DOI: 10.5061/dryad.vq83bk40v
title: The most remarkable migrants: Systematic analysis of the Western European insect flyway at a Pyrenean mountain pass

description: 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, with ‘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 with larger annual migration flows associated with
 higher autumn temperatures in Northwest Europe. 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.
description: # The most remarkable migrants: Systematic analysis of the Western
 European insect flyway at a Pyrenean mountain pass
 [https://doi.org/10.5061/dryad.vq83bk40v](https://doi.org/10.5061/dryad.vq83bk40v) This dataset is four years of insect migration observations during the months of September and October moving through the 30m wide Pass of Bujaruelo, 2300m up in the Pyrenees. We collected this data using a video trap and a malaise trap. Meteorological data was collected from meteoblue forecasting. ## Description of the data and file structure The data is organised into an excel file with separate sheets for different aspects. These aspects are: Number of insects in the pass; Nutrient transfer calculations; Migratory strength data; Hoverfly species in the pass; Total assemblage; Butterfly migration data; Intercept trap data. **Number of insects in the pass** This dataset is an overview of the numbers of insects recorded migrating through the Pass of Bujaruelo. This contains all the data from the four years of data collection. The columns are: Year; Date; Ordinal date (an ordinal format for the multiples of a day elapsed since the start of year); Columns D to H are various environmental variables at the Pass of Bujaruelo obtained from MeteoBlue forecasts and models; Daily video counts (the number of insects recorded by the video camera each day); Small Insect Migratory Traffic Rate = (count/30m)/8hrs (this is the calculation of the migratory traffic rate in numbers of small insects per metre per hour moving through the pass); Daily butterfly counts; Butterfly Migratory Traffic Rate = (count/30m)/8hours (this is the calculation of the migratory traffic rate in numbers of butterflies per metre per hour moving through the pass). **Nutrient transfer calculations** This dataset contains a list of the species and families of migratory insects recorded moving through the Pass of Bujaruelo. An estimate for the weights of these insects were determined and listed in column D, the citation (if used) noted in column C. The total number of individual insects were then calculated in column E. The total biomass (in kg) of these insects calculated in Column F, then the estimated totals of Nitrogen and Phosphorous (nutrients limiting to plant growth) are in columns G and H. Finally, brief calculations on the biomass of the insects per day are included.  **Migratory strength data** This dataset contains information on the migratory strength of the insects moving through the Pass of Bujaruelo. The southward score purports to the percentage of insects moving southwards on days when there is a headwind or a tail wind. 100% of the Pieris butterflies, for example, moved southwards during a headwind, and 100% moved southwards during a tail wind as well. The n/a values are because no insects representing the insect groups were recorded during tailwinds in the pass (it is thought that they were capable of migrating at higher altitudes during these weather conditions - see main manuscript). **Hoverfly species in the pass** This dataset contains a list of all hoverflies (Syrphidae) recorded migrating through the Pass of Bujaruelo including their estimated percentage of the total assemblage and their estimated numbers across the four years of data collection. **Total assemblage** This dataset contains an overview of the total assemblage of migratory insects in the pass. All recorded insects are listed here, including the ones deemed as non-migrants. Their percentage composition of the total assemblage is also included.  **Butterfly migration data** This dataset contains a breakdown of the butterfly migration data collected each year. Including the raw numbers for headwind and tailwind days. Calculations for scaling the counts to account for the time between counts (*32) and to allow comparison with intercept trap data (/15) are also included.  **Intercept trap data** This dataset contains an overview of the intercept (malaise) trap data. It contains raw counts for invertebrate families recorded including breakdowns for head and tailwind days for each year. The malaise trap was bidirectional which is why there are separate rows for south and north counts for each year. Blank spaces are present when that family was not recorded in the given year.  Below the year by year breakdown are summaries of the total (four years) data collection. This includes breakdowns for head vs tail wind days, as well as north vs south trap direction collections. ## Code/Software R code is also provided for analysing the impacts of environmental conditions on the migration of the insects.