Hot weather leads to unwanted weight loss in young Rockjumpers

Submitted by Johan on 3 September 2021.

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In this study, we examined whether population declines in the Cape Rockjumper were caused by decreased provisioning rates at higher temperatures, leading to smaller nestlings and thus lower fledging or post-fledging success, and finally lowered population recruitment. Cape Rockjumpers Chaetops frenatus, endemic to the mountains of southwest South Africa, have decreased reporting rates have been strongly correlated with areas of their habitat experiencing the greatest warming (Milne et al., 2015).

Figure 1 Male (top) and female (bottom) Cape Rockjumpers bringing food to a nest.

We used camcorders set up at Blue Hill Nature Reserve in the Western Cape province of South Africa to examine parental care at the nest across a range of daily maximum temperatures. We collected nestling mass before and after filming on each film day and recorded the number of provisions per day. In addition, these cameras allowed for novel recorded footage of prey item type, contact calls, and social interactions at the nest. While the adults were always a bit wary of the cameras on setup, they always began feeding within 10 minutes of pressing “start”.



Nestling mass gain and provisioning rate both decreased at higher temperatures, although for nestling mass gain this only occurred above a specific temperature threshold (22.4 °C) –– possibly from smaller prey items (Barras et al., 2021) or increased direct physiological costs on nestlings (Van de Ven et al., 2020) only felt above a specific temperature.

We were also able to record a few interesting and fun situations, such as these 16-day old nestlings “partially fledging”, as one of them wanders away from the nest for a bit before returning for in time for a snack.



While we did not set up our HD cameras close enough to the actual fledging date to catch the event, our infrared predator cameras were able to capture a few fledglings and show how unsteady they are straight out of the nest.



As a side analysis, we examined how the presence of a helper individual (in all instances a male) changed the group dynamics of parental care. For these groups, while the helper did share in provisioning, this only provided help to the breeding male. Females did the same amount of provisioning regardless of group size. However, both males and females will share in all parental care (besides laying the egg itself!).



Given how any factor that results in a decrease in nestling mass gain could result in lower fledge success and decreased post-fledging survival, our finding of decreased nestling mass gain at increasing temperatures may be partially responsible for decreases in Cape Rockjumper populations. As such, it is likely that with climate change we will continue to see such decreases in the future. As a mountain-endemic, Rockjumpers are unable to relocate, so identifying areas of sensitivity to climate and potential conservation continue to be an important avenue of research.

Text, photos and videos by: Krista Oswald

Read the full paper online:


 Barras, A.G., Niffenegger, C.A., Candolfi, I., Hunziker, Y.A., Arlettaz, R., 2021. Nestling diet and parental food provisioning in a declining mountain passerine reveal high sensitivity to climate change. Journal of avian biology.

Milne, R., Cunningham, S.J., Lee, A.T., Smit, B., 2015. The role of thermal physiology in recent declines of birds in a biodiversity hotspot. Conservation Physiology 3, cov048.

Van de Ven, T., McKechnie, A.E., Er, S., Cunningham, S., 2020. High temperatures are associated with substantial reductions in breeding success and offspring quality in an arid-zone bird. Oecologia 193, 225-235.