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arrival dates advance more than in long-distance migrants (Rubolini et al., 2007). Climate change has impacted birds also in other ways, for example, altitudinal range shift (Kirchman and van Keuren, 2017). Despite many difference discussion, there is consensus that climate warming already has effect on phenology of bird species and populations.
I investigated long-term fluctuations in dates (1991–2016) of spring migration of the Eurasian Golden Oriole (Oriolus oriolus L.), a long-distance migrant, wintering south of the Sahara to examine migration phenology in relation to average spring air temperatures over the period of 26 years. This species is common breeding bird in northwestern Croatia.
MATERIAL AND METHODS
MATERIJALI I METODE
Research took place in Mokrice area (46°00´N, 15°55´E) in north-western Croatia from 1991 to 2016. Study area situated at an altitude approximately 140 m above sea level. The investigated area is a mixed farming area with small deciduous forest, hedges, orchards, lawns, arable land etc. This study was conducted in the small deciduous mixed forests consisting of Hornbeam (Carpinus betulus L.) and Pedunculate Oak (Quercus robur L.). The shrub cover is formed by Blackthorn (Prunus spinosa L.) and Common Elder (Sambucus nigra L.). Data is available for spring arrivals of Eurasian Golden Oriole detected by the first song. Bird singing was observed each year consistently from 10 April on the daily basis. Bird singing was observed twice a day; in early morning and early afternoon. Dates were converted to numerical values such that 1 indicates 1 April. In this study, I focused on the long-term data. Long-term data have been important for documenting impact of temperature change in spring phenology (Askeyev et al. 2009; Dolenec and Dolenec, 2011a; Dolenec, 2018a). In some papers, shifts have significantly been correlated with changes (increase) in local air temperature (Biaduń et al., 2009; Dolenec and Dolenec, 2011b). Spring temperature data (mean April–May temperature, 1991–2016) were supplied by the station of Maksimir (123 m a.s.l.) – Meteorological Office in Zagreb, about ca. 20 km from the study area (April–May, mean = 9.2 ± 1.37 ºC; range = 6.5 to 11.9 ºC).
The relationship between the timing of migration and mean spring air temperatures and year in study area was assessed using a simple linear regression and Pearson correlation tests (r). Statistical data processing was performed using the SPSS 13.0 for Windows.
The relevant variables used for this paper were the arrival date, mean spring temperature (April–May) and year. Correlation between study period (1991-2016) and arrival date was negative significant (r = – 0.48, n = 26, P = 0.013; y = 481.39 – 0.23x; Figure 1). The spring migration of Eurasian Golden Oriole advanced 0.23 days per year (statistically significant) on average