Spatial patterns of habitat distribution of Corvidae (the case of urban-rural gradient)

Abstract

The spatial distribution and abundance of Corvidae species in Zhytomyr region was studied in terms of the urban-rural gradient. We selected Rook (Corvus frugilegus L.), Eurasian Jackdaw (C. monedula L.), Hooded Crow (C. cornix L.), Common Raven (C. corax L.), Eurasian Magpie (Pica pica L.) and Eurasian Jay (Garrulus glandarius L.) for our observations during 2009–2012. Some 38 survey routes totaling more than 8000 km were surveyed in 21 settlements in Zhytomyr region. Among them 13 routes were in Zhytomyr city. The average density of Rooks was 55.9 birds/km2. We found a strong correlation between Rook density and rural-urban gradient and observed that the numbers of wintering Rooks in cities significantly increased due to the influx from villages. The peak number of Rooks in villages was registered in the breeding and post-breeding seasons while in the cities it was high in winter and during the spring migration. The average density of Eurasian Magpies in the study area was 8.7 birds/km2 and we registered weak correlation with the urban-rural gradient. Their maximum density in urban habitats was in the winter period whereas the highest density in rural habitats was fixed in the summer months. The density of Magpies varied insignificantly within a narrow range during the three years of research, which suggests the species has successfully adjusted to the transformed landscapes. Average density of Hooded Crows in towns was 6.6 birds/km2. The linear relationship between the urban-rural gradient and the density of this species was rather weak. In the breeding period, the birds’ density was considerably higher in urban habitats. We also registered that the average density of Hooded Crows changed insignificantly but gradually increased during the study period. The average density of Eurasian Jackdaws was 9.7 birds/km2 and had high annual dynamics. It also changed significantly during the three years of research. The average density of Eurasian Jays was 2.4 birds/km2 and varied significantly in different settlements, although the fluctuation range was small (0.1–9.3 birds/km2). We registered an expansion in the breeding distribution of Eurasian Jays in recent years i.e. the birds began to nest in old urban neighborhoods and small central parks of large cities. However, the number of birds naturally decreased along the gradient of landscape transformation. The impact of urban-rural gradient on Eurasian Jay density was significant but the correlation was weak and negative. The density of this species was highest in small villages (4.9 birds/km2) and the lowest in medium-sized cities (1.5 birds/km2). The seasonal density of Jays varied significantly and the greatest value was registered in the post-breeding period. In urban areas the birds’ density was much higher in winter compared to the nesting period, and in villages we observed the reverse situation. The number of Eurasian Jays remained almost stable in all the settlements during the study period indicating the stability of the local populations. The average density of the Common Raven was 1.3 birds/km2 and the highest density was registered in small villages (2.2 birds/km2), the lowest – in the townships (small urban type settlements) (0.6 birds/km2). The urban-rural gradient significantly affected the spatial distribution of Common Ravens, but we cannot claim an increase or decrease in their numbers along the gradient of urbanization, since the value of this indicator also depended on habitat conditions in each specific settlement. The spatial distribution of Common Ravens varied seasonally and the highest density was typical in winter due to food migrations towards human settlements.