Flight muscle atrophy and predation risk in breeding birds

Abstract

Summary 1.The speed with which small birds can get airborne is critical to the effectiveness of their escape response when attacked by a predator. However, take-off ability is likely to be affected by physiological changes occurring as a result of egg formation. 2. To investigate whether reduced take-off velocity is a cost of reproduction, the physiological costs of egg production in the Zebra Finch ( Taeniopygia guttata ) were experimentally manipulated by varying both the number of eggs a female laid and the quality of her prelaying diet. The effect of changes in postlaying flight muscle condition and body mass upon alarmed flight take-off velocity (a measure of escapeability in birds) was subsequently measured. 3. Changes in muscle condition were found to correlate positively with changes in various measures of flight velocity: treatments that caused the greatest declines in muscle condition during egg-laying were associated with the greatest declines in flight performance over this period. In contrast, breeding attempts that caused the smallest declines in muscle condition were associated with improvements in flight performance (i.e. birds flew faster at the end of laying than at the start). 4. These effects were independent of changes in body mass, and occurred postlaying, suggesting that the cost of egg production lies primarily in the formation of the eggs, rather than in carrying them as other studies had suggested. The observed trade-off between muscle loss resulting from egg production and escape ability could have important implications for the evolution of optimal clutch size in birds.

The trade-off between growth rate and locomotor performance varies with perceived time until breeding

“…Similar reproductive costs have been shown to include a temporary impairment of locomotor abilities during the breeding season across a range of organisms [e.g. whelks (Brokordt et al, 2003); passerine birds (Lee et al, 1996;Veasey et al, 2000;Kullberg et al, 2002)]. In the present study all groups showed on average a poorer swimming endurance (coupled with a slower recovery) after the breeding season, but this was accentuated in the groups that had earlier exhibited the fastest growth rate.…”

Costs of compensation: effect of early life conditions and reproduction on flight performance in zebra finches

“…The flight performance of LH birds prior to reproduction was not different to that of other birds. However, whilst there was a reduction in mean escape flight speed after breeding in all four diet groups, presumably due to the demands of egg production, incubation and chick rearing (Veasey et al 2000(Veasey et al , 2001Kullberg et al 2002b), this reduction was significantly greater in the LH females (in comparison to HH and LL females) and smaller in LH males (vs. HH and LL males). Analogous results of an effect of growth rate on locomotor performance only being evident under more challenging conditions were obtained by Royle et al (2006), who found that the early growth trajectory of fish did not influence their initial swimming endurance when first tested at adulthood, but it did influence their ability to improve with further training: fish that had earlier undergone a phase of compensatory growth were unable to further increase their stamina, suggesting that the rapid growth had reduced their muscular or metabolic capacity (as hypothesised by Arnott et al 2006).…”

“…Therefore, one potential component of the costs associated with egg production is a reduction in the locomotor performance of the female, which could reduce the effectiveness with which she can escape from a predator attack. Indeed, it has been shown that the more a female transfers nutrients from muscles into her eggs, the more her flight performance declines over the laying period (Veasey et al 2000); accordingly, there is a negative relationship between clutch size and post-laying flight performance within individual females (Veasey et al 2001). While egg production costs will only affect females (Kullberg et al 2002a), flight ability can also be reduced by the need to build up body reserves during incubation (Kullberg et al 2002b), which could potentially affect both sexes.…”

Effects of pregnancy on locomotor performance: an experimental study on lizards

“…For example, such costs have been demonstrated or inferred in crustaceans (Winfield and Townsend 1983;Berglund and Rosenqvist 1986), scorpions (Shaffer and Formanowicz 1996), insects (Isaacs and Byrne 1998;Gu and Danthanarayana 2000), reptiles (Shine 1980;Seigel et al 1987;Cooper et al 1990), birds (Lee et al 1996;Veasey et al 2000Veasey et al , 2001, and mammals (McLean and Speakman 2000). This diversity of examples suggests that locomotor "costs" of pregnancy may offer a powerful opportunity for comparisons across disparate kinds of animals.…”