Sparrowhawks’ small numbers down to natural chain of life
SINCE the Sparrowhawk is a common and widespread bird of prey, why don’t we see large numbers of them about our towns and countryside?
The answer lies in the concepts of energy flow, food chains and pyramids of numbers.
Life on Earth is powered by the Sun. Light shines down from the Sun and makes plants grow; in other words, the solar energy is converted into food energy. Sunshine gets stored in the seeds of the amazing and very familiar structure that is the Dandelion clock.
Goldfinches feed almost exclusively on seeds and they are common on people’s garden lawns delicately extracting seeds from a Dandelion clock. A hunting Sparrowhawk swoops over the garden wall and the unfortunate Goldfinch’s ability to feed is brought to a sudden and unexpected end.
But death is not an end; the flesh of the Goldfinch gives live to the Sparrowhawk. The energy that came from the Sun was converted to make the Dandelion seeds that fed the Goldfinch made the meat that fed the bird of prey.
In physics, the Law of Conservation of Energy tells us that energy is conserved by changing from one form to another. By a series of conversions, the energy from the sun was transferred from our nearest star first to the Dandelion then to the Goldfinch and ended up in the Sparrowhawk.
Since the energy flowed along that particular path via feeding these three life forms are said to form a food chain. The Dandelion, Goldfinch and Sparrowhawk are individual links in a three-link chain.
If a Sparrowhawk eats a few Goldfinches every day, if his or her partner does the same and since their chicks need to be fed too, it follows that it takes a large population of Goldfinches to support a small number of Sparrowhawks. It also follows that it takes a pretty large crop of Dandelions to feed all the Goldfinches.
The population sizes involved can be modelled as a pyramid of numbers. The large population of Dandelions makes up the broad base section of the pyramid. The smaller population of Goldfinches makes up the narrowing middle section of the pyramid and the small population of Sparrowhawks is represented by the pointed top.
Because the Sparrowhawk is a top predator at the end of its energy flow pathway or food chain it must always exist a relatively small numbers.
The Sparrowhawk is a common and widespread but never numerous bird of prey.