The most remarkable migrants – systematic analysis of the Western European insect flyway at a Pyrenean mountain pass

Abstract

In 1950 David and Elizabeth Lack chanced upon a huge migration of insects and birds flying through the Pyrenean Pass of Bujaruelo, later describing the spectacle as combining both grandeur with 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, while a lack of quantitative data has prevented insights into the ecological impact of this mass insect migration and into the factors affecting it. To address this, we revisited the site in autumn over a 4 year period and systematically monitored diurnal insect species and numbers. We document an annual mean of 17.1 million day-flying insects from 5 orders moving south, dominated by the Diptera, with "mass migration" events associated with warmer temperatures, the presence of a headwind, sunlight, low windspeed, and low rainfall. Finally, using observed environmental thresholds for migration, we estimate an annual "bioflow" of at least 14.6 billion day-flying insects migrating south over the whole Pyrenean Mountain range, highlighting the importance of this route for seasonal insect migrants.

Bat activity correlated with migratory insect bioflows in the Pyrenees

“…Numbers of nocturnal insects caught in the trap were largest when a southwesterly wind was present creating a headwind that prevented the insects from flying over the high peaks, when night-time temperatures were warmer, and when levels of precipitation were low or none. Interestingly, these meteorological conditions are broadly similar to the conditions needed for diurnal migration of other migratory species to occur through the pass, except of course for the presence of sunlight [ 19 ]. Like the nocturnal insects, the bats tended to show more activity when a southwesterly wind was present.…”

“…Although autumnal preferred migratory direction is towards the south, the pass is orientated southwest/northeast. Despite this, migration still occurs through the pass due to the channelling effect of the steep-sided Pyrenean valley, forcing the migrants through the pass under certain weather conditions [ 19 ]. We documented the nocturnal migration of insects for three consecutive autumns (September–October) in the years 2019–2021 while bat migration was studied during September 2019.…”

“…The first records of co-migration occurred in 1950 when ornithologists David and Elizabeth Lack recorded birds and insects migrating diurnally through the pass in large numbers [ 18 ]. Follow-up research between 2018 and 2021 revealed that huge numbers of diurnally migrating insects move through the pass each autumn following the Western European flyway [ 19 ].…”

The efficiency of varying methods and degrees of time compensation for the solar azimuth

“…(a) Estimating the sun's azimuth for separate locations Sun azimuth data from September, a peak month for autumn migration, were obtained for four known migration locations of different latitudes through which a diverse range of bird and insects migrate (the data year [2018] does not affect results). The highest latitude was the Falsterbo peninsula, Sweden (55.36°N, 12.81°E) [18][19][20], followed by the Pyrenean mountain pass of Bujaruelo (42.70°N, 0.06°W) [21,22], the Maghreb, Morocco (31.46°N, 7.83°W) [23][24][25][26], and lakes in Panama (9.17°N, 79.85°W) [5,10]. The data were downloaded from sunearthtools.com at 5 min intervals, so to estimate the sun's azimuth for the intermediate minutes, spline models were fitted to daily azimuth data using the smooth.spline(…, all.knots=T) and predict() functions from base R version 4.2.1.…”

“…Migratory activity is not constant and can vary between taxa from all day, to tailing off around sunrise and sunset, to concentrated at a particular daylight hours [21,[27][28][29]. To show how migratory efficiencies change in organisms where migration peaks at midday, as seen in many insects and some birds [27,28], we used radar data of hoverflies migrating in the UK in September [29].…”