The bird does not so much fly as fold through the air above the cork oaks at Tarifa. It is just past nine in the evening and the last light is leaving the Sierra del Cabrito in long copper sheets. A European nightjar, Caprimulgus europaeus, lifts from a low branch and is gone before the eye can fix on it.
Dr. Ana Bermejo, a field ornithologist with SEO/BirdLife, has been waiting for this moment since February. She lowers her binoculars and notes the time in a battered red field book. The nightjars are early this year, she says, by perhaps four days.
Tarifa sits at the narrowest point of the Strait of Gibraltar, fourteen kilometres of water between Europe and Africa. For migrating birds it is the cheapest crossing on the continent, and for nightjars returning from wintering grounds in the woodlands of Mozambique and Zambia it is the necessary one.
They come through in pulses between mid-April and late May, riding the easterly levante when they can and the westerly poniente when they must.
Bermejo's study, funded by the Junta de Andalucia and running since 2019, has fitted thirty-eight adult nightjars with miniature geolocators weighing less than a gram. The recovered tags tell a story of astonishing precision. One male, captured at the same juniper grove in three consecutive Junes, had each time wintered within four kilometres of the same patch of miombo woodland near Tete.
The nightjar is a bird built for liminal hours. Its plumage is bark and ash and lichen. Its eye is enormous and dark. Its mouth, when opened, is wider than its head, and into it disappear moths, lacewings, beetles, the occasional confused bat.
By day it lies along a branch lengthwise, eyes closed to slits, and is more or less invisible. By night it hawks insects with a fluttering, erratic flight that has earned it the local Spanish name engañapastores, the shepherd-deceiver.
“By day it lies along a branch lengthwise, eyes closed to slits, and is more or less invisible.”
An older folklore held that nightjars stole milk from goats. Pliny believed it. The birds were named for it: capri-mulgus, goat-milker. The truth is duller and stranger. They merely hunted the moths that gathered around the warm flanks of livestock at dusk.
Bermejo and her field assistant, a young biologist from Granada named Marta Lopez, walk a transect each evening through a stretch of dehesa south of Los Barrios. The cork oaks here are widely spaced. The ground is thin and stony. Cistus and wild lavender perfume the warm air.
They are listening for the male's territorial song, a long mechanical churr that can run unbroken for five minutes and carries close to a kilometre on still nights. To the untrained ear it resembles a distant two-stroke engine. To Lopez, who learned to identify individuals by subtle differences in pitch, it is as legible as handwriting.
On the night of 27 April she identifies four distinct males singing within a single hectare. Two are residents from the previous season. One is new. The fourth she suspects is a bird from a study site in the Sierra de Aracena, sixty kilometres north.
The species is in trouble across much of its European range. The British Trust for Ornithology has documented a thirty percent decline in the United Kingdom since the 1970s. In Germany and the Netherlands the numbers are worse.
The reasons are familiar and tangled. Loss of heathland breeding habitat. The collapse of insect populations. Light pollution along migration corridors. The drying of Sahelian stopover sites.
Andalucia, for now, remains a stronghold. The dehesa system, that ancient mosaic of grazed oak savannah maintained for centuries by pig farmers and cork harvesters, produces exactly the conditions nightjars require. Open ground for hunting. Scattered trees for roosting. Thin soils that hold heat into the evening and bring out the moths.
Bermejo worries about a recent push to plant industrial olive groves on former dehesa land. She has written, with characteristic restraint, two papers documenting the loss.
Just before midnight Lopez catches a female in a mist net rigged across a forest track. The bird is calm in the hand. Her crop is full. She has been hunting heavily and is likely feeding nestlings.
Bermejo fits a numbered ring and a fresh geolocator, weighs and measures the bird, and releases her into the dark. The whole procedure takes under four minutes. The female flies low and silent toward the ridge.
Later, walking back to the field station with head torches off so as not to disturb the hunting birds, Lopez says quietly that the nightjars feel to her like a kind of message. Not from anywhere in particular. Just a message that the night is still being used.
The strait is loud with shipping. The wind farms above Tarifa turn ceaselessly. Somewhere out over the water a small grey bird is folding itself between continents, fed for a thousand kilometres on moths it caught above Andalusian oaks.
Bermejo locks the station door and turns off the porch lamp without being asked.
