A growth cost of begging in captive canary chicks

Abstract

Nestling birds solicit food from adults by using begging displays that appear paradoxically costly and wasteful. Theoretical work suggests that the evolution of such exuberant offspring behavior reflects parent-offspring conflict over the supply of parental investment. Originally, extravagant begging was seen as a means of psychological trickery by which offspring could wheedle additional resources from resistant parents. Subsequently, costly begging came to be viewed as the hallmark of resolved parent-offspring conflict, serving either to prevent escalated scramble competition or to enforce honest signaling. However, the theoretical assumption of costly solicitation has been called into question by the low level of energy expenditure measured empirically during begging. This finding has prompted new theoretical work that shows that begging can be cost-free and yet still resolve parent-offspring conflict. Here, I report that begging is more costly than recent work suggests. My experimental evidence from captive canaries demonstrates a marginal cost of begging through impaired growth. Furthermore, I argue that previous studies of energy expenditure during solicitation do not measure the cost of begging, as defined theoretically. More generally, my results may account for the evolution of nestling growth rates, as well as the observation that begging is typically most flamboyant in older offspring.

Figure 1

The mass gained daily by unmanipulated canary chicks during the nestling period. Filled circles indicate the days on which the cost of begging experiment was subsequently performed. Means with standard error bars are shown.

Figure 2

The marginal effect of excessive begging on mass loss due to energetic expenditure during the experiment (a) and mass gain (after controlling for start mass) during the subsequent 24 h (b), both in relation to chick age. Regression lines are shown.

Figure 3

The effect of increased postural begging intensity on mass loss due to energetic expenditure during the experiment.

Figure 4

The interaction between chick age and begging treatment on the number of fecal sacs produced by nestlings during the experiment. Means with standard error bars are shown.

Figure 5

The average proportion of the daily energy budget spent on begging by nestling house wrens in relation to the proportion of their daily energy budget spent on growth. Data were calculated from raw data presented in Table 3 in ref. .