nov 2023 to train Yinka Abayomi on insect identification and to complete the processing of the remaining invertebrate collection for Yinka’s MSc thesis.
  • 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.
  • 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 Professor Irene Tieleman in Groningen

Highlights

  • Chima’s Junior Research Fellowship was extended for two years pending a successful visa application; we are grateful to the Carnegie Developing Emerging Academic Leaders Programme for their continuing support.
  • Claire Spottiswoode secured support for the establishment of the Max Planck centre at the Fitz which will provide research funding and additional two years of funding for Chima beyond the Carnegie Developing Emerging Academic Leaders Programme Junior Research Fellowship.
  • Chima gave a keynote oral presentation at the 14th European Ornithologists Union Congress in Lund, Sweden. .

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

Research team 2023
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)

Students: Rebecca Muller (PhD, UCT), Matt Lobenhofer (CB MSc, UCT).