In our study, we confirmed all three of the predictions posed in the introduction. First, cats in urban areas hunted more birds compared to cats living in the rural environment. Second, temporal variations in the prey brought home by cats seemed to follow changes in prey availability throughout the year; rodents were most often brought home in the autumn (with a peak around September and October, when the highest densities were reached) and birds in the spring (with a peak around June, when the young leave the nests), while reptiles were caught least often during late autumn and winter (from November to February, when they are inactive). Third, the temporal dynamics of the prey brought home by rural and urban cats were clearly different. Therefore, our data suggest a facultative feeding strategy in free-ranging house cats.

However, it should be kept in mind that the methodological approach used in this study may be somewhat biased. When free-ranging cats hunt, they may injure, capture or kill their prey and either leave it in the field, consume it in the field or bring it home (Fitzgerald and Turner 2000), so methods that are based on consumed prey (scat/stomach content analyse) give different results than those based on prey brought home (Krauze-Gryz et al. 2012a). Indeed, cats usually kill more prey than they actually bring home (Loyd et al. 2013), so our study addressed only part of their hunting activity. Nevertheless, prey brought home may be assumed to be an index representing a minimum number of animals killed and can show a general pattern in cat diets (Woods et al. 2003; Tschanz et al. 2010). The other limitation of this study is the division of the relatively small number of sites (n = 26) divided into the two environments (rural vs. urban). A larger number of sites distributed across different levels of urbanization would allow us to investigate variation over the full urbanization gradient and test for possible nonlinear relationships between local habitats and the hunting ecology of cats.

As in previous studies (Churcher and Lawton 1987; Barrat 1997; Fitzgerald and Turner 2000; Woods et al. 2003; Kays and DeWan 2004; Morgan et al. 2009; Tschanz et al. 2010; Krauze-Gryz et al. 2012a), mammals (mostly rodents) dominated the prey brought home by cats in both studied environments. However, their dominance was far lower in the urban environment, where the share of birds in the prey clearly increased. Several former studies also indicate that pattern; in Finland, birds accounted for 24% of the prey of urban cats in contrast with 14% in the diet of rural cats (Kauhala et al. 2015). In the suburbs of Auckland (New Zealand), rodents dominated house cat kills in a habitat located on an urban/forest fringe, while birds were the most commonly caught urban vertebrate prey (Gillies and Clout 2003). The observed differences in the composition of prey brought home by cats living in urban and rural environments most likely reflected prey availability, which was driven by differences in land-use. Within the bird group, the share of pigeons and sparrows in the prey brought home by cats was higher in the urban environment. Sparrows were also the most frequent (Gillies and Clout 2003; Morgan et al. 2009) or among the most frequent (van Heezik et al. 2010) birds in urban and suburban habitats in New Zealand. Bird communities are usually less species rich in cities, but their abundance is relatively high compared to other environments (Rosin et al. 2016) with sparrows and pigeons being more common than in rural landscapes (Crooks et al. 2004). Thus, their share in the diet of opportunistic predators is usually higher in urban than rural environments, which was observed in studies of tawny owl (Goszczyński et al. 1993), martens and foxes (reviewed in Bateman and Fleming 2012).

Of the rodents killed by cats, the occurrence of mice was higher in the urban environment. Urbanization influences rodent species composition with the striped field mouse Apodemus agrarius, being a dominant species in considerably human-transformed habitats in Warsaw (Babińska-Werka et al. 1979; Gortat et al. 2014). Voles, on the other hand, are absent from the central quarters of Warsaw (Gryz et al. 2008), and their share in the rodent community decreases along an urbanization gradient (Andrzejewski et al. 1978; Gortat et al. 2014). Although populations of the striped field mouse reach very high densities in urban habitats, the species may be relatively difficult to catch due to dense vegetation (i.e., in parks, where rodents can hide in ivy cover), as observed for the tawny owl (Goszczyński et al. 1993). The two less common prey categories, soricomorphs and reptiles, also decreased in cat prey with increased urbanization. Our findings that the proportion of insectivores was higher in the rural than in the urban environment were similar to those of Kauhala et al. (2015). Generally, shrews, being highly sensitive to fragmentation (Vergnes et al. 2013), are scarce in highly transformed, typically urban habitats (Andrzejewski et al. 1978; Gryz et al. 2008; Gortat et al. 2014).

We observed a clear temporal pattern in the composition of cat prey as different prey categories were caught in different proportions in different seasons. However, this seasonal variation was different in rural and urban environments, showing that urbanization affected the temporal patterns in the foraging ecology of cats. In our study, and similar to the results of other, rodents were most often caught in autumn and early winter (Barrat 1997; Weber and Daily 1998), and this variability is most likely driven by distinct changes in their abundance, which is the highest in autumn (Goszczyński 1977). Interestingly, this seasonal change in the number of rodents brought home by cats seemed to be smaller in the urban environment. According to Chernousova (2001), small mammal communities in cities are less dynamic, and rodent abundance remains relatively high compared to those in more natural areas. Another possible explanation for the autumnal increase in the numbers of rodents caught by cats in rural areas is their higher availability due to agricultural operations. After crops are harvested in the farmland (summer-autumn), rodents become much easier to hunt, and this is the time when cats more frequently penetrate fields (Goszczyński 1977; Krauze-Gryz et al. 2012b). The number of birds killed in rural areas fluctuated greatly, reaching a peak in spring (June) and a minimum in late autumn (November), while the number caught in the urban environment remained stable throughout the year. The peak in the number of birds killed by cats in rural areas, which primarily occurs in spring, probably reflects the killing of juveniles (e.g., Liberg 1984; Churcher and Lawton 1987; Barrat 1997; Baker et al. 2005; van Heezik et al. 2010), and several other generalist predators hunt birds most often during this season (e.g., Mirski et al. 2016). In winter, bird abundances are lower in rural areas due to seasonal migration, while in cities, most of the common species are sedentary (tree sparrow Passer montanus; house sparrow Passer domesticus; domestic pigeon Columba livia domestica), while migratory species are replaced by birds that winter in the city (e.g., Żmihorski et al. 2010); urbanization stabilizes winter bird communities (Suhonen et al. 2009). For example, birds can be hunted by cats next to bird feeders (Dunn and Tessaglia 1994). Woods et al. (2003) showed that cats living in households where birds were provided feed caught birds more often. The number of reptiles brought home by cats fluctuated greatly in rural areas. In the urban environment, where they were rather accidental prey, so the interseasonal change was rather small. Nevertheless, they appeared among the cat preys through spring and summer. Similarly, in Finland, reptiles were mainly brought home in the breeding season (Kauhala et al. 2015); in other seasons in the northern latitudes, when temperatures often drop below zero Celsius degrees, reptiles remain inactive and thus unavailable to cats.

Our study suggests that the two critical impact periods of cats on native fauna is spring (for birds) and autumn (for rodents), while during winter, cats catch fewer prey (especially in the rural environment) and are less active (Goszczyński et al. 2008). Thus, their overall predation pressure is relatively low. Rural cats, which are mainly kept as ‘mousers’ (Krauze 2008), are perceived as effective rodent killers, but in addition to killing synanthropic rodents, such as house mice Mus musculus, and rats Rattus spp., they prey on numerous species of voles as well as legally protected or rare species (e.g., shrews; hares Lepus europaeus; red squirrels Sciurus vulgaris; least weasels Mustela nivalis) (Krauze-Gryz et al. 2012a; this study). The effect of predation by free-ranging domestic cats on prey populations can be severe as their numbers are kept artificially high by supplemental feeding (Sims et al. 2008). At the same time, they do not show normal numerical or functional responses to prey density (Coleman and Temple 1993) as they switch between household food and natural prey depending on accessibility (Liberg 1984; Weber and Daily 1998). As a result, cats can efficiently compete with wild predators (Krauze-Gryz et al. 2012b). Urban and suburban cats mainly focus on birds, which results in serious predation rates (Lepczyk et al. 2004) or increases in sub-lethal factors, such as a reduction in fecundity (Beckerman et al. 2007) or food delivery to chicks (Bonnington et al. 2013). Special attention should be paid to the influence of house cats on the fauna of nature reserves located in or adjacent to cities or suburbs as the presence of free-ranging domestic cats, which are likely the most abundant predator, can reduce the effectiveness of these protected areas as a tool for protecting nature (Wierzbowska et al. 2012).