Trade-Offs Between Reproductive and Somatic Investment in Male Birds (2000)

Strategic allocation decisions, in particular trade-offs between reproductive and somatic investment, are central to life history theory. The allocation of resources to body mass maintenance is beneficial for avoiding starvation, but is costly because resources are diverted away from the current reproductive effort, and mass-dependent costs are increased. Stochastic dynamic models have been developed to investigate optimal routines of mass regulation and behaviour in male birds displaying to attract a mate. What is central to these models, and to any empirical investigation of the trade-off, is the dynamic interaction of body mass regulation and display behaviour routines. Previous experimental work has focussed on each of these factors in isolation, but never in unison. This work addressed this shortfall by examining the dynamic interaction of investment in reproduction versus somatic investment in male zebra finches (Taeniopvgia guttata), a theme discussed more fully in chapter 1. Chapter 2 provides details of software and hardware that I created in order to enable body mass and display hopping activity of males to be automatically recorded. Further, it describes software and hardware that controlled the lighting and temperature of the experimental environment and finally it discusses software I created to perform post-hoc processing of the data which were generated. The lighting controller sought to mimic the variations in light intensity associated with twilight. In chapter 3, I describe an experiment which tested the effect of naturalistic light intensity control versus 'all- or-nothing' lighting on the mass regulation and display behaviour of male zebra finches. In the experiment described in chapter 4, I presented male zebra finches with four stimulus treatments which inherently varied in their potential to be courted, and I recorded diel patterns of body mass regulation and hopping activity. The stimulus cage contained either: no stimulus bird, a female bengalese finch, a male zebra finch, or a female zebra finch. There were significant effects of treatment on mean body masses and display hopping activity, with lowest mean masses and highest display activity during the female zebra finch treatment. Mass trajectories did not differ between treatments, instead the effect of treatment was manifested as a complete shifting in the y-axis of the mass curves. Hopping activity trajectories did however vary between treatments with the female zebra finch stimulus evoking a marked peak around dawn. This apparent anomaly may be attributed to either strategic compensatory foraging aimed at guarding mass trajectory, variation in the mass-dependent costs of display behaviour, a reorganisation of time and energy budgets, or some combination of these factors. This work provided the first experimental evidence of the trade-off between reproductive and somatic investment in birds. The diel patterns of display activity exhibited by many male passerines could be due to temporal variation in either male-state or in female-state (i.e. her attractiveness and/or receptivity). These effects are naturally confounded, but by desynchronising the photoperiods of the female stimulus birds from the males it was possible to disentangle the effects of the time of presentation and time-state of the female on male mass regulation and display activity. Female zebra finches were presented to the males for a discrete three hour period at dawn or midday, and in each case females were either in a dawn or a midday time-state (making four treatments in total). The time of presentation had no effect on overall display rates, but the peaks of display activity were shifted to coincide with the time when the female was present. Despite no difference in mean hopping activity, there was a significant effect of presentation time on mean mass, which I attributed to time of day differences, due to male-state, in the costs of display. Female time-state was for the first time shown to influence display activity of males. Dawn-state females were more attractive/responsive and evoked a greater display effort, but only in the context of the males' dawn, as no effects of female time-state were apparent at midday. Arbitrary symmetry has been shown to influence female choice in birds, particularly in zebra finches. We investigated the effects of arbitrary symmetrical and asymmetrical traits of females (applied as coloured leg bands) on the dynamic interaction between body mass regulation and display activity in male zebra finches (chapter 6). We found no significant effects of leg band treatment on either overall levels of body mass and display hopping, or on their diel trajectories, all of which were highly consistent between treatments. There was however a significant and consistent effect of female identity (irrespective of leg banding) on male display hopping activity. This fact, coupled with evidence from chapters 4 &amp; 5 that the trade-off exists and can be detected by the present experimental design, points towards arbitrary symmetrical traits of female zebra finches having no effect (or a very weak effect) on male choice, unlike the situation that occurs when the sex roles are reversed.