We investigated temporal changes in diet composition of the Montagu’s Harrier Circus pygargus breeding in natural habitat (calcareous peat bog) in SE Poland. We characterized diet composition in a three-year period (2007-2009), based on pellet analyses. We investigated whether diet composition was affected by years or stage of breeding. We compared diet of the studied population between 2000s and 1990s and with other populations. We found that the food of the studied population was dominated by insects and mammals (by number) and mammals and birds (by biomass). Biomass and abundance of main prey items differed between studied years because of different air temperatures. We found some interannual differences in contribution of some prey items including higher number of thermophilic prey (insects and amphibians) in warmer years. Comparison of pellet composition in the 1990s and 2000s revealed significant increase in the abundance of thermophilic prey (insects, reptiles) and decrease of mammals including Microtus voles and birds. Those changes may be linked to habitat changes in areas neighboring peat bogs and climate change induced changes in prey communities. The studied population was able to respond to changes in foraging habitats and prey composition by opportunistic foraging on easily available prey. The diet of the studied population is the most similar to the geographically closest populations foraging in similar habitats and characterized by high contribution of insects.

We reconstructed diet composition of the Montagu’s Harrier breeding in calcareous peat bogs near Chełm in SE Poland based on prey remains in pellets. We collected them from spring to summer in 2007, 2008 and 2009. Pellets were collected from the railway embankment crossing the “Bagno Sereberyjskie” reserve.

We disintegrated collected pellets in order to extract all the solid material, for example, bones, beaks, scales and insect remains. We determined prey items using keys for the identification of mammals and birds. We identified mammals by bones and teeth, birds by bones, beaks and feathers, and lizards by skulls, jaws, skin and scales. We grouped Insects not identified to the species level into the three most frequent taxa (Coleoptera, Hemiptera, Tettigonideae). To avoid possible biases associated with a secondary predation, we excluded remains of small beetles from the analysis when they were found together with lizard scales or bird remains in the same pellets. As many collected pellets in the field were broken, to avoid multiplication of prey number from fragmented pellets we pooled all pellet fragments collected during one visit to form one sample. We estimated abundance of particular prey items in all pellets collected during one sampling. We estimated biomass of particular prey item categories based on literature data.

We collected pellets in the consecutive stages of the Monatgu's Harier breeding season. We considered the period 15 April - 15 May as a pre-laying period, 16 May – 15 June as incubation, 16 June – 15 July as chick rearing period, 16 July – 15 August as post-fledging period.

To characterize climatic/meteorological conditions in particular stages of the Montagu’s Harrier breeding period we collected data from the closest meteorological station in Włodawa (40 km north of the studied area) from https://www.tutiempo.net/. We considered the following daily parameters: average temperature [^oC], maximal temperature [^oC], minimal temperature [^oC], average relative humidity [%], and total precipitation [mm].

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