What drives dependence on parents post-fledging?

One of the most relevant stages in the life cycle of altricial birds is the post-fledgling dependence period (hereafter PFDP). In a similar way to human being when it starts to walk, young birds do not become independent from their parents immediately after flying from the nest, but rather stay with them for some time to gain experience, learn the necessary skills for flying and foraging and, why not, for benefitting for some extra time of comfortability, and free resources! Indeed, the development and duration of this period has the potential to shape, and affect, future survival probability, dispersal movements and reproduction outcomes (Cox et al. 2014, Naef-Daenzer and Grüebler 2016). Thus, studying this period, and understanding the whys and the hows of its variation, is key to address any potential conservation issues in birds.

However, the development and the duration of the PFDP are generally understudied, since birds are difficult to track when they leave the nest (Naef-Daenzer and Grüebler 2016). This becomes more challenging in bigger species such as raptors, which occupy large ranging areas and have high mobility rates. Bearing in mind that raptors are declining worldwide, it is imperative to gain knowledge on this type of aspects of their life cycle (McClure et al. 2018). We performed an extensive literature review on the state of knowledge of the duration of the PFDP in the most representative group of diurnal raptors, the family Accipitridae, which includes hawks, eagles, buzzards, and kites, among others.

What we found is in line with other studies accounting for information gaps in raptors (Buechley et al. 2019): less than half of the raptors have reliable studies on the duration of the PFDP, and only a quarter of them have two or more studies on this topic. More dramatically, this information is lacking in more than two thirds of the species of conservation concern (Figure 1), especially in the southern hemisphere, where few studies have been carried out on this topic.

Figure 1. Summary of the results of the review of the length of the PFDP, sorted by subfamily and conservation status (LC, Least Concern; NT, Near Threatened; VU, Vulnerable; EN, Endangered; CR, Critically Endangered). In all cells of the table, the total number of species is given in parentheses.

Additionally, we performed a meta-analysis to get to know which ecological, and life-history variables could affect the length of the PFDP, while controlling for species phylogeny. First, we found that heavier (larger) species show longer post-fledging dependence periods than smaller ones (Fig. 2A). According to classic laws of anatomical and physiological scales, the bigger you are, the more time you need to fully develop (Blueweiss et al. 1978); thus, it is sensible to think that larger raptors require more time to achieve independence from their parents. Also, since these species usually lay smaller clutches, a prolonged parental care after fledging could be a strategy to enhance the survival of the few offspring (Martin 1996, Russell et al. 2004).

Figure 2. Results of the models to account for the variation (at the species level), in the duration of the PFDP, according to: (A) species’ average weight; (B) interaction between the species’ average breeding latitude and the species’ average weight (orange represents species situated at the mean values, violet and sky blue represent species situated 1 standard deviation above (+1 SD) and below (-1 SD) respectively); (C) (c) interaction between the species’ average breeding latitude and the migration status (red represents migrant, blue represents resident); (D) interaction between the species’ average breeding latitude and the breeding hemisphere (red represents north, blue represents south).

Second, raptors breeding at lower latitudes and in the southern hemisphere showed longer PFDPs (Fig. 2B). This is likely a result of the environmental stability of the tropics when compared with temperate areas, and the reduced continentality of the southern temperate areas, when compared with their northern counterparts (Oliver 2005). A low seasonality in food availability, higher ambient temperatures, and the lack of a severe winter (or a prolonged time available for breeding), results in a more stable environment that favours a slow-life strategy of extending the parental care after fledging (Martin 1996, Russell et al. 2004). Third, migrant species showed shorter PFDPs than resident ones (Fig. 2C), because the former have a constrained time devoted to parental care between fledging and emigration (Bennet and Owens 2002, Bildstein 2006).

Interestingly, these ecological and life-history drivers do not come alone, but rather interact to shape the duration of the PFDP. For instance, for large species (violet, Fig. 2B), a difference of a given number of degrees of latitude produces a variation in the duration of the PFDP of weeks, or even months, but for smaller species, this variation might be of only days (sky blue, Fig. 2B), which again has to do with anatomical scaling laws. Also, the effect of latitude is marked in resident species (blue, Fig. 2C), but inexistent in migrants (red). In the latter case, the environmental conditions that impose (and constrain) the start of migration are determinant, regardless of the latitude where the species breeds. Last, the fact that breeding latitude has a different effect in the two hemispheres (Fig. 2D) might be confounded with the scarcity of research carried out in the southern hemisphere (blue), and with its lower proportion of mainland (and thus, lower number of raptors).

Figure 3. Results of the models to account for the variation in the duration of the PFDP, with studies of all species pooled together, according to: (A) breeding latitude (absolute values); (B) interaction between absolute breeding latitude and migration status (red represents migrant, blue represents resident); (C) interaction between absolute breeding latitude and breeding hemisphere (red represents north, blue represents south).

Most of the aforementioned results are similar when we replicate the analyses with all raptors pooled together (Fig. 3), highlighting the paramount importance of ecological, and life-history, drivers of variation in the extent of the PFDP in raptors, over other phylogenetic characteristics. Furthermore, the joint (and interactive) significance of breeding latitude and hemisphere, and migration status on the duration of the PFDP found in our study calls for urgent research on the possible negative effects of global change on this fragile stage of raptors’ life cycle (Martínez-Ruiz et al. 2023). Overall, our findings should encourage researchers to focus their investigations on the study of this poorly known stage of raptors’ life cycle, in order to better tackle the conservation issues that they face nowadays.