Breeding limits foraging time: evidence of interrupted foraging response from body mass variation in a tropical environment

Abstract

Birds should store body reserves if starvation risk is anticipated; this is known as an 'interrupted foraging response'. If foraging remains unrestricted, however, body mass should remain low to limit the predation risk that gaining and carrying body reserves entails. In temperate environments mass gain in female birds during breeding is often attributed to egg formation and mass loss after incubation to flight adaptation or the effect of reproductive workload, rather than as a result of an adaptive interrupted foraging response to the limited foraging time or unpredictable foraging conditions that breeding demands. In tropical environments, foraging conditions vary more within the breeding season than in temperate environments, and so studies in tropical environments are more suited to decouple the potentially confounded effects of increase in body reserves versus egg formation on the body mass of breeding birds. In this study, we test whether breeding results in an interrupted foraging response in a tropical savannah system using body mass data collected over a 15-year period from female Common Bulbuls Pycnonotus barbatus. This species breeds both in the wet and dry season, despite fewer resources being available in the dry season. Breeding stage predicted female body mass: body mass peaked abruptly during incubation, but was not closely associated with the egg-laying stage, and declined during brood rearing. Breeding females were heavier in the dry season than in the wet season. In the dry season, heavier birds were more likely to incubate eggs or brood chicks. These observations suggest that increased body reserves are required to buffer the consequence of limited foraging time or impoverished foraging conditions, which may be most pronounced during incubation and in the dry season, respectively. Such mass increases are consistent with an interrupted foraging response, which may apply to temperate zone birds experiencing foraging restrictions during breeding.

Seasonal differences in baseline innate immune function are better explained by environment than annual cycle stage in a year‐round breeding tropical songbird

“…But lower haptoglobin concentration cannot be interpreted as an overall down‐regulation of immune function in breeding birds, because ovotransferrin and nitric oxide concentrations, and haemolysis titre were higher in breeding birds and haemagglutination titre was not affected by breeding state. Note however, that in a diet manipulation experiment on Common Bulbuls, we observed that low haemolysis titre and high haptoglobin concentration are associated with loss of body mass (C. J. Nwaogu, A. Galema, W. Cresswell, M. W. Dietz, & B. I. Tieleman, unpublished data), and breeding Common Bulbuls are more likely to be in better condition (Nwaogu et al., ), so high haemolysis titre and low haptoglobin concentration may indicate less challenge. Similarly, incubating Common Eiders Somateria millissima in better body condition have higher NO x concentration (Bourgeon, Raclot, Le Maho, Ricquier, & Criscuolo, ).…”

“…In Nigeria, there is a single wet and dry season annually, and environmental factors differ between and within these seasons. Food is usually more abundant in the wet season (Ngozi Molokwu, Olsson, Nilsson, & Ottosson, ; Nwaogu, Dietz, Tieleman, & Cresswell, ), but pathogens and disease vectors should also be more abundant in the wet season due to high environmental productivity (Pascual, Bouma, & Dobson, ; Young, Garvin, & McDonald, ). However, social contact may increase with less food and water during the dry season because animals cluster around fewer resources, and this may facilitate disease transmission (VanderWaal, Gilbertson, Okanga, Allan, & Craft, ), even if pathogen abundance is low.…”

Rapid plastic breeding response to rain matches peak prey abundance in a tropical savanna bird

“…Tarsus length (in mm, which represents body size) and time of capture (linear and quadratic) were included as explanatory variables and bird identity and year as random variables. Body mass (g) with body size as covariate was used as a proxy of individual body reserves (as Nwaogu, Dietz, Tieleman, & Cresswell, 2017); we also tested body mass alone (without tarsus as covariate) and a "scaled body mass index" (Peig & Green, 2009), which produced qualitatively similar results (details in Table S2). intervals up to 24 weeks for rainfall and up to 32 weeks for temperature.…”

Multitasking and the evolution of optimal clutch size in fluctuating environments

“…Specifically, E i is the basic energy reserve and cxt r is the product of the number of offspring produced in each breeding attempt (i.e., the clutch size) (c), the cost per offspring (x), and the time required for breeding successfully (t r ). Hence, the amount of energy and time required to commence breeding are positively correlated to clutch size (e.g., Eden, Horn, & Leonard, 1989;Møller, 2007;Nwaogu, Dietz, Tieleman, & Cresswell, 2017;Smith, Källander, & Nilsson, 1987). For simplicity, we also assume that clutch size is a fixed strategy and remains the same for each individual in each nesting attempt (see Supporting Information Figure S1 for reducing clutch size throughout the season).…”