In general, then, the smaller (and slower) the bird, the more it can be affected adversely by unfavourable winds, and the greater the safety margin of fuel it is likely to need on migration. Loss of flight range through flying against a headwind cannot be prevented, but it can be reduced by flying somewhat faster than Vmi if sufficient power is available. Small birds are better off than large ones in this respect, on account of their greater power margin, which gives greater endurance. This is of special importance for landbirds blown out to sea, when they may be better off slowing down to Vmp to conserve energy and thereby remain airborne as long as possible on the chance of reaching land. In fact, the best strategy for a bird attempting to achieve maximum range would be to fly slower with a tailwind and faster against a headwind (Figure 3.4). Such adjustments have been shown to occur. For example, the ground speed of night migrants tracked by radar increased with a tailwind, but less than expected (3-4 knots for a 10-knot

Figure 3.4 Flight speed in relation to power requirement in different wind conditions. The 'power curve' is U-shaped, so that the maximum range on a given fuel reserve will come near the bottom of the power curve (Figure 3.1). It will not come at the very bottom because the slight increase in power needed to fly faster is compensated by the extra length of journey achieved. The best possible speed for cruising is given by the longest tangent from the power curve which passes through the origin (Vm(o)) in still air). The graph shows the situation in still air. However, with a wind blowing, the graph would need to be altered to take into account the real distance over the ground achieved by the bird. This can be done by shifting the origin of the graph to the left if the bird is being helped by a tailwind (VW) and to the right if the bird is being hindered by a headwind. The resulting tangents, giving the best speed for the maximum range, show that the bird should fly slower with a tailwind (Vmr(W)) and faster against a headwind, compared to the maximum range speed in still air (Vmr(0)). In practice, the amount by which the best speed is varied turns out to be much less than the wind speed encountered. The effect of headwinds and tailwinds will be much greater on slower-flying than faster-flying species which in general have high maximum range speeds. From Pennycuick (1975).

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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