Demonstrated negative effects of increased temperatures on avian reproductive success suggest a mechanism by which climate change may impact species persistence. High temperatures can result in reduced parental care and reduced nestling condition in passerines with dependent young, resulting in lowered fledging success and population recruitment. We examined provisioning rate and nestling condition in a South African mountain endemic, the Cape Rockjumper Chaetops frenatus, whose population declines correlate with warming habitat. Our aim was to determine whether Rockjumper reproductive success could be affected by high air temperatures. We set up video cameras on nests at three nestling age classes (≤ 7 days old; 8–12 days old; ≥ 13 days old) for 8 hours on 37 separate days. We successfully collected full-day footage on 25 of the 37 days (4 days with predation, 8 with equipment failure). Nestlings were weighed at the beginning and end of each film day, barring the 4 days with mid-day predation (N = 65 nestling measures from 33 of the 37 days). Average mass gain across all nestlings per nest was positively correlated with provisioning rate (0.78 g provisions^-1 hr^-1, CI: 0.26 – 1.30), and provisioning rate decreased at increasing temperatures (-0.08 provisions hr^-1 °C^-1, CI: -0.15 – -0.01). Daily change in mass of individual nestlings was negatively correlated with air temperatures above a significant temperature threshold (22.4 °C; -0.30 g °C^-1, CI: -0.40 – -0.19).  This suggests nestling energy requirements were not being met on higher temperature days –– perhaps because nestling energy and water demands for thermoregulation are elevated and provisioning rate is not correspondingly maintained or increased. These results suggest that higher temperatures negatively affect nestling mass gain. While in our study this did not directly affect fledging rates, it may affect post-fledging survival.  

Table S1  Raw data for body mass (M_b; g) collected before and after film setup (~8:45 – 17:15 SAST) of nestling rockjumpers at 14 nests on 33 days (N=65 nestling measures). Data collected for each day includes: territory designation, date, month, year, maximum daily temperature (“tmax”), nestling age class (≤ 7 days old; 8–12 days old; ≥ 13 days old), brood size (“nestlings”), group size (“adults”), nestling change in M_b, nestling starting M_b, total length of time between measuring (min), percent daily change in mass per nestling (“Eqn2”; 100 ((M_b2 - M_b1)/M_b1) / ((t₂ - t₁)/480)), and daily change in mass per nestling (“Eqn1”; M_b2 - M_b1) / ((t₂ - t₁)/480): t₂-t₁ is the number of minutes between morning (t₁ @ 9:00) and evening (t₂ @ 17:00), M_b1 is initial morning M_b, and M_b2 is evening M_b.

Table S2 Raw data for video footage collected from 25 days of filming 13 nests of rockjumpers between ~9:00 – 17:00 SAST. Data collected for each day includes: territory designation, total length of video (min), date, month, year, maximum daily temperature (“tmax”), nestling age class (≤ 7 days old; 8–12 days old; ≥ 13 days old), brood size (“nestlings”), group size (“adults”), proportion of visits with adult panting (panting per hour; “dissipation”), rate of adult provisioning (visits per hour; “provisioning”), time adults spent in the nest (“nest_min”), and average mass of nestlings from start to end of film time (“avg mb”).

Table S3 Raw data for male proportion of nest behaviours collected from 32 days of at 13 nests rockjumpers between ~9:00 – 12:00 SAST. Data collected for each day includes: male proportion of provisioning visits (“m.f.provisioning”), male proportion of time spent brooding (“m.f.time”), territory designation, date, month, year, group size (“adults”), and age class (≤ 7 days old; 8–12 days old; ≥ 13 days old).