The Post-fledging Dependence Period
Introduction
We can split birds into two different groups depending on their development when they hatch. On the one hand, precocial birds are characterized by hatching rather developed, they born with open eyes and with down, being able to leave the nest a few days after hatching. An example of this kind of birds is partridges (Alectoris rufa) or mallards (Anas platyrhynchos). On the other hand, altricial birds are less developed when they hatch. They need to spend more time in the nest than precocial birds, as they are usually born with the eyes closed and with no dawn. For instance, Passerines belong to this group. These two strategies determine different aspects of the life of birds, being one of the most important, the amount of parental investment devoted to the offspring. Precocial species invest more in the first stage of reproduction, therefore they have more developed offspring that need less care after hatching. Instead, altricial birds invest less in the beginning, thus they have to devote more care after hatching.
Parental care is crucial for the nestlings of altricial birds, as during their first weeks of life are entirely dependent on their parents. Parental care can be divided into two different stages. In the first one, takes place between the hatching of the nestlings and their first flight outside the nest. During this stage, they grow a lot and gone from being a little ball of down to an individual with almost all the features of an adult. The second one occurs once the nestlings make their first flight outside the nest and finishes when the fledglings are independent of their parents. In this stage, known as the post-fledging dependence period, the fledglings start flying in the surroundings of the nest remaining dependent on their parents. Both periods are significant for the proper development of the offspring. However, most of the scientific research has focused on the first stage, and the second one has been rather overlooked, probably due to the difficulties associated with studying this period.
The post-fledgling dependence period
We can define the post-fledging dependence period, as the time between the first flight of the nestlings and their full independence from their parents. Drawing a small parallel, we could say that the post-fledging dependence period is equivalent to the adolescence in humans. In this stage, fledglings as teenagers, remain dependent on their parents but gradually become independent from them. In this stage, fledglings enjoy the care provided by the parents in the form of food or protection from predators. Besides, during the post-fledging dependence period, they develop crucial future skills such as flying or hunting. For example, common kestrel (Falco tinnunculus) fledglings can be detected perching in trees or stakes during July and August (Fig. 1), when they employ most of their time hunting or tying to hunt small insects or even inanimate objects (i.e. stones or cow dung). Both, the skills and the benefits obtained from their parents are crucial for the fledglings. During the post-fledging dependence period, fledglings increase their nutritional state, their competitive abilities or foraging skills, things that can determine their survival probabilities. In fact, in the common kestrel, the fledglings with longer dependence periods have enhanced probabilities of survival to adulthood.
Figure 1: Common kestrel (Falco tinnunculus) fledglings, perching on the nest-box
The duration of this stage is variable among the different species, for instance, in raptors, the post-fledging dependence period ranges from a few weeks in species like the red kite (Milvus milvus) to nearly a year like in the crowned eagle (Stephanoaetus coronatus). Interestingly, this variability among the duration of the post-fledging dependence period between species also occurs within species, as not all the fledglings of a certain species remain dependent the same number of days. However, considering all the benefits that can be obtained during this stage, why don't fledglings remain dependent as long as possible?
Who decides, the parents or the offspring?
As we have seen, fledglings benefit from the care provided by the parents during the post-fledging dependence period, and therefore we can expect that they try to maximize the time they remain dependent. However, this period requires a great energy investment from the parents, that may have negative consequences for them. Reproduction costs are a widely studied phenomenon, and it has been reported in many studies that increased investment in current reproduction (e.g. laying larger clutches), can have negative effects in the following reproductive events. Although to a lesser extent, this trade-off has also been described for the post-fledging dependence period. In the Savannah sparrows (Passerculus sandwichensis), it has been shown that those parents providing longer post-fledgling dependence periods have lower survival probabilities. In addition, in the common kestrel, it has recently been shown that those fathers that provide longer post-fledgling dependence periods mate with females laying reduced clutches in the following breeding season. This parent-offspring conflict roots on the fact that parents and offspring share a different amount of genes. While parents share the same amount with all the offspring (50%), each descendant shares more genetic material with themselves (100%) than with their parents (50%). Thus, this generates a divergence in their evolutionary interests. Whether for the fledglings the best option is to maximize the duration of the post-fledging dependence period, for the parents is to provide enough care so the offspring can survive without suffering costs that decrease their future survival and/or reproduction. In this way, parents are those determining the duration of the post-fledging dependence period. The most common approach to finish this stage is a progressive reduction in the amount of care that they provide, as a motivation way for the offspring. However, although less commonly, this motivation is not that subtle, and parents use physical aggression towards the offspring to end the post-fledging dependence period.
Figure 2: Common kestrel (Falco tinnunculus) fledgling in Campo Azálvaro during the month of August
Which factors modulate the duration of the post-fledgling dependence period
We have previously seen that we can expect seeing the offspring maximizing the duration of the post-fledging dependence period, and that is a parental choice ending it, to avoid suffering enhanced costs. However, several factors modulate the duration of this stage within the fledglings of a certain population. One of the most important factors behind this variation is the quality of the parents. Some studies have shown that parents of higher quality can provide longer post-fledgling dependence periods. On the one hand, it has been reported that factors as the parental physical condition or the laying date (both quality proxies) modulate the duration of the post-fledging dependence period. On the other hand, it has been experimentally shown that in nests under supplementary feeding, there are longer post-fledgling dependence periods. In this way, its shown that the offspring with more higher quality parents have longer post-fledgling dependence periods. This association between parental quality and the ability to provide longer post-fledgling dependence periods can be explained by several factors. Still, we can briefly say that this association roots on the fact that higher-quality individuals can invest more on their offspring, probably because they are able of monopolizing larger amounts of limited resources, like food or breeding sites.
In addition to parental quality, another factor that can modulate the duration of the post-fledging dependence period are the characteristics of the offspring. Considering that the resources available to the parents are limited, we can expect an enhanced investment in the fledglings of the higher quality, as they will confer higher benefits to the parents in terms of fitness. For instance, in the Arabian babbler (Turdoides squamiceps), a social species inhabiting in the Middle East, it has been reported a sex-dependent divergence in the parental investment. Parents bias their care towards male fledglings when they live in small groups, and towards females when they live in large groups. This behavior can be explained by the fact that males are the philopatric sex, remaining in the natal group as adults, and females tend to disperse more. By biasing the amount of provided towards females when they live in large groups parents increase the survival probabilities of the dispersing individuals. Thus, they favor those offspring that will not compete with them for limited resources in the future.
Finally, environmental heterogeneity can also modulate the length of the post-fledging dependence period. For example, under benign environmental conditions, it has been shown that there are shorter dependence periods than when the environmental conditions are good. One idea behind this pattern is that when under benign environments fledglings can look for themselves more easily than when the environmental conditions are bad. This will also be beneficial for the parents, as they will be able to reduce the cost of reproduction by providing shorter post-fledgling dependence periods when the environmental conditions are good.
Wrapping-up
The post-fledgling dependence period is a crucial stage for both the offspring and the parents, as it can determine the survival of the former and the future reproduction of the latter. There is huge variability in its length between the different bird species, mostly associated with their ecological features. This variability also occurs within a single species and is modulated by several factors, including environmental conditions, parental quality and the characteristics of the offspring among others. However, our knowledge about this important life stage is incomplete and limited to a small number of species. This limitation roots on the fact that is a very difficult period to study. In many birds, especially in small-sized ones, in nearly impossible to follow and observe the fledglings once they are out of the nest, and also the amount of care they receive from the parents. Therefore, more studied are needed to shed light on this important life stage that remains rather overlooked, to better comprehend the life-history strategies of birds in particular and of animals in general.
David López-Idiáquez (4 - April - 2019)
*This is a reproduction of an article published in Naturalmente