It’s Not How You Get There, But When
When migrating, animals like the great white pelican have to walk the fine line between saving time and saving energy. (Image Credit: Ray in Manila, CC BY 2.0, Image Cropped).
Landscape-dependent time versus energy optimisations in pelicans migrating through a large ecological barrier (2019) Efrat et al., Functional Ecology, https://dx.doi.org/10.1111/1365-2435.13426
We have all seen the amazing scenes in nature documentaries of the great seasonal migrations undertaken by many different species on this planet. By migrating between two different habitats, migrating animals are thought to maximize both how many resources they have access to, and to minimize their exposure to harsh environmental conditions.
Despite these benefits gained by migrating animals, there are risks associated with these seasonal, long-distance travel events. Migrating animals, like the great white pelican (Pelecanus onocrotalus), have to decide what is better: traveling for a shorter distance or using less energy by taking a less strenuous – but longer – path. Today’s authors tracked the great white pelican during its seasonal migration over the Sahara to study how these birds made decisions about their travel.
What They Did
The authors trapped pelicans in Israel before they began the part of their migratory journey that would take them over the Sahara desert. After they were captured, the pelicans were fitted with data loggers that allowed the authors to track their location and acceleration data, the acceleration data being used to distinguish between the low-energy gliding flight and the high-energy flapping flight.
To determine what type of flight strategy the pelicans used in their migration, the authors simulated two different flight paths and compared these to the average route of all of the pelicans. The “direct-pass” route was the straightest and shortest route, and passed mostly over the Sahara Desert, while the “corridor-pass” route was longer and went along the Nile River, an environment in which pelicans could use less energy and have access to more food resources. If pelicans’ flight paths were more similar to the direct-pass route, that would indicate that these birds optimize time during migration, as opposed to the lower-energy corridor-pass option.
Did You Know: GPS Trackers
Studies like this one benefit from the amazing amount of technology available for tracking animals across the globe. Additionally, due to their size, the pelicans can carry large GPS trackers that give researchers an abundance of information.
But when researchers work with smaller animals, trade-offs have to be made between how much information they can get and how many animals they can track. GPS trackers are expensive; any tracker that can give a lot of information will be relatively large and be a burden or a hindrance for smaller animals. Additionally, due to their cost, not as many can be used at once. In contrast, smaller trackers, like radio-telemetry tags, can be put on many individual insects and used to track their movement. But these only relay what radio tower that insect flew by, that is, if it even flew by the tower at all…
What They Found
The pelicans tended to act as time–optimizers,,flying in straight-line paths along the Nile when possible, yet taking shortcuts by flying 300-400km across the Sahara when the Nile took large bends towards the west. This was surprising, as the authors predicted that these large birds that are adapted to live in aquatic habitats would prefer to fly along the Nile.
Interestingly, the birds tended to use more high-energy flight when faced with adverse weather conditions (such as unfavorable winds), but only when they were flying over the desert. When the birds were flying along the Nile, they did not engage in high-energy flight when faced with these same adverse weather conditions.
The authors only analysed about two-thirds of the autumn migration of the great white pelican, as they did not have enough data for the rest of the migratory period or any other part of the annual migration cycle.
Another issue is that pelicans are highly social and different group characteristics might affect their behavior. For example, the group that a given pelican is in may affect how fast it flies and where it decides to stop. Unfortunately, the pelicans seem to (as far as the authors know) change their group almost daily, it is impossible to use these data when considering migration pathways and strategies.
The finding that a large, wetland specialist like the great white pelican tends to optimize migration time by flying over the desert is surprising, and goes to show how important it is for researchers to go out and test hypotheses about the natural world.
The authors suggest that future migration studies need to incorporate and consider not only species-specific biological characteristics and the environmental conditions during the migratory period, but also the landscape that any migratory animal travels over. All of these factors have the potential to interact and affect the outcome of decision-making by the migratory animal.