I wouldn’t mind going there myself. But you know who really loves going to Spain? It’s not the British. It’s the invertebrates.

There is a path through they Pyrenees, the Pass of Bujaruelo, which is where all the insects funnel through on their way to summering in Spain. It’s been known for a long time that this is the place to find swarms of insects flocking south.

The Pyrenean mountain range and the Pass of Bujaruelo. (a) The fieldwork location (red marker) within the Pyrenean mountain range; image taken from Google Maps. (b) The valley that channels the insects over the Pass of Bujaruelo (red arrow) and the surrounding geological formations.

In autumn 1950 David and Elizabeth Lack chanced upon a huge migration of insects and birds flying through the Pyrenean Pass of Bujaruelo, from France into Spain, later describing the spectacle as combining both grandeur and novelty. The intervening years have seen many changes to land use and climate, posing the question as to the current status of this migratory phenomenon. In addition, a lack of quantitative data has prevented insights into the ecological impact of this mass insect migration and the factors that may influence it. To address this, we revisited the site in autumn over a 4 year period and systematically monitored abundance and species composition of diurnal insect migrants. We estimate an annual mean of 17.1 million day-flying insect migrants from five orders (Diptera, Hymenoptera, Hemiptera, Lepidoptera and Odonata) moving south, with observations of southward ‘mass migration’ events associated with warmer temperatures, the presence of a headwind, sunlight, low windspeed and low rainfall. Diptera dominated the migratory assemblage, and annual numbers varied by more than fourfold. Numbers at this single site hint at the likely billions of insects crossing the entire Pyrenean mountain range each year, and we highlight the importance of this route for seasonal insect migrants.

Who is caravaning over the mountains? Everyone. But mostly flies, and mostly pollinators. Lots of midges, gnats, hoverflies, all these small underappreciated flies that do a lot of the work of pollinating (it’s not just bees, you know.)

Classification of the migratory assemblage. Average ratios of insects showing migratory behaviour collected in the intercept trap and butterfly counts over 4 years sorted by (a) order and (b) family. (c) Migratory behaviour of insect groups based on southward scores. ‘High-altitude’ migrant insect groups (red), and ‘Flight Boundary Layer’ migrant insect groups (blue). Missing values indicate fewer than 100 individuals in the sample of head- or tailwinds. Southward scores of 100 indicate all individuals were going south, 0 that the same number were going south as north, and negative values indicate that more were going north than south under a particular wind condition.

We’re talking millions of migrants, probably billions. Of course, they estimate that the total mass of all those insects was about 140kg per season. One or two Cantabrian brown bears ambling across the pass would outweigh all those insects in mass, but would probably have a negligible ecological impact in comparison.

This was a study of one pass with a convenient bottleneck to enable effective counting. Insects are flooding across the length of the Spanish/French border, and further, some of them keep going across the Mediterranean and take tours of Morocco.

Numerous studies have found a consistent south or southwest bias in migratory insect headings across Europe, suggesting insects from a large geographical area are filtered down into the Iberian Peninsula, passing through the Pyrenees each season. This makes insect migration bottlenecks within the Pyrenees important locations for censusing species and monitoring numbers. We estimate that the total number of insects moving across the Pyrenees mountain range reaches into the tens of billions. This number is of comparable size to those from radar studies across a similar-sized area. Insects are known to cross the Pyrenees not only in the centre where the Pass of Bujaruelo is situated, but also along the coasts. Williams et al. observed some southward movement of butterflies in October along the coast at Argelès-sur-Mer, where the Pyrenees descend to the Mediterranean Sea. Similar southward movements occur along the Atlantic seaboard. To accurately quantify the total number of migratory insects crossing the Pyrenees, extensive deployment of monitoring resources and techniques is needed, including the use of vertical-looking radars.

I’m reading that and thinking that if I were a spider, I’d want to set up a nice web across the Pass of Bujaruelo. Unfortunately, this study didn’t look at spiders. They weren’t migrating, after all, they were just setting up shop in the pass and taking advantage of all the tourists.