Abstract:
Rapid anthropogenic climate change potentially severely reduces avian breeding
success. While the consequences of high temperatures and drought are reasonably
well-studied within single breeding seasons, their impacts over decadal time scales
are less clear. We assessed the effects of air temperature (Tair) and drought on the
breeding output of southern yellow-billed hornbills (Tockus leucomelas; hornbills) in the
Kalahari Desert over a decade (2008–2019). We aimed to document trends in breeding
performance in an arid-zone bird during a time of rapid global warming and identify
potential drivers of variation in breeding performance. The breeding output of our study
population collapsed during the monitoring period. Comparing the first three seasons
(2008–2011) of monitoring to the last three seasons (2016–2019), the mean percentage
of nest boxes that were occupied declined from 52% to 12%, nest success from 58%
to 17%, and mean fledglings produced per breeding attempt from 1.1 to 0.4. Breeding
output was negatively correlated with increasing days on which Tmax (mean maximum
daily Tair) exceeded the threshold Tair at which male hornbills show a 50% likelihood
of engaging in heat dissipation behavior [i.e., panting (Tthresh; Tair = 34.5◦C)] and the
occurrence of drought within the breeding season, as well as later dates for entry into
the nest cavity (i.e., nest initiation) and fewer days post-hatch, spent incarcerated in
the nest by the female parent. The apparent effects of high Tair were present even in
non-drought years; of the 115 breeding attempts that were recorded, all 18 attempts
that had ≥ 72% days during the attempt on which Tmax > Tthresh failed (equivalent
to Tmax during the attempt ≥ 35.7◦C). This suggests that global warming was likely
the primary driver of the recent, rapid breeding success collapse. Based on current
warming trends, the Tmax threshold of 35.7◦C, above which no successful breeding
attempts were recorded, will be exceeded during the entire hornbill breeding season
by approximately 2027 at our study site. Therefore, our findings support the prediction
that climate change may drive rapid declines and cause local extinctions despite the
absence of direct lethal effects of extreme heat events.
