Barcelona has a new resident in its green spaces: the Rosy Eye Green Hairstreak butterfly (Callophrys avis), an insect never before recorded in the city. The discovery was made in the Horta Labyrinth Park during a survey by the uBMS urban butterfly citizen observatory. This finding increases the number of diurnal butterfly species observed in Barcelona to 52, which is approximately a quarter of all species found in Catalonia.
The discovery is particularly important because this is not just any species. The Rosy Eye Green Hairstreak is closely linked to specific ecological conditions. It is a univoltine species, meaning it has only one generation per year, flying exclusively between March and May. It relies heavily on plants such as the strawberry tree and rockrose. Its presence is therefore good news for urban biodiversity and an indicator of the quality of the habitats where it can survive.
The Callophrys avis, or Rosy Eye Green Hairstreak butterfly, is commonly known as 'cellarossa' and belongs to the Lycaenidae family. Its main features include bright green undersides to its wings, crossed by a faint white line. Its distinguishing characteristic is a halo of rust-coloured, orange, or yellowish scales surrounding its eyes.
Butterflies as Environmental Indicators
Diurnal butterflies, like the Rosy Eye Green Hairstreak, are considered some of the best bioindicators of environmental quality. They respond quickly to changes in vegetation, climate, and the management of green spaces. Their high sensitivity to these changes provides valuable information about the state of urban biodiversity. In their larval stage, these species depend exclusively on a small number of plants. When these plants disappear, their populations can decline sharply. Monitoring their evolution helps to better understand the health of nature within the city.
Beyond their ecological value, butterflies play a fundamental role in urban ecosystems as primary consumers and as a food source for numerous predators and parasitoids. They also help connect citizens with urban nature due to their ease of identification and their high educational and aesthetic value.
Citizen Science and Monitoring Efforts
This discovery was made possible by the work of nearly 40 volunteers participating in the uBMS (urban Butterfly Monitoring Scheme) citizen observatory. This project is coordinated by CREAF and has operated in Barcelona since 2018 in collaboration with the Municipal Institute of Parks and Gardens. Between March and November, this network of volunteers conducts weekly butterfly monitoring. Visual censuses are carried out in 29 parks and gardens to record species present and track their evolution over time.
In addition to the uBMS observatory's monitoring, Barcelona also participates in the Catalan Butterfly Monitoring Scheme (CBMS), promoted by the Granollers Museum of Natural Sciences. This scheme conducts monitoring along two routes in Montjuïc and Carmel.
Challenges and Future Outlook
Despite the increase in observed species, data shows that urban biodiversity is still heavily influenced by the most adaptable species. The five most common butterflies account for 64% of all observations. This indicates that the city continues to favour generalist species, while more demanding species require better-preserved and connected habitats.
To address this, Barcelona is promoting naturalisation measures such as creating urban meadows and grasslands, implementing low-intensity forest management in areas like Montjuïc, the Tres Turons, Collserola, and the Rec Comtal, and establishing biodiversity refuges. The aim is to improve ecological connectivity and offer better conditions for species with more specific requirements, such as the Rosy Eye Green Hairstreak.
Accumulated data also shows the impact of climate and urban management. In 2020, with less intervention in green spaces during lockdown and a wetter year, butterfly populations increased notably. In contrast, the prolonged drought between 2021 and 2023 caused populations to decrease. Since 2024, however, records indicate a progressive recovery. The presence of the Rosy Eye Green Hairstreak confirms that even in a large city, small changes in green space management can open the door to more wildlife. It also reminds us that butterflies not only bring beauty, they are a key part of urban ecosystems and a valuable tool for understanding Barcelona's biodiversity.
