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.

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