Temperatures in Excess of Critical Thresholds Threaten Nestling Growth and Survival in A Rapidly-Warming Arid Savanna: A Study of Common Fiscals

Frequency, duration, and intensity of hot-weather events are all predicted to increase with climate warming. Despite this, mechanisms by which temperature increases affect individual fitness and drive population-level changes are poorly understood. We investigated the link between daily maximum air temperature (t max ) and breeding success of Kalahari common fiscals ( Lanius collaris ) in terms of the daily effect on nestling body-mass gain, and the cumulative effect on size and age of fledglings. High t max reduced mass gain of younger, but not older nestlings and average nestling-period t max did not affect fledgling size. Instead, the frequency with which t max exceeded critical thresholds (t crit s) significantly reduced fledging body mass (t crit  = 33°C) and tarsus length (t crit  = 37°C), as well as delaying fledging (t crit  = 35°C). Nest failure risk was 4.2% per day therefore delays reduced fledging probability. Smaller size at fledging often correlates with reduced lifetime fitness and might also underlie documented adult body-size reductions in desert birds in relation to climate warming. Temperature thresholds above which organisms incur fitness costs are probably common, as physiological responses to temperature are non-linear. Understanding the shape of the relationship between temperature and fitness has implications for our ability to predict species’ responses to climate change

Relationships between numbers of days t_max > t_crit during the nestling period and fledging parameters.

<p>Model estimates and 95% CIs for relationships between number of days on which t_max > t_crit (plotted on the x-axis) and A: fledging mass; B: fledging tarsus length; and C: age-at-fledge. The y-axis shows the effect on each fledging parameter of a single day during the nestling period on which t_max > t_crit. For example in A: a single day of t_max > 38°C will reduce fledgling body mass by 5.3 g. As number and intensity of hot days within the nestling period increases, the size of fledglings decreases (A & B) and nestlings take longer to fledge (C). White circles indicate 90% CIs include zero; grey circles indicate that 90% CIs exclude zero; black circles indicate that 95% CIs exclude zero.</p

Factors affecting nestling % daily mass change (Δm), estimates of effect sizes, standard errors (SE), and 95% confidence intervals (95% CI).

*<p>N = 38 observations of 18 broods from 14 pairs. Units for estimates of effect size are % daily body mass change (Δm).</p

Top five models for nestling % daily body mass gain (Δm).

*<p>ppn  =  provisions per nestling. Dev  =  model deviance. Global model: nestling age + ppn + t_max + nestling age* t_max + ppn* t_max + nestling age*ppn. Random term: brood identity nested within pair identity. N = 38 observations of 18 broods from 14 pairs.</p

Factors affecting total daily provisioning rate, estimates of effect sizes, standard errors (SE), and 95% confidence intervals (95% CI).

*<p>N = 28 observations of 12 broods from 12 pairs. The model was run with a Poisson error structure and log-link function. Effect size estimates are not back-transformed, therefore no units are presented.</p

Relationships between t_max on each day of the nestling period and fledging parameters.

<p>Model estimates and 95% CIs for relationships between t_max on each day of the nestling period (day 1  =  day of hatch) and A: fledging mass (g); B: fledging tarsus length (mm); and C: age-at-fledge. The y-axis shows the effect of a 1°C increase in t_max on the day of the nestling period indicated on the x-axis. For example in A: a 1°C increase in t_max on day eight of the nestling period reduced fledgling body mass by 1.2 g. White circles indicate 90% CIs include zero; grey circles indicate that 90% CIs exclude zero; black circles indicate that 95% CIs exclude zero.</p