Physiology, activity and costs of parental care in birds

Abstract

Parental care is assumed to be costly in that it requires sustained, high-intensity activity sufficient to cause costs of reproduction (decreased survival and future fecundity of parents). Costs of reproduction are, in turn, thought to have a physiological basis where intense activity causes a decrease in parental condition. However, attempts to identify the physiological basis of costs of reproduction have produced mixed results. Here, I argue that in birds, the central idea that parental care represents sustained, high-intensity work might be incorrect. Specifically: (a) the duration of intense activity associated with chick-rearing might be quite limited; (b) flight, the most obvious sustained, high-intensity activity, might only represent a small component of an individual's overall activity budget; (c) some (high-quality) individuals might be able to tolerate costs of intense activity, either owing to their physiological state or because they have access to more resources, without perturbation of physiological homeostasis; and (d) individuals might utilise other mechanisms to modulate costs of activity, for example, mass loss, again avoiding more substantial physiological costs. Furthermore, I highlight the important fact that life-history theory predicts that reproductive trade-offs should only be expected under food stress. Most birds breed in spring and early summer precisely because of seasonal increases in food abundance, and so it is unclear how often parents are food stressed. Consequently, I argue that there are many reasons why costs of reproduction, and any physiological signature of these costs, might be quite rare, both temporally (in different years) and among individuals.

Physiological predictors of reproductive performance in the European Starling (Sturnus vulgaris)

“…Our overall goals were to assess whether the physiological variables are related to workload or immediate fitness consequences, and if they mediate future survival or reproductive success. Although workload variables (categorized here as nest visit rate and brood size; Fig 1 ) should reflect how hard females are working during parental care, the links may not be via energetics [ 22 , 82 , 83 ] and the consequences may be through other physiological avenues. Additionally, fitness costs indicate potential life history consequences: current reproduction, future fecundity (in second brood or in second year), survival and cumulative productivity (over 2 years; see Fig.…”

Maternal glucocorticoid levels during incubation predict breeding success, but not reproductive investment, in a free-ranging bird

“…We predicted that if there were a relationship between CORT and reproductive investment and success, it would most likely emerge post-hatch, given the higher maternal energy expenditure required during chick rearing than during incubation ( Nilsson and Råberg, 2001 ; Humphreys et al, 2006 ; Sakaluk et al, 2018 ; but see Williams, 2018 ). However, female CORT levels during chick rearing were unrelated to any measure of reproductive investment (nest box visits and nestling growth rate) nor any measure of reproductive success (nestling mass at day 14 and fledging success).…”

Oxidative status and stress during highly energetic life‐history stages of Chinstrap Penguins: breeding versus molting

“…Reproduction is one of the most energy‐demanding life‐history stages (Green et al. , Costantini , but see Williams ). Several investigators have examined how reproduction causes oxidative stress in both captive (birds: Alonso‐Alvarez et al.…”

“…Reproduction is one of the most energydemanding life-history stages (Green et al 2009, Costantini 2014, but see Williams 2018). Several investigators have examined how reproduction causes oxidative stress in both captive (birds: Alonso-Alvarez et al 2004, Wiersma et al 2004, mammals: Ołdakowski et al 2012) and wild (Christe et al 2012, Wegmann et al 2015 animals.…”