In order for a bird to keep airborne, it requires large chest muscles, a large heart and a light skeleton. Flying is demanding for any bird.
Kestrels hunt do most of their hunting while flying. Kestrels can be seen hovering to save energy, before swooping down and catching their prey. Kestrels can also be seen on perches looking for their next meal. While flying while hunting takes more energy, it is also more lucrative, resulting in more prey.
Birds have many advantages over terrestrial mammals so these have to be worth the energy demands put on their bodies. In order to make it worthwhile, birds have found some fascinating ways of reducing the energy needed.
What do kestrels eat?
The kestrel will eat voles, mice, and shrews, along with smaller birds. However, most of their diet is made up of voles which they hunt while they are feeding on grass. Voles feed every two hours and will leave their burrows at these times. Kestrels will time their hunting to coincide with these excursions.
Flight-hunting vs perch-hunting
The kestrel (Falco tinnunculus Linnaeus), which is found in Europe, Africa, and Asia is well known for the way it hovers in the sky, with its wings outstretched or beating fast while looking for its prey. Kestrels use this method called flight-hunting, but also use another method of hunting called perch-hunting. They will sit on a tree or fence post before swooping down to catch their prey.
Flight-hunting has been shown a more effective hunting method than perch-hunting. Kestrels are able to catch their prey at a rate of 2.7 per hour while in flight, while just 0.14 mammals are caught per hour from perches. This is due to the larger area that the kestrel can survey while flying than from a perch. Flight-hunting does have its drawbacks though as it uses up lots of energy.
Kestrels will only flight-hunt when it is worthwhile and is done less in winter when more energy resources are required to keep warm. Perch-hunting is more prevalent in winter for another reason. During summer they need to find food not only for themselves but also for rearing a family, but this is not needed in winter. Perch-hunting in summer will not yield enough food to ensure the survival of their young
Kestrels, and other birds, ensure that the amount of time they spend flight-hunting achieves the maximum return. Because kestrels feed on voles, they need to make sure there is a bountiful supply around when they are hunting. Voles live in burrows but come out to feed on grass. Voles feed every two hours and so the kestrels coincide their hunting with the voles feeding times.
Kestrels are clever in that they time when they are going to eat as well. Although they may catch voles during the day they won’t eat them until dusk when the voles don’t venture out. By doing this the kestrel can maintain their weight throughout the day and ensure that they are not using extra energy to carry the extra weight. It is estimated that catching a vole in the morning but not eating it until that evening can save 7% of the kestrel’s energy requirement.
Kestrels can often be seen hovering in the sky and this reduces the energy they expend during hunting. By making use of the wind, kestrels are able to soar. The kestrel is not the only bird to use this technique as an energy-saving method.
While in flight, kestrels bodies are moved around by the wind, but while the body is buffeted the head stays in one place. This allows the kestrel the best chance to pick out their prey from a long distance but also helps them to soar. As their head does not move, they can detect the airflow around them and control their flight to ensure they continue to soar.
Soaring reduces the energy requirements considerably, saving up to 70% of the energy it would expend if the bird was flapping its wings. This allows the kestrel to stay in the air for longer, giving them a greater chance when searching for food.
Kestrels can use slope-soaring as a means to stay in the air longer and reduce the energy used. Kestrels hover into the wind to increase the airflow over their wings. With its wings outstretched, the kestrel can ensure there is enough lift to stay in the air without expending energy. By using the geography of the land below and hunting over ground that slopes into the wind, the airflow is directed upward. As the wind is directed upwards, the resulting draught lifts them higher.
Kestrels aren’t the only birds that use these draughts, and these winds don’t only occur over land. Many sea birds including gannets and gulls use waves from an island to soar high.
Weather fronts can also cause an updraught where two air masses meet. Kestrels can also be caught in thermals, a column of rising air, caused by the earth heating up. As the sun heats the earth up, if the temperature is higher than the ground next to it then the air rises to lift the bird up.
Kestrels are very clever in the way they hunt. They are extremely mindful of the energy requirements on their bodies and will look to use any changes in air pressure that can help them hover for longer without using energy.