Bibio marci
In 3 days ~10% in a week ~20%
Bibio marci or St. Mark's fly or hawthorn fly, is a species of fly from the family Bibionidae. It is found across much of Europe. Their common name comes from the fact that the adults usually emerge around St Mark's Day, 25 April.
Adult specimens are usually 12-14mm in length. They can be observed hovering with their prominent legs dangling below. The adults feed on nectar and are considered an important pollinator. The body is entirely shiny black and covered in dark hairs, and the wings are translucent or milky-white with prominent dark veins along the leading costal margin.
The species exhibits sexual dimorphism in eye structure: Males have large eyes that meet at the top of the head (holoptic), divided horizontally into upper and lower sections. Females have smaller compound eyes set far apart on the sides of the head (dichoptic).
Like most bibionid larvae, they grow up in grassy areas and are herbivores and scavengers feeding on dead vegetation or living plant roots. Adult females usually lay their eggs in soil or decomposing vegetation. Bibio marci larvae are known to be root damage pests of celery, asparagus, roses, saxifrages... Wikipedia →
| # | Park | Country | Daily sighting probability |
|---|---|---|---|
| 1 | Bavarian Forest National Park | 🇩🇪 Germany | |
| 2 | Wadden Sea National Park | 🇩🇪 Germany |
Rare
Under 5% of wildlife watchers report a Hawthorn Fly sighting in Wadden Sea National Park on a given day, based on 5 verified sightings from GBIF citizen-science data from 2019 to 2026. It is the 699th most-seen of the 818 insects we can rate there. Over three days the chance rises to about 10%, and over a week to about 20%, if each day is a fresh chance.
April
In that month about 10% of wildlife watchers report a Hawthorn Fly in Wadden Sea National Park on a given day, against under 5% in May to March, the lowest months. That is one of Wadden Sea National Park's driest months, with about 45 mm of rain against 140 mm in November.
Between 2 and 5 pm
Across all the parks we cover, that three-hour window holds the most Hawthorn Fly sightings with a recorded time, and about 8 in 10 of them come between 6 am and 6 pm: it is a daytime animal.
2nd of 2
Of the 2 parks where we can rate Hawthorn Fly sightings, Wadden Sea National Park ranks 2nd at under 5%. The best odds are in Bavarian Forest National Park (Germany, about 10%).
GBIF citizen-science records
It is estimated from how often the Hawthorn Fly is recorded against all the insects logged in Wadden Sea National Park (5 verified sightings from 2019 to 2026), calibrated against known sighting rates. Figures are rounded to the nearest 10% because citizen-science data supports honest approximations, not decimals, and months with records from fewer than five people show "—". How it works.
