Citizen Science Enhanced Monitoring of Phenological Mismatches in Migratory Species Along Latitudinal Gradients

Abstract

Climate change is quickly changing the time synchrony between migrant species and the seasonal
periodicities of their habitats. Greece's complex topography, featuring over 3,000 islands and steep
mountain ranges, produces distinct phenological signals (e.g., in the European roller Coracias garrulus
and other trans-Saharan raptors). This paper uses more than 25,000 citizen science checklists (2010
2025) on platforms such as eBird and iNaturalist to observe the arrival of 18 indicator species. The
study uses a combination of these records and satellite-derived Normalized Difference Vegetation
Index (NDVI) to measure the progression of the so-called Green Wave, moving southwards
(Peloponnese) to northwards (Rhodope Mountains) respectively. Generalized Additive Models
(GAMs) statistical analysis demonstrates important changes in the migration phenology. Findings
show that despite the average of 0.42 days of advance in the yearly beginning of spring (green-up),
the long-range migrants have merely changed arrival dates by 0.18 days/year, which leads to an
increasing potential timing of the mismatch. This level of asynchrony is strongest at higher latitudes
(> 40oN) with the breeding thermal range being smaller. Regression models indicate that there is a
significant relationship (R2 = 0.68, p < 0.01) between warmer mean temperatures in April and a greater
magnitude of mismatch. These results indicate that northern Greece has been found to have the most
severe thermal delays, which can negatively affect the survival of fledglings because of the loss of
food coincidence. The research highlights the strength of citizen science in supplying high-resolution
spatiotemporal data required to monitor ecological changes over complicated landscapes. The findings
suggest that conservation of stopover sites in the Aegean flyway should be prioritized in order to
counter the cost of the increasingly complex phenological gaps of fitness.