An experimental approach to the brood reduction hypothesis in Magellanic penguins

Abstract

In many bird species, eggs in a brood hatch within days of each other, leading to a size asymmetry detrimental to younger siblings. Hatching asynchrony is often thought of as an adaptive strategy, and the most widely studied hypothesis in relation to this is the ‘brood reduction hypothesis’. This hypothesis states that when food resources are unpredictable, hatching asynchrony will allow the adjustment of the brood size maximizing fledging success and benefitting parents. The Magellanic penguin Spheniscus magellanicus is an appropriate species to test this hypothesis because it has a 2‐egg clutch that hatches over a 2‐d interval with a broad range of variation (–1 to 4 d), it shows facultative brood reduction, and food abundance between breeding seasons is variable. We performed a manipulative study at Isla Quiroga, Argentina, during three breeding seasons (2010–2012) by forcing broods to hatch synchronously (0 d) or asynchronously (2 or 4 d). Years were categorized based on estimated food abundance. Our study provided mixed results because in the low estimated food abundance year asynchronous broods did not have higher nestling survival than synchronous broods, and the second‐hatchling in asynchronous broods did not die more often than those in synchronous broods. On the other hand, younger siblings of 4‐d asynchronous broods starved earlier than those of synchronous broods, and 2‐d asynchronous broods fledged heavier young than synchronous broods. Asynchronous hatching would seem to benefit reproduction in this species, not with respect to survival, but in terms of the advantages it can accord to nestlings and, in terms of lower costs, for parents raising nestlings.

Food provisioning in Magellanic penguins as inferred from stable isotope ratios

“…Unfortunately, there were no accoustic surveys for schooling fish stocks during the study period. However, the 2012 season seemed to be a period of low abundance of these stocks, as inferred by other schooling fish predator's diet and the poor Magellanic penguin breeding success during this breeding season …”

“…However, the 2012 season seemed to be a period of low abundance of these stocks, as inferred by other schooling fish predator's diet 44 and the poor Magellanic penguin breeding success during this breeding season. 45 The differences in ED to parents, and hence in food provisioning, would depend on the rate of provisioning, specifically how often each adult returns with food to feed the brood, and the amount of food The random effect structure is shown in parentheses transferred in each feeding event. The latter could be affected by the larger size of males.…”

Compensatory effect of egg size dimorphism on hatching asynchrony in Magellanic penguin

“…The magnitude of hatching asynchrony is known to influence sibling competition in many bird species (Merkling et al 2014). In Magellanic penguins, last‐hatched chicks are at a competitive disadvantage throughout the pre‐fledging period (Barrionuevo and Frere 2017). Here, we demonstrated that this disadvantage could be compensated by egg size dimorphism in favor of the last‐hatched chick, agreeing with what was reported for altricial birds (Rosivall et al 2005, Bitton et al 2006).…”

“…If the nest had been categorized as ‘control', we measured each egg (E1 and E2), marked it with the laying order and returned it to the nest. If the nest had been categorized as ‘manipulated' when E1 was laid, we measured and marked the egg and replaced it with a plaster replica (the real E1 was safely placed in a plastic container inside the colony, under a bush sheltered from the sunlight but exposed to ambient temperature (Barrionuevo and Frere 2017)). The replica eggs were made of gypsum and were the same size and shape as penguin eggs, and were accepted by all females (Wagner et al 2013, Barrionuevo 2015).…”

Food supply and provisioning behavior of parents: Are small hoopoe nestlings condemned to die?

“…In contrast, when the availability of food is unpredictable, birds typically lay optimistic clutches that produce optimistic broods that, in case of limited availability of resources, parents can reduce or adjust to environmental conditions ( Lack 1947 , 1954 , 1968 ). Brood reduction, however, does not occur randomly among siblings and the existence of asynchronous broods, in terms of, for instance, body size or competition skills, would facilitate brood size adjustment to environmental conditions ( Lack 1947 ; Mock and Parker 1997 ; Barrionuevo and Frere 2017 ; Hildebrandt and Schaub 2018 ). The most common way for parents of producing a brood hierarchy is to start incubation before clutch completion, which results in asynchronous hatching and broods that include nestlings of different ages and sizes.…”