The adaptive value of parental responsiveness to nestling begging

Abstract

Despite extensive theoretical and empirical research into offspring food solicitation behaviour as a model for parent-offspring conflict and communication, the adaptive value of parental responsiveness to begging has never been tested experimentally. Game theory models, as well as empirical studies, suggest that begging conveys information on offspring state, which implies that parental investment can be better translated to fitness by responding to begging when allocating resources rather than by ignoring it. However, this assumption and its underlying mechanisms have received little or no attention. Here we show by experiments with hand-raised house sparrow (Passer domesticus) nestlings that a 'responsive parent' will do better than a hypothetical 'non-responsive' mutant (that provides similar food amounts, but irrespective of begging). This is neither because food-deprived nestlings convert food to mass more efficiently, however, nor because responsiveness reduces costly begging. Rather, responsiveness to begging is adaptive because it reduces two opposing risks: one is wasting time when returning too soon to feed already satiated nestlings and the other is repeatedly overlooking some nestlings as a result of the stochastic nature of a random, non-responsive strategy. This study provides the first experimental evidence for the adaptive value of parental responsiveness to offspring begging.

Figure 1

Time interval to the next feeding visit as a function of the pair's ‘begging posture sum’ (sum of the visit posture score for both nestlings, see §2). (a) All the visits in the RT group. (b) All the visits in the NRT group. Six increasing circle sizes represent <20, 20–40, 40–60, 60–80, 80–100 and 100<overlapping data points. Correlation values for the separate pairs' data are given in the text.

Figure 2

Comparison of the RT and NRT nestlings in respect to: (a) mass gain; (b) apparent AMC; (c) apparent MEC; (d) average begging posture; (e) the proportion of untaken food (due to feeding refusals) and (f) mass gain per food consumed (g ml⁻¹). Means with standard error bars are shown.

Figure 3

The difference in the proportion of untaken food between each NRT pair and the RT pair it was matched to, plotted against the initial difference in mass between the pairs. Dashed lines indicate no difference between NRT pairs and their respective RT pairs.

Figure 4

Controlling for the effect of errors in estimating daily food requirements on the proportion of untaken food. (a) The proportion of untaken food plotted against the deviation of the actual amount of food offered to each nestling from the food amount required according to its initial mass (see §3c for details), for RT (open circles) and NRT (filled circles) nestlings. The regression line for both treatment groups together is shown. (b) Mean±s.e. residuals from the regression line shown in (a) for RT (open bar) and NRT (filled bar) nestlings. Data for all nestlings are shown here, but we conservatively used the mean residual for each pair to compare statistically between treatment groups.