# Long-Term Phenological Shifts in Butterfly Species from Transylvania, Romania—A Case Study

## TL;DR

This study shows that butterflies in Transylvania are emerging earlier in spring and flying longer in autumn due to climate change.

## Contribution

The study provides the first long-term phenological data on butterflies in Eastern Europe using historical and recent records.

## Key findings

- Spring-emerging butterflies now begin their flight periods about 15 days earlier.
- Autumn-active butterflies have extended their flight periods by an average of 23 days.
- Phenological shifts correlate with multi-decadal temperature and precipitation trends.

## Abstract

Climate change can significantly impact insects, such as butterflies, by shifting their geographic ranges, altering the number of generations they produce annually, or changing their flight periods. To date, most evidence of these effects comes from Western and Central Europe, where long-term monitoring programs have provided the necessary data to detect climate-driven trends. In contrast, Eastern Europe—often considered a biodiversity haven due to the relatively lower rate of land-use change—has lacked such data, making it difficult to assess climate change impacts in the region. In this study, we utilize an alternative data source: historical museum collections combined with contemporary records and long-term climate data. This approach allows us to investigate how the phenological cycles of 16 butterfly species with varying life history traits have responded to climate change. Our findings reveal that butterfly species emerging in spring now begin their flight periods approximately 15 days earlier, while those active in autumn have extended their flight periods by an average of 23 days. These shifts in flight periods may have significant ecological consequences, particularly through altered synchrony with host plants and other critical resources.

Insects can respond rapidly to climate change through population fluctuations, range shifts, altered voltinism, life cycle changes, flight period adjustments, behavioural shifts, and changes in habitat or food preference—often varying by region due to local environmental and anthropogenic factors. While the phenological cycles of diurnal lepidopterans have been extensively studied in countries with large monitoring networks, eastern and southeastern Europe remain under-researched. This study provides the first insights into phenological shifts in 16 butterfly species in Cluj-Napoca (Transylvania, Romania) between 1921 and 2023, using a unique dataset combining historical and recent records. The species studied include spring-emerging, multivoltine, and migratory butterflies. Phenological trends were analyzed in relation to long-term climatic data. Results show that spring species now emerge approximately 15 days earlier, and autumn species extend their flight periods by up to 23 days. These changes correlate with multi-decadal trends in temperature and precipitation. We also discuss changes in voltinism and migratory behaviour and the potential impacts of climate change on butterfly populations in the study region.