How climate change might influence the starvation-predation risk trade-off response

Abstract

Climate change within the UK will affect winter starvation risk because higher temperatures reduce energy budgets and are likely to increase the quality of the foraging environment. Mass regulation in birds is a consequence of the starvation-predation risk trade-off: decreasing starvation risk because of climate change should decrease mass, but this will be countered by the effects of predation risk, because high predation risk has a negative effect on mass when foraging conditions are poor and a positive effect on mass when foraging conditions are good. We tested whether mass regulation in great tits (Parus major) across the UK was related to temporal changes in starvation risk (winter temperature 1995-2005) and spatial changes in predation risk (sparrowhawk Accipiter nisus abundance). As predicted, great tits carried less mass during later, warmer, winters, demonstrating that starvation risk overall has decreased. Also, the effects of predation risk interacted with the effects of temperature (as an index of foraging conditions), so that in colder winters higher sparrowhawk abundance led to lower mass, whereas in warmer, later, winters higher sparrowhawk abundance led to higher mass. Mass regulation in a small bird species may therefore provide an index of how environmental change is affecting the foraging environment.

Figure 1.

How starvation risk and predation risk might combine to change mass in great tits as temperature increases. As the temperature increases, so less mass is put on as insurance against starvation during winter (1. solid and dotted lines main graph). But the effects of temperature interact with predation risk so that where hawk abundance is high, and winter temperatures are low (2. inset graphs: mass decreases with hawk abundance), mass is lowered (so average mass is decreased from the solid line to the dotted line). But where hawk abundance is high, and winter temperatures are also high (3. inset graphs: mass increases with hawk abundance), mass is increased (so average mass is increased from the solid line to the dotted line). For example, starvation risk (low temperature) may add +1 g and predation risk remove −0.5 g, so overall the bird is fatter because of starvation risk, but not as fat as it would be in the absence of predation risk.

Figure 2.

Residual mass of great tits during the winter with year. The mean of residuals (+1 s.e.) were plotted from a GLM of mass with month as a factor, and hawk abundance, sex, age, wing length, latitude, longitude, hours of daylight and day length as covariates. The illustrated change is significant (F_1,8 = 24.7, p = 0.001, R² = 0.73; B = −0.023 ± 0.005).

Figure 3.

The amount of mass gained or lost for great tits as abundance of sparrowhawks increases for 10 different winters 1994–1995 to 2004–2005. The model shown in table 1 was run (excluding year and the year × hawk abundance interaction) for each year of the study separately, and the parameter estimate for hawk abundance ± 1 s.e. are plotted. A reference line where hawk abundance had no effect on mass on average is plotted; points below the line show years where greater hawk abundance led to decreases in mass and above where greater hawk abundance led to increases in mass. That some points lie well above and below this line indicates that the effect of hawk abundance differs depending on year (illustrating the significant interaction of year × hawk abundance in the model in table 1).

Figure 4.

Mass gain or loss in response to higher hawk abundance plotted with mean daily temperature for the 10 winters of the study. Mass gain or loss was taken as the parameter estimates for the year × hawk abundance interaction for the model in table 1: note the values use 2004–2005 as the reference value. A dotted reference line where hawk abundance had no effect on mass on average is plotted: that points consistently lie below this line at low temperatures, and above at higher temperatures, illustrates the likely significant interaction of hawk abundance with temperature.

Figure 5.

Change in mean daily winter temperature with year.