By Christiaan Both [expedition member Dec 2018]
I thought I knew pied flycatchers pretty well. After having studied them for about twenty breeding seasons, having ringed thousands of nestlings and adults, and having carried out quite some crazy experiments, I considered pied flies among the easiest bird species to study in the field. However, this is only part of the story.
Most of us, Europeans, have only a rather limited view on pied flycatchers (and most other migrant birds). Although they are more African than European species, we know exceptionally little about the seven months of the year that they reside at their wintering grounds. That is why Janne and her team are here: to uncover the role of wintering ecology and conditions of this species.
In contrast to the breeding season, flycatchers are not easily studied here at their wintering grounds in the Ivory Coast. It is not that they are rare, and difficult to be found. Actually, they likely are the most common of all bird species found here at the forest savanna of Comoé National Park around our field station. Their densities may even be rather similar to breeding densities in our nest box areas in The Netherlands. But their behavior is less conspicuous. During the breeding season they live in the fast lane. Everything they do is meant to maximize reproductive success: they sing, display, build nests and work hard to raise their families.
This busy life allows us to obverse and catch them easily. Here at their wintering grounds they seem to be in a rather quiet phase of their annual cycle. They are faithful to a rather small area, but even if you know where they should be, they are not easily observed. Do they spend a lot of time just resting in a tree? Are ecological conditions at this moment easy for them? Which conditions are important in constraining their numbers: food, predation, diseases, competition with local residents? And does this change during their stay here?
Our work involves studying individuals. We are interested in how local ecological conditions affect how individuals can prepare for migration during the spring, how this affects their departure timing, and whether and how this carries over to breeding success and survival. If we can answer these questions, we will be better able to understand their capabilities of adapting to large-scale environmental changes, such as habitat destruction or climate change. Unfortunately, we are still unable to study the same individuals during both winter and breeding season. However, the tiny dataloggers we deploy the flycatchers with, show us when they depart, where they go for breeding, and hopefully even how successful their reproduction is. Our challenge here is to retrap individuals with loggers from last year, and deploy new loggers again. At the breeding grounds this is an easy task, but here it is hard work, requiring good catching skills.
How much connection exists between the conditions individuals experience during the breeding season and their wintering ecology? In general habitat structures seem similar: in the breeding season we find the highest densities in rather open woodland, and the woodland savanna, and especially the edges of forest and savanna do resemble these pretty well. Flycatchers however vary largely in breeding habitat structure, and one can wonder whether even at the individual level a resemblance exists. Do flycatchers that grow up in more open habitats develop foraging strategies tuned to these conditions, and do they therefore also occupy more open savanna habitats during the wintering season? More in general, how do young flycatchers select their first wintering site, giving the apparent diversity in habitats? And may this habitat choice in winter affect subsequent breeding habitat choice?
A result from the breeding grounds that intrigues me for a long time is that a high proportion of first year individuals do not breed, which is not expected for such a short-lived species. Could it be that young birds wintering in a low quality habitat may have difficult to depart in time? Or do they just have to learn their spring route, which differs from the autumn route? This question likely will be answered with the tracks obtained from Comoé. We may even see from the geolocator-data whether young individuals that arrive too late at the breeding grounds do attempt to breed or not. As these dataloggers record light levels, breeding individuals residing in nest holes will show regular low light levels during the day. In this way, our field work here will clearly teach us important insights about the breeding season.