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Article

Migration Phenology and Spatial Distribution of Soaring Birds in Greece: From Citizen Science Data to Implications for Monitoring and Conservation Strategies

by
Anastasios Bounas
1,2,*,
Nikos Tsiopelas
1,
Angelos Evangelidis
1 and
Christos Barboutis
1
1
Hellenic Ornithological Society/BirdLife Greece, 52 Ag. Konstantinou Str., GR-10437 Athens, Greece
2
Department of Biological Applications and Technology, University of Ioannina, GR-45110 Ioannina, Greece
*
Author to whom correspondence should be addressed.
Birds 2025, 6(1), 6; https://doi.org/10.3390/birds6010006
Submission received: 1 November 2024 / Revised: 20 December 2024 / Accepted: 13 January 2025 / Published: 16 January 2025
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Simple Summary

Every year, millions of birds migrate through Greece on their way between Europe, Asia, and Africa. Understanding when and where these birds travel is important for their protection, but studying them across the whole country is challenging due to limited resources and Greece’s complex landscape of islands and mountains. In our study, we used observations reported by citizen scientists (mainly birdwatchers) on the eBird platform. By analyzing thousands of these submitted reports from 2010 to 2023, we identified the timing and spatial occurrence of 18 different migratory bird species. We found that while some birds migrate at different times in the spring, many species move together in early September during the autumn migration. Important migration paths include Greece’s coastlines, islands, like Crete, and mountainous areas in the mainland, which serve as crucial flyways. Our research shows that by combining the efforts of citizen scientists with advanced analyses, we can better understand bird migration patterns. This knowledge helps us protect these birds by informing conservation plans and ensuring that key habitats along their migration routes are preserved.

Abstract

Timing and spatial distribution patterns of migratory birds are crucial for their conservation, particularly in Greece, which serves as a vital migratory corridor between Europe, Asia, and Africa. Traditional monitoring methods face challenges due to resource limitations and the country’s complex geography. This study aimed to determine the migration phenology and spatial distribution of 18 species of raptors and soaring birds in Greece using citizen science data from eBird, analyzed with generalized additive models (GAMs). We processed 15,940 checklists for spring migration and 9131 for autumn migration from 2010 to 2023. GAMs successfully modeled the migration phenology for most species, revealing variable peak migration dates in spring and more synchronized migration in autumn, with most species migrating in early September. A spatial analysis highlighted the importance of coastal areas and islands (particularly the Aegean islands and Crete) as key migratory routes and stopover sites. Validation with standardized counts from the Antikythira Bird Observatory showed some discrepancies, emphasizing the limitations of relying on a single monitoring site and the value of broad-scale citizen science data. Our findings demonstrate the effectiveness of integrating citizen science data with robust analytical techniques to fill knowledge gaps, providing valuable insights for designing monitoring programs and informing conservation strategies.

1. Introduction

Every year, millions of raptors and other soaring birds undertake extensive migratory journeys between their breeding grounds in the Palearctic region and their wintering sites [1]. Seasonal changes in resource availability drive these migrations and are among the most remarkable phenomena in the natural world [2]. Ecological barriers, like the Mediterranean Sea, can challenge species’ migration. Large raptors and other soaring birds rely heavily on thermal currents and updrafts to conserve energy during long-distance flights, converging through specific flyways [3]. The vast expanse of the Mediterranean Sea poses a challenge for these species, as thermals over large bodies of water are minimal, making sustained flapping flight energetically costly. However, some smaller raptors undertake long sea crossings to move between Europe and Africa [4].
Situated at the crossroads of Europe, Asia, and Africa, Greece occupies a strategic position in the Eastern Mediterranean that is a vital migratory corridor for a multitude of bird species, including raptors and other soaring migrants [5]. The country’s complex geography, characterized by an intricate mainland coastline, numerous islands, and rugged terrain, creates a network of landmasses that serve as leading lines for migrating birds. Islands and peninsulas, in particular, provide alternative routes through which birds minimize sea crossings through an island-hopping strategy [6,7,8], rendering them especially important for species that are willing to undertake shorter sea crossings or for inexperienced juveniles that may not yet have established migratory routes [4,9,10].
Despite significant advances in technology and the extensive study of large-scale raptor migration [11], monitoring migration across broad geographic areas, particularly in regions with complex landscapes like Greece, remains a logistical and resource-intensive challenge. The advent of citizen science and the participation of the public in scientific research has become increasingly significant in conservation science, making available an unprecedented volume of data that can be used to address changes in migration and distribution over broad spatial and temporal scales even in data-poor regions [12,13,14,15]. These databases allow for the study of species across their entire range over several years by utilizing the collective efforts of birdwatchers worldwide, efforts that would be impractical due to time and resource limitations if conducted solely by professional scientists [16]. Among the most well-known platforms facilitating ornithological citizen science is Cornell University’s eBird platform (www.ebird.org), which provides essential data for bird migration studies. eBird is a comprehensive database hosting a vast volume of bird count data, known as “checklists” [17]. Researchers have effectively utilized these semi-structured survey data to investigate a range of topics, including patterns of species occurrence [18,19], bird distribution mapping [20], migration dynamics [21], population monitoring efforts [22,23], and conservation decision-making [24,25]. Therefore, the participation of citizen scientists in Greece presents an invaluable opportunity to fill critical gaps in our understanding of raptor migration patterns.
In this study, we aim to determine the migration phenology and spatial distribution patterns of raptors and other soaring migrants in Greece using citizen science data analyzed through generalized additive models (GAMs). We further validate GAM-based findings with data from a systematic raptor migration observatory. Overall, this study provides a scientific basis for designing and implementing monitoring programs and informing policy decisions by competent authorities responsible for wildlife conservation.

2. Materials and Methods

2.1. Data Collection

We used observation data from eBird [17] by downloading the most recent eBird Basic Dataset (www.ebird.org/science/download-ebird-data-products; EBD version released in June 2024; accessed on 24 June 2024). Records were filtered in R [26] using the package “Auk” [27] to include only complete, non-duplicate checklists occurring in Greece. We kept only records from the last 15 years (from 2010 onward) and used checklists from February to May and August to November to assess phenology for spring and autumn migration, respectively. To minimize variability in detectability across checklists, we restricted them to stationary or traveling counts lasting less than 5 h, covering distances under 5 km, and with fewer than ten observers per checklist [28]. The checklist dates were converted to days of the year (doy) for further analyses.
We focused our analysis on 18 migratory species of raptors and other soaring birds that are frequently encountered during migration in Greece. This selection was guided by three key aspects: (i) the species’ prevalence in citizen science datasets (e.g., frequency of detection in eBird checklists); (ii) their conservation relevance, as some of these species are listed as Critically Endangered or Endangered at national or international levels (e.g., Egyptian Vulture Neophron percnopterus); and (iii) their representation of different functional groups with different migration strategies and habitat preferences: harriers (n = 3), eagles (n = 5), falcons (n = 3), other raptors (n = 4), and other soaring birds (n = 3). Some of the species breed locally in Greece, while others are transiently breeding in northern latitudes (Table 1).
To validate estimations from citizen science observations, we also analyzed count data from a standardized long-term monitoring program of autumn raptor migration held in the Antikythira Bird Observatory, run by the Hellenic Ornithological Society. Migration counts take place annually from mid-August to the beginning of October. Data were retrieved for the same 18 species for the last 11 years (2013–2023). The description of the migration phenology of the species using standardized counts is presented as a median and an interquartile range to characterize the central tendency and variability of migratory timing [29]. We calculated the Pearson’s correlation coefficient (r) to assess the relationship between the two datasets for each species. Correlation analyses were conducted separately for spring and autumn migration.

2.2. Statistical Analyses

The daily proportion of eBird checklists that reported the presence of each species was modeled as a function of doy using generalized additive models (GAMs) in the mgcv package in R [30]. We fitted a separate model for each species, with the proportion of presence as the response variable and day of the year as a cyclic cubic smooth. We used a binomial error structure and fit the models using Restricted Maximum Likelihood (REML). The daily proportion of presence data was weighted by the number of checklists per day [31]. The model fit was evaluated by checking convergence by visually inspecting residual plots and examining diagnostic information produced using the gam.check() function within mgcv. If any problems were detected, the model for that species was excluded from further analysis [32].
To identify the species’ migration phenology, we followed the approach presented in [31]. Briefly, we first defined the migration period by calculating the second derivative of the fitted GAM [12]. Since the second derivative of a function is the change in the slope, peaks in the second derivative represent the beginning of migration (increase in the proportion of lists reporting each species’ presence) or the end of migration (decrease in the proportion of lists reporting each species’ presence). We visually inspected each curve to avoid capturing false starts and ends since second derivatives are dependent on the shape of the smoothed curve estimated by the GAM. After identifying the start and end points, we calculated the area under the curve between those points and calculated the phenology metrics based on quantiles of that migration period. We defined the migration range as the duration (days) between the 0.1 and 0.9 quantiles (corresponding to the days when 10% and 90% of migration has occurred). The 0.5 quantile (migration midpoint) corresponds to the median day of migration. Specific details can be found in the Supplementary Materials and commented code provided in [31].
To visualize species occurrence in Greece and assess any hotspots while accounting for sampling effort, we created frequency of occurrence maps for each species, i.e., we calculated the observation frequency (number of checklists where a species was observed divided by the total number of checklists) within each cell of a 20 km grid. This standardization accounts for differences in sampling effort across space, as higher observation frequencies in areas with more checklists reflect increased species occurrence rather than higher sampling effort. The analysis was performed in QGIS v.3.22.6.

3. Results

A total of 15,940 checklists for the spring migration period and 9131 for the autumn migration period were obtained from eBird, spanning from 2010 to 2023 throughout Greece. During the spring migration, the most frequently encountered species included the White Stork (5.6% of the total checklists), followed by the Short-Toed Eagle (5.3% of the total checklists). In contrast, the rarest observed species were the Egyptian Vulture and the Imperial Eagle, with both species being present in less than 0.1% of the total checklists. During autumn, the most prevalent migrants were the Marsh Harrier (8.8% of the total checklists), followed by the Short-Toed Eagle and the Honey Buzzard (4.6% and 2.8% of the total checklists, respectively).
Generalized additive models (GAMs) were successfully fitted for 16 out of the 18 species for both the spring and autumn migrations. Models for the Egyptian Vulture and the Lesser Kestrel in spring and for the Imperial Eagle and the Eleonora’s Falcon in autumn were excluded due to convergence issues or poor diagnostic results upon inspection. The explained deviance ranged from 3% to 85% across species, with most species showing a significant effect of day of the year on detection probability (p < 0.05; Table S1). However, certain species exhibited low explained deviance, for example, the Black Kite and the Osprey in spring (explained deviance = 3.1% and 5.9%, respectively) and the Great White Pelican in autumn (explained deviance = 9.8%), showing weak associations between migration timing and day of the year (doy). Peak migration during spring was highly variable among species (Figure 1; Table S2), but for the majority, it occurred in late April. Species such as the Imperial Eagle, the Black Kite, and the Marsh Harrier exhibited earlier migration midpoints during the spring compared to others (early March). Latest spring migrants included the Eleonora’s Falcon, the Great White Pelican, and the Honey Buzzard (mid-May). The species with the longest spring migration range periods included the White Stork (49 days) and the Booted Eagle (46 days). For the autumn migration, peak dates were less variable than in spring, with most of the species showing migration midpoints during the first half of September (Figure 2; Table S3). The latest autumn migrants included the Great White Pelican and the Red-Footed Falcon (late September–early October). The quickest passage was estimated for the Marsh Harrier and the Red-Footed Falcon (17 days), whereas the Black Kite exhibited the most extended autumn migration range (49 days).
Data from the autumn raptor migration counts at the Antikythira Bird Observatory provided a comparative dataset for validating the GAM-derived phenology metrics. In total, 38,306 individuals from the 18 study species were observed during systematic observations from 2013 to 2023 (Figure 3; Table S4). The Honey Buzzard was the most numerous species (78.8% of all observations), followed by the Marsh Harrier (13.2% of all observations). The rarest species include the Egyptian Vulture, the Red-Footed Falcon, the Lesser Kestrel, and the Short-Toed Eagle (<0.2% of all observations). Although some peak migration dates were comparable between the two datasets, we found an overall weak positive correlation (Figure 4) between the GAM-derived phenology metrics and the observatory data (Pearson’s correlation coefficient, r = 0.3, p = 0.16). For some species, particularly those with low detectability at the Antikythira Bird Observatory, the estimated peak migration dates differed substantially between the two data sources. For example, Osprey GAMs estimated their peak passage to occur 14 days later than standardized counts, whereas for the Levant Sparrowhawk and the Lesser Spotted Eagle, the peak passage was estimated to occur 25 and 20 days earlier.
Regarding the spatial distribution of spring migrants (Figure 5), both the eastern and western coastlines of Greece consistently show high frequencies of observations for multiple species, suggesting that these areas serve as important migratory routes for soaring species. Additionally, the Aegean islands and Crete appear as important stepping stones for migratory species, especially sea-crossing raptors, such as harriers and falcons. Mountainous and semi-mountainous regions in mainland Greece appear as high-frequency areas for large raptors. For autumn migrants (Figure 6), the importance of coastal areas, especially around Attica and the Peloponnese, is further highlighted, as they are used as critical corridors for multiple species heading south. Greek islands, particularly Crete and the Dodecanese, as well as Ionian islands, stand out again as key sites for sea-crossing raptors, such as the Honey Buzzard, the Short-Toed Eagle, and the Marsh Harrier.

4. Discussion

Understanding the migration phenology and spatial distribution of raptors and other soaring migrants is essential for their conservation, particularly in regions like Greece that serve as critical migratory pathways. This study was motivated by the need to address knowledge gaps due to challenges in traditional monitoring methods, such as resource and geographical limitations. By leveraging citizen science data from eBird and applying GAMs, a robust statistical framework to analyze complex and non-linear ecological data, we aimed to provide a comprehensive analysis of migration patterns at a national scale. Our findings demonstrate the effectiveness of this approach, revealing detailed migration phenology estimates for multiple species and identifying key migratory corridors. Furthermore, validating these models with systematically collected data enhances the credibility of the findings, not only highlighting the utility of citizen science data in ecological research but also offering practical insights for designing new monitoring actions and informing conservation strategies.
Our findings align with the general understanding of raptor migration in the Mediterranean [33,34,35,36]. The most frequently encountered species in autumn eBird checklists were the Marsh Harrier, the Honey Buzzard, and the Short-Toed Eagle, directly confirming previous observational estimates [35]. During the spring migration period, peak migration dates ranged widely, reflecting species-specific migration strategies and ecological requirements. For example, Greece hosts both migratory and wintering populations of certain species, such as the Imperial Eagle and the Marsh Harrier [37,38]. Therefore, in spring, distinguishing between migrating individuals and wintering can be challenging, and so the apparent variability in migration timing might be inflated. However, it should be noted that such variation in timing among observation sites in Greece has been reported for certain raptors [35]. On the other hand, the later migration of species such as the Eleonora’s Falcon is consistent with its unique breeding ecology. The Eleonora’s Falcon breeds later in the year to coincide chick-rearing with the autumn passage of migratory passerines, which serve as its primary food source [39]. In contrast, autumn migration exhibited less variability in peak dates, with most species migrating during the first half of September. This convergence suggests that autumn migration is more synchronized among species, possibly due to geographical factors, and many species might funnel through specific corridors, like the eastern Aegean or Ionian flyways, resulting in more synchronized migration timing [35,40], or congregate in areas before migration, exploiting seasonal resources [41,42,43]. Moreover, significant information can be derived from the skewness of migration curves; the right-skewed migration curves observed for species like the Levant Sparrowhawk in spring are consistent with a sudden influx of migrants early in the migration period. This pattern has been suggested for the species, with its main migratory flow passing through the Bosphorus during both spring and autumn [35,44]. This migration strategy, avoiding flying over the sea but rather performing a detour around the coast of the Aegean Sea, explains the discrepancy between GAM-derived and standardized count estimates of peak migration for the species, as mostly some juveniles migrate on a broad front during autumn across the island of Antikythira [45].
One potential explanation for the observed discrepancies in phenology estimates between the Antikythira Bird Observatory and eBird checklists is the difference in data collection protocols. While the ABO conducts systematic, daily migration counts, eBird checklists are opportunistically submitted. This difference in temporal effort may influence phenology estimates; however, our approach of weighting daily eBird checklist data in the GAMs addresses much of this bias. Furthermore, such discrepancies between GAM-derived phenology metrics and standardized counts for certain species highlight the complementary nature of these two data sources and the limitations of relying on a single monitoring site to represent national-scale migration phenology. Antikythira’s geographical position makes it a key site for observing species that often perform sea crossings, but it may not adequately represent species that prefer overland routes. For example, the Short-Toed Eagle and the Lesser Spotted Eagle both avoid crossing the Mediterranean Sea, performing a long detour and crossing the sea at the Bosphorus, so a few individuals—often juveniles—migrate through Antikythira [45,46]. Moreover, the Honey Buzzard performs a loop migration strategy, concentrating over the island of Antikythira in autumn but bypassing it in spring [40], whereas the Red-Footed Falcon migrates on a wider front and in a larger time period during spring, as opposed to the more concentrated autumn migration [47]. As such, citizen science data, which encompass a broader spatial coverage, may provide more accurate estimates for species that do not frequently pass over Antikythira. This emphasizes the need to establish additional monitoring projects along other critical flyways. Such initiatives should be launched in places such as the southwest end of the Peloponnese, east Crete, or the islands of Karpathos or Rhodes, Olympos mt., and the prefecture of Evros in northeastern Greece in order to sufficiently cover the majority of the migratory avian soaring species.
The use of GAMs applied to eBird data proved overall effective in modeling migration phenology for most species, demonstrating the potential of citizen science data to uncover phenological patterns in the absence of extensive standardized monitoring. However, some limitations do exist. For example, in some cases, it is very challenging to distinguish the migration timing for species that also breed in the country. A characteristic example was the Marsh Harrier during autumn, where the model estimated a quick passage, resulting in a discrepancy with the standardized count results. Furthermore, limitations of GAMs are mainly connected to observer bias. A characteristic example comes from the Lesser Kestrel, for which we were not able to calculate spring migration phenology metrics using this modeling approach. The reason for that was the fact that Greece hosts both migrating individuals and local breeders of the species, combined with increased birdwatching activity in urban areas, where the species breeds [48,49]. Therefore, for such species, the use of alternative methods, such as a ring-recovery analysis or the establishment of standardized spring monitoring schemes in key areas, would be much more effective [50]. Furthermore, our frequency of occurrence maps revealed gaps in spatial data coverage, with zero checklists in less accessible or less popular regions among birdwatchers. The absence of eBird checklists in these areas underscores the need for increased participation of citizen scientists across the entire country. Encouraging birdwatching activities in underrepresented regions can enhance data quality and provide a more comprehensive understanding of migration patterns at a national scale. Finally, while the GAMs provided useful insights into migration phenology for most species, some species exhibited low explained deviance, suggesting that factors beyond day of the year are influencing the migration phenology of these species. For instance, weather conditions (including wind speed and direction) are known to strongly influence the movement of soaring birds and may not be fully captured by day of the year alone [51].
Migrating birds often follow mountain ranges, sea coasts, and other “leading lines” [52]. Our results showed high frequencies of observations along the coastlines and islands during both spring and autumn migration, emphasizing their role as key migratory pathways. In fact, the significance of small islands and coastal sites in the Mediterranean as refueling sites has already been stressed in different spatial scales and for many different species [53,54,55,56]. We found an increased frequency of occurrence of the Red-Footed Falcon around coastal sites. The species indeed prefers lowlands covered with cropland and coastal areas with mosaics of cropland and natural vegetation as stopover sites during migration, which are sites that can be critical for sea-crossing raptors [47]. Therefore, deterioration of these vital sites in Greece may have disproportionate consequences for the migratory species that they support [57]. Moreover, the higher prevalence of the Egyptian Vulture during autumn, along with the observations—of mostly juvenile birds—in Antikythira [45], results from the fact that some inexperienced birds undertake sea crossings in their first migration [58]. This behavior, coupled with increasing winter occurrences of the species in Crete [59], highlights the need for targeted conservation measures in these areas, including habitat protection and monitoring of threats, such as poisoning and collision with infrastructure. The identification of key migratory routes has significant implications for renewable energy development, particularly wind farms. Coastal areas, islands, and mainland locations identified as critical corridors for sea-crossing raptors should be carefully considered in the planning and placement of wind energy projects. Offshore and onshore wind farms pose collision risks to migrating birds, and their placement should be informed by migration data to minimize impacts [60,61]. Environmental impact assessments must incorporate migration patterns to ensure that the energy infrastructure does not adversely affect raptor populations, whereas the implementation of mitigation measures, such as turbine shutdowns during peak migration periods or micrositing, can reduce the collision risks [62,63].
In the face of accelerating environmental changes, such as climate change and habitat fragmentation, accurate information on migratory patterns is essential. This research underscores the importance of integrating citizen science with robust analytical techniques to enhance ecological knowledge and inform decision-making processes, ultimately contributing to the preservation of raptors and the ecosystems they inhabit. By effectively mapping the migration phenology and spatial distribution of raptors and soaring birds in Greece, this study provides crucial insights that fill existing knowledge gaps and identify key migratory corridors requiring additional monitoring effort. The findings demonstrate the valuable role of citizen science in ecological research and highlight the potential for collaborative efforts to inform monitoring strategies and conservation policies. Furthermore, this study underscores the importance of establishing further monitoring projects for raptor migration, covering the key migratory corridors identified here, and the need for citizen participation in currently insufficiently represented regions in order to sufficiently monitor raptor and soaring bird migration at a national scale. Such integrative approaches are vital for protecting migratory species, ensuring that conservation actions are timely, targeted, and effective.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/birds6010006/s1, Table S1: Key outputs from the performance of GAMs for each species and migration period (spring and autumn); Table S2: GAM-derived spring migration phenology metrics for 16 species included in our analysis based on citizen science data. Values refer to day of the year (doy); Table S3: GAM-derived autumn migration phenology metrics for 16 species included in our analysis based on citizen science data. Values refer to day of the year (doy); Table S4: Autumn migration phenology for 16 species in Greece based on standardized counts from the Antikythira Bird Observatory. Values refer to day of the year (doy).

Author Contributions

Conceptualization, A.B., N.T. and C.B.; methodology, A.B. and C.B.; validation, A.B., N.T., A.E. and C.B.; formal analysis, A.B.; investigation, A.B.; resources, A.E.; data curation, C.B.; writing—original draft preparation, A.B.; writing—review and editing, N.T., A.E. and C.B.; visualization, A.B.; project administration, N.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the GREEN FUND in the framework of the funding program Natural Environment & Innovative Actions with the citizens 2023.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available in eBird (https://science.ebird.org/en/use-ebird-data/download-ebird-data-products, accessed on 30 October 2024). Data from the Antikythira Bird Observatory are available on request from the Hellenic Ornithological Society (https://ornithologiki.gr/en/, accessed on 30 October 2024).

Acknowledgments

We acknowledge the contributions of the many citizen scientists who submitted observations to eBird and the staff and volunteers of the Antikythira Bird Observatory for their dedicated monitoring efforts, namely Soner Bekir, Giacomo Biasi, Stratis Bourdakis, Tonia Galani, Giorgos Kouthouridis, Elisabeth Navarrete, Nikolas Probonas, Lavrentis Sidiropoulos, Konstantinos Vlachopoulos, and Aris Vouros. Their collective contributions have been instrumental in advancing our understanding of raptor migration in Greece. This is contribution No. 43 from the Antikythira Bird Observatory, Hellenic Ornithological Society/BirdLife Greece.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. GAM-derived spring migration curves along with migration midpoints (0.5 quantile, dotted line) for 16 species in Greece based on citizen science data. The shaded areas under each curve show the migration range (0.1 and 0.9 quantiles). The colors denote morphological groups (green = harriers, red = eagles, yellow = other raptors, orange = falcons, blue = other soaring birds).
Figure 1. GAM-derived spring migration curves along with migration midpoints (0.5 quantile, dotted line) for 16 species in Greece based on citizen science data. The shaded areas under each curve show the migration range (0.1 and 0.9 quantiles). The colors denote morphological groups (green = harriers, red = eagles, yellow = other raptors, orange = falcons, blue = other soaring birds).
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Figure 2. GAM-derived autumn migration curves along with migration midpoints (0.5 quantile, dotted line) for 16 species in Greece based on citizen science data. The shaded areas under each curve show the migration range (0.1 and 0.9 quantiles). The colors denote morphological groups (green = harriers, red = eagles, yellow = other raptors, orange = falcons, blue = other soaring birds).
Figure 2. GAM-derived autumn migration curves along with migration midpoints (0.5 quantile, dotted line) for 16 species in Greece based on citizen science data. The shaded areas under each curve show the migration range (0.1 and 0.9 quantiles). The colors denote morphological groups (green = harriers, red = eagles, yellow = other raptors, orange = falcons, blue = other soaring birds).
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Figure 3. Relative intensity of autumn migration for 16 species in Greece based on standardized counts from the Antikythira Bird Observatory. The vertical lines correspond with the species’ first and third quantile passage dates (dashed) and median migration dates (solid). The colors denote morphological groups (green = harriers, red = eagles, yellow = other raptors, orange = falcons, blue = other soaring birds).
Figure 3. Relative intensity of autumn migration for 16 species in Greece based on standardized counts from the Antikythira Bird Observatory. The vertical lines correspond with the species’ first and third quantile passage dates (dashed) and median migration dates (solid). The colors denote morphological groups (green = harriers, red = eagles, yellow = other raptors, orange = falcons, blue = other soaring birds).
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Figure 4. Scatter plot and correlation between GAM-derived (citizen science data) estimates of peak migration days and peak migration estimates based on standardized counts from the Antikythira Bird Observatory for the 16 species included in the analysis. Shading represents standard error.
Figure 4. Scatter plot and correlation between GAM-derived (citizen science data) estimates of peak migration days and peak migration estimates based on standardized counts from the Antikythira Bird Observatory for the 16 species included in the analysis. Shading represents standard error.
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Figure 5. Frequency of occurrence maps during spring migration for the 16 species included in the analysis in a 20 km grid across Greece. The color scale is used to represent frequency of occurrence ranges from 0 (a species is not present in any of the eBird checklists submitted at the grid cell) to 1 (a species is present in all eBird checklists submitted at the grid cell).
Figure 5. Frequency of occurrence maps during spring migration for the 16 species included in the analysis in a 20 km grid across Greece. The color scale is used to represent frequency of occurrence ranges from 0 (a species is not present in any of the eBird checklists submitted at the grid cell) to 1 (a species is present in all eBird checklists submitted at the grid cell).
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Figure 6. Frequency of occurrence maps during autumn migration for the 16 species included in the analysis in a 20 km grid across Greece. The color scale is used to represent frequency of occurrence ranges from 0 (a species is not present in any of the eBird checklists submitted at the grid cell) to 1 (a species is present in all eBird checklists submitted at the grid cell).
Figure 6. Frequency of occurrence maps during autumn migration for the 16 species included in the analysis in a 20 km grid across Greece. The color scale is used to represent frequency of occurrence ranges from 0 (a species is not present in any of the eBird checklists submitted at the grid cell) to 1 (a species is present in all eBird checklists submitted at the grid cell).
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Table 1. The 18 species included in our analysis. Rows in bold represent species that breed in Greece.
Table 1. The 18 species included in our analysis. Rows in bold represent species that breed in Greece.
Common NameScientific NameGroup
Pallid HarrierCircus macrourusharriers
Montagu’s HarrierCircus pygargusharriers
Marsh HarrierCircus aeruginosusharriers
Booted EagleHieraaetus pennatuseagles
Imperial EagleAquila heliacaeagles
Lesser Spotted EagleClanga pomarinaeagles
Short-toed EagleCircaetus gallicuseagles
OspreyPandion haliaetuseagles
Black KiteMilvus migransother raptors
Honey BuzzardPernis apivorusother raptors
Levant SparrowhawkAccipiter brevipesother raptors
Egyptian VultureNeophron percnopterusother raptors
Eleonora’s FalconFalco eleonoraefalcons
Lesser KestrelFalco naumannifalcons
Red-footed FalconFalco vespertinusfalcons
White StorkCiconia ciconiaother soaring birds
Black StorkCiconia nigraother soaring birds
Great White PelicanPelecanus onocrotalusother soaring birds
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MDPI and ACS Style

Bounas, A.; Tsiopelas, N.; Evangelidis, A.; Barboutis, C. Migration Phenology and Spatial Distribution of Soaring Birds in Greece: From Citizen Science Data to Implications for Monitoring and Conservation Strategies. Birds 2025, 6, 6. https://doi.org/10.3390/birds6010006

AMA Style

Bounas A, Tsiopelas N, Evangelidis A, Barboutis C. Migration Phenology and Spatial Distribution of Soaring Birds in Greece: From Citizen Science Data to Implications for Monitoring and Conservation Strategies. Birds. 2025; 6(1):6. https://doi.org/10.3390/birds6010006

Chicago/Turabian Style

Bounas, Anastasios, Nikos Tsiopelas, Angelos Evangelidis, and Christos Barboutis. 2025. "Migration Phenology and Spatial Distribution of Soaring Birds in Greece: From Citizen Science Data to Implications for Monitoring and Conservation Strategies" Birds 6, no. 1: 6. https://doi.org/10.3390/birds6010006

APA Style

Bounas, A., Tsiopelas, N., Evangelidis, A., & Barboutis, C. (2025). Migration Phenology and Spatial Distribution of Soaring Birds in Greece: From Citizen Science Data to Implications for Monitoring and Conservation Strategies. Birds, 6(1), 6. https://doi.org/10.3390/birds6010006

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