Follow the flight of the blackpoll warbler

Diversity is a mark of richness and environmental health - and birds are its flag-bearers.

Macaws.
Good migrations: macaws fly over a river in Peru. Photograph: Frans Lanting / Gallery Stock

Bird variation set both Charles Darwin and Alfred Russel Wallace on the path to discovering natural selection and its role in evolution. Wallace observed it on the Molucca Islands, Darwin on the Galapagos. Diversity is a mark of richness and environmental health and birds are its flag-bearers. Their disappearance, like the collapse of the canary in the mine, is our warning signal.

Yesterday, wondering where blue tits and great tits have gone this summer (am I providing the wrong kind of peanuts?), I found the Shakespearean phrase “nature’s riches” used for a site explaining the rich geological pickings of the Arctic, now being unlocked by global warming. It could equally refer to the tropical forests being destroyed to make monocultures such as palm oil. Maybe only migrating birds know (though it must be a special kind of “knowing”) the true force of the paradox that areas of greatest biodiversity are also areas of greatest conflict – and how fast biodiversity is being lost in our time.

This is true above all between Arctic and tropic. These words first occur in the late 14th century. In Greek, arktikos means “belonging to bears”. Not polar bears: people knew the night sky and the constellation of the Great Bear. Tropic, from the Latin tropicus and the Greek trope, meaning “turn”, refers to the daytime sky where the sun, after reaching its northernmost or southernmost point, stands still (solstitium) and turns back.

If all birds stayed in the tropics, many would die out through competition for food and nesting sites, so species evolved to migrate. As resources regenerate each spring in the north, millions of birds trade tropic for Arctic: less competition for insects, longer daylight hours to find them. When Arctic food dwindles in autumn, they return to the replenished tropics.

These journeys are risky but better than the alternative. Blackpoll warblers winter in Venezuela, then fly north in spring over the Gulf of Mexico up to Alaska, where they build little cup nests in fir trees. They take a different route back: they fly south-east over the Great Lakes and from the New England coast some follow the shoreline down to Florida, some island-hop to South America, but most fly east to pick up winds hurling them over the Caribbean non-stop, day and night, for 88 hours to Venezuela. Four days over the Atlantic in peak hurricane season, facing spiralling winds, clouds, rain. Many crash exhausted in the sea. Yet it’s worth the risk because this route is 1,500 miles shorter and survivors establish a winter feeding territory earlier.

Nature is prodigal, said Darwin. So much waste. “The face of nature may be compared to a yielding surface, with 10,000 sharp wedges packed closely together and driven inwards by incessant blows, sometimes one wedge being struck, and then another with greater force.”

His understanding of the conflict driving nature’s apparent harmony was triggered by reading “An Essay on the Principle of Population” by Robert Malthus. This led him to the concept of natural selection and also to ideas about the evolution of instincts and human psychology.He was young: after South America, his mind was racing. “The principle of population is strife”; “All forms compete against others”; “You can understand the true conditions of life only if you use your imagination to hold on to a sense of the ruthlessness of the natural forces that could waste the bright surface.”

This side of his work inspired Freud’s insight into psychic conflict and the origins of human aggression. “The human mind is shaped by its animal past,” said Darwin. Nature’s ruthlessness is ours, too: our bodies, our psyches.

Unlike a blackpoll warbler, we can’t fly away when resources are gone. In our time, it seems, Arctic and tropic will change their meanings again. But biodiversity may no longer be part of the equation.

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