Trade-offs in the early development of tree swallows (Tachycineta bicolor): offspring characteristics, offspring quality, and parental care

Abstract

Life-history theory predicts that because organisms have limited resources available to them, they must make decisions to prudently allocate resources in a way that maximizes fitness.Therefore, there is expected to be a trade-off between current reproductive effort and future survival and reproduction, with those individuals investing in a current breeding attempt doing so at a cost to their own survival and/or future fecundity. Altricial birds rearing young are consequently expected to be prudent in their allocation of resources between broodmaintenance and self-maintenance, and their allocation choices may be influenced by numerous offspring characteristics that potentially indicate offspring "quality" or condition.My thesis had three main goals: 1) to understand if the smallest nestling tree swallows in a nest are of inherent poor "quality", or if they are the smallest simply due to being outcompeted by their older and larger siblings; 2) to investigate whether, in an environment with parasites, it is the parents or their offspring who bear the costs associated with parasitism; and 3) to better understand the function of gape and flange colouration in nestling tree swallows. Overall, I wanted to understand how each of these nestling characteristics affect, and are affected by, parental life-history trade-offs. By pairing observational data with a cross-fostering experiment, I determined that both total egg mass and yolk mass increased with order of laying. N estlings that hatched earlier in the hatching sequence and were manipulated to be smaller within the size hierarchies in nests performed in a similar fashion as if they had retained their size advantage, but had lower immune responses, which may be because these earlier-hatched nestlings hatched from earlier-laid eggs with smaller yolks. By manipulating parasite levels in nests and then comparing nestling immune function and growth, and parental provisioning rates with control nests in a three-year study, I observed no 11 significant effects of larval blow flies parasitizing nestlings, except parasitized nestlings showed slightly lower haematocrit levels in one year. Deleterious effects of parasitism may have been compensated for by parents, as the parents of parasitized nestlings fed their young at marginally higher rates than the parents of parasite-free nestlings. By manipulating either gape redness or flange colouration (both within ultraviolet and human-visible wavelengths) of nestling tree swallows and carrying out feeding trials, I determined that parents did not preferentially feed nestlings with redder gapes. Instead, they fed nestlings with flanges that reflect in the ultraviolet more than nestlings with non-ultraviolet-reflective flanges, although only when nestlings were older. Parents also appeared to feed nestlings with blackened flanges less throughout the nestling period. Overall, my studies indicate that 1) tree swallows may allocate extra nutrients to later-laid eggs to alleviate detrimental effects of within-brood size hierarchi...

Avian hatching patterns in response to low-quality breeding environments and ecological stochasticity: Adaptation or constraint?

“…Alternatively, lack of significant treatment effects in tree swallows may have been the result of differential investment in eggs. In some years, tree swallows increase yolk size with laying order (Unger 2016), suggesting that egg investment may vary with environmental conditions and food availability during the laying period. If hatching asynchrony is an unintended consequence of constraints, then females may try to counter this through increased investment in last-laid eggs.…”

“…This is consistent with other studies of growth patterns of tree swallows (Zach 1982;Clotfelter et al 2000;Johnson et al 2003), which may support a brood reduction strategy; however, fledging success and age of fledging were unaffected by brood hatching spread, suggesting that last-hatched young were still able to leave the nest despite any disadvantages of being reared in asynchronous nests. In such cases, last-hatched nestlings may be able to compensate for their size by other traits, such as by having stronger immune systems (Unger 2016) or higher circulating levels of testosterone to help them compete with their older siblings (Schwabl 1996). Alternatively, conditions at HVC and the REF site may not have been severe enough in 2014 to result in large-scale brood reduction.…”