10 September 2024: Day 4, Tarifa and birding at the Strait of Gibraltar, WINGS Spain in Autumn. Click here for a map showing where I am today.

We are now at the Strait of Gibraltar, the closest point between Europe and Africa. At its narrowest the Strait is only 13 km (8.1 miles) across, flanked on each side by mountains.

The passage is longer east-to-west than it is wide –> 58 km (36 miles) from beginning to end. Its current, water flow and variable depth give scientists clues as to how it was formed.

Water flows through the Strait more or less continuously, both east and west, but the balance of water moves eastward from the Atlantic into the Mediterranean. That’s because the Mediterranean evaporates faster than tributaries and precipitation can fill it. The Atlantic pours in but the Mediterranean stays saltier than the Atlantic Ocean.

In fact, Mediterranean waters are so much saltier that they sink below the constantly incoming Atlantic and form a warm highly saline layer of bottom water called a thermohaline. You can see this in the schematic of the Camarinal Sill which is west of the Narrows.

While the Camarinal Sill is the shallowest point (280 m = 920 ft deep), the deepest point is at the Narrows (900 m = 3,000 ft deep). Deep water is black on this underwater topo map.

With only 25km (15.5 miles) between the two points the flow often generates internal (subsurface) waves that can be seen from satellites, though they don’t look like waves in surface navigation.

And there is certainly a lot of surface navigation! Every time we look at the Strait there are ships on it. The Strait is the one of the busiest and most crowded waterways in the world. Half the world’s trade and one third of the world’s oil and gas pass through the Strait, according to a CityAM article in 2015.

Incredibly, the Strait of Gibraltar did not exist in the Miocene era when saber-toothed tigers roamed the earth. Here’s how the Strait came to be.

About 6 million years ago plate tectonics(*) closed the connection between the Mediterranean and the Atlantic. Without an influx of water the entire Mediterranean Sea evaporated, leaving behind salt and gypsum deposits. This Messinian Salinity Crisis lasted roughly 700,000 years. Then, rather suddenly, a leak developed between Africa and Europe. It quickly eroded a deep channel to the Mediterranean and eroded backwards (upstream) to create the Gulf of Cadiz. This video of an experimental dam failure shows how that looks.

The Zanclean Flood happened quite rapidly! The Mediterranean filled up within two years!

90% of the Mediterranean Basin flooding occurred abruptly during a period estimated to have been between several months and two years, following low water discharges that could have lasted for several thousand years. Sea level rise in the basin may have reached rates at times greater than ten meters per day (30 feet per day).

— Wikipedia: Zanclean Flood account

The result is an underwater topography carved by the flood. This diagram shows the marine floor but is not to scale (described below). It comes from ScienceDirect: The Zanclean megaflood of the Mediterranean – Searching for independent evidence — Daniel Garcia-Castellanos et al.

Cartoon (not to scale) showing the timing and the flood erosional and depositional features expected in the eastern and western basins of the Mediterranean as a result of a large water input through the Gibraltar gateway, showing 5 stages: 0) Initial level before significant Atlantic inflow occurred; 1) time of maximum erosion rate at the Strait of Gibraltar; 2) the western basin level reaches the Sicily Sill. 3) eastern basin level reaches the Sicily Sill; and 4) the Mediterranean is filled to the normal oceanic level. — ScienceDirect: The Zanclean megaflood of the Mediterranean – Searching for independent evidence — Daniel Garcia-Castellanos et al)

Everyone had to run away from the flood while the Strait was being formed.

(*) p.s. Plate tectonics closed the Strait 6 million years ago. At present the African Plate is subducting under the Eurasian Plate in the Mediterranean region. Africa is slamming into Europe, albeit very slowly.

9 September 2024: Day 3, birding in Chipiona and driving to Tarifa, WINGS Spain in Autumn
Click here to see (generally) where I am today.

Yesterday we went birding at local hotspots near Chipiona. One of the best was Playa de Montijo at the mouth of the Guadalquivir River. We arrived just after high tide so the cobbles beyond the sand (not shown above) were exposed yet the birds were still close.

(embedded Google map of Playa de Montijo, Chipiona, Spain)

Since I wrote this article before this trip, the birds shown below are a selection of what was seen a year ago in September 2023. We saw them!

Of the 11 species of living oystercatchers, only the Eurasian oystercatcher (Haematopus ostralegus) has a distribution in Europe and Asia north of the equator. It is a Near Threatened species. We saw several noisy groups bowing and shouting, 25 in all.

A few whimbrelds fed near the oystercatchers. Interestingly the whimbrel (Numenius phaeopus) has recently been split into Eurasian whimbrel and Hudsonian whimbrel. Some taxonomic authorities do not accept the split and have kept them as one species. Alas, eBird is one of them so I won’t gain this Life Bird until eBird says changes its mind.

The bar-tailed godwit (Limosa lapponica) is the most cosmopolitan of godwits, breeding in northernmost Eurasia and Alaska and spending the winter on the coasts of Africa, southern Asia, Australia and New Zealand. Its close relatives — black-tailed, Hudsonian and marbled godwits — are all New World birds.

The common greenshank (Tringa nebularia) resembles a greater or lesser yellowlegs except that its legs are green. All of them are in the Tringa genus.

Last September there was quite a collection of birds on this rocky bar including Eurasian oystercatchers, a black-bellied plover, common terns and the rare-to-the-area elegant tern. We saw them all plus a rare roseate tern.

A bonus for me was the slender-billed gull (Chroicocephalus genei).

Great birding from the beach.

8 September 2024: Day 2, birding near Chipiona, WINGS Spain in Autumn. Pelagic eBird Hotspot = Salida Pelágica desde Chipiona Click here to see (generally) where I am today.

This morning we’re at the coast in lovely weather. But out to sea, beyond the continental shelf, the wind is blowing hard and the sea is too rough for our planned pelagic birding tour in the Gulf of Cadiz in a boat that comfortably holds 12 people.

No problem. There are plenty of birds to see from land, some of which we would have seen on the boat and I’m willing to pass up an opportunity to learn about seasickness.

One of birds we cannot see from land is the critically endangered Balearic shearwater (Puffinus mauretanicus).

The Balearic Shearwater is one of the rarest birds in Europe, being one of just two species there to be listed as Critically Endangered on the IUCN Red List. Its successful conservation will require considerable effort, across country borders. As its name implies, as a breeder this shearwater is confined to the Spanish-owned Balearic Islands in the western Mediterranean, where its population is usually estimated at just 1,800–2,500 breeding pairs (of which about 50% nest on Mallorca).

… Counts at sea (especially through the Strait of Gibraltar) suggest it is possibly more numerous, which has led to a revised estimate of 24,000–26,500 individuals.

— Birds of the World: Balearic shearwater account

This bird might be really rare. Or there might be 10 times as many as we thought.

Balearic shearwaters are regularly seen in autumn in the Gulf of Cadiz as they leave their breeding grounds on the Balearic Islands in the Mediterranean …

… and move to the Atlantic and the North Sea for the winter.

To me shearwaters all look the same so I would never have known I was looking at a Balearic shearwater without a guide. They’re a little like confusing fall warblers. 😉