This is a quote from Dey et al 2014:
Hatching asynchrony is thought to be adaptive because...
What exactly does adaptive mean here? Does it mean hatching asynchrony has fitness benefits? Or does it mean hatching asynchrony is likely to be selected for?
The question is probably more complicated than it seems because, if I am not wrong, the word adaptation here is understood at the group level.
Definitions of adaptation
Unfortunately, there is not such thing as a single, standard definition of adaptation. But for most cases the accurate definition the author is using is not of much importance as all of the usual definitions totally fit in the sentence without changing the meaning of the concept they want to express. In you case however, the concept of adaptation seems to be a bit more complicated (and interesting) because it refers to a group-level adaptation.
Note that most people does not really care about the exact definition they're using and this might yield to some confusion. It will probably be a bit hard to know exactly what the authors (Dey et al.) of your article had in mind.
Definition of Group-level adaptation
I am here going to talk about how the term adaptation can be understood for a (social) group. This is important as I suspect the authors to use the term adaptation on a group phenotypic trait (emergent trait if you want).
Pareto optimality
To my experience, the concept of group-level adaptation is defined in relation to the concept of Pareto optimality which is a concept used in evolutionary game theory. Shortly speaking, in a system that is at Pareto optimality no individual can by any change of the trait under consideration increase its fitness without decreasing the fitness of another individual. In other words, a population that is at Pareto optimality has the greatest achievable population mean fitness. Therefore, it makes sense to consider a group to be (perfectly) adapted when it is at Pareto optimality.
Nash equilibrium
Depending on the underlying evolutionary game, the Pareto optimality may or may not be a Nash equilibrium. Shortly speaking, a population is at Nash equilibrium if no individual can increase its fitness by changing the trait under consideration. Therefore, group-level adaptation can only be stable if the Nash equilibrium is Pareto optimal. An example of game where the Nash equilibrium is the free-market game.
In your case
At first sight, because asynchrony is a group level trait, it seems to me that the sentence "Hatching asynchrony is thought to be adaptive [..]" means that hatching asynchrony is thought to be at Pareto optimality. In other words, it means that hatching asynchrony is thought to be the state of the system where the mean fitness of the group is maximized, that is no individual can increase its fitness without decreasing the fitness of another individual by being synchronous.
A trait is said to be adaptive when it causes fitness to increase. Fitness is generally understood as the (relative) contribution to future generations in terms of offspring or genes. The trait is selected for by the environment and hence increases fitness.
In the paper of Dey et al. this is the fitness of the parent birds. Hatching asynchrony causes size differences between chicks which may (1) "facilitate brood reduction in unfavourable environmental conditions", (2) "provide insurance against the failure of core offspring" and (3) "reduce competition among nestlings". Dey et al. also cite a reason why hatching asynchrony may be sustained in a population despite being maladaptive: initiating incubation before the clutch is completely laid is selected for and causes asynchronous hatching as a by-product. In this case, hatching asynchrony merely correlates with fitness, but does not cause fitness to increase and is thus not adaptive.
Your questions mean basically the same. Birdcare.com says:
The situation in which all the eggs in a clutch do not hatch at (more or less) the same time, as is more usual among birds, but have their hatching spread over several days. It is well seen in the various types of raptor, and is an adaptation to a type of food supply which may fluctuate. During seasons when food is short the later hatched young will probably starve as the earlier hatched young, being larger and stronger, deprive them of food, and so the size of the brood is reduced to a level in balance with the available food supply. In years of plenty all the young may be able to survive. In 'synchronous hatching' all the eggs hatch at more or less the same time.
The fitness benefit of hatching asynchrony on the species level is due to increased range of individual fitness among the young birds. The available resources do not need to be distributed equally among the offspring during unfavorable seasons such that fewer, but at least some of the hatchlings (the 'strongest') can survive. You see that this is the same as beeing selected for because on average the number of agents passing their genes to the next generation is higher with asynchronous hatching than with synchronous hatching. Of course, the success of this strategy is strongly dependent on environmental factors. In very stable habitats it might turn out to be disadvantageous on the long run.
