We studied two different crow populations, one in northern Italy (1989–91 and 2001, 2002) and one in northern Spain (1995–2003). In Italy we studied non cooperative crows at two different agricultural sites, 10 km apart (45°N, 9°E). The two areas (10 km2 each) were similar, with a flat landscape dominated by rice fields, maize and, to lesser extent, meadows. Agriculture was intensive, with fields irrigated regularly and natural vegetation confined to the shores of irrigation canals. Crows nested in poplar tree (Populus sp. L.) plantations or in small patches of relict oak (Quercus robur L.) forest, with similar breeding densities at the two sites (6·3 and 6·2 pairs km2). Crows in Italy are socially monogamous, and non-breeders live year-round in non-territorial flocks. Occasional observations (n = 2, 6% of observed territories) of a ‘third bird’ in a territory have been reported (Baglione et al. 2002a; V. Baglione unpublished data), but the helping role of those crows could not be confirmed.
The habitat in rural northern Spain (42°N, 5°W) was markedly different, with low-intensity agricultural use (no irrigation) and a mosaic of crops (mainly wheat and oat), meadows, uncultivated land for grazing of sheep and cattle and oak (Quercus pyrenaica Willdenow) forest patches. Here 75% of crow territories are held by cooperative groups of up to nine individuals (mode 3 birds). Extra birds are either non-dispersing offspring that stay with their parents on the natal territories or immigrants that form coalitions with resident territorial birds. Groups may have up to three helpers involved in feeding the nestlings (for details on the Spanish crow society see Baglione et al. 2002a).
We measured habitat saturation (HO) in Italy and Spain as number of competitors per breeding vacancy arisen in a year time span (Kokko & Lundberg 2001). To do so in the two populations, we collected data on the number of individuals competing for territory vacancies and the proportion of breeding vacancies available for each year of study.
Both in Italy and Spain, we searched all new nests at the beginning of the breeding season. Afterwards, regular surveys allowed the detection of any late nesting attempts. We used different methods to estimate the number of non-territorial crows of the two populations. In 1989, 2001 and 2002 we counted crows in the Italian study areas at least three times throughout the breeding season, following a standard itinerary with a car. The extreme structural simplicity of the landscape allowed a very accurate bird census. We also surveyed the surroundings of the study sites, where we commonly observed other stable flocks of floaters. As same-year bird counts within the study plots gave almost identical results (variation between 1 and 3%), we believe that we assessed the actual number of birds settled within the study sites and that our censuses were not inflated by individuals occasionally coming from elsewhere. We therefore regarded the maximum number of crows observed during the censuses as the number of crows living on the study areas. The number of potential competitors for territories was obtained by subtracting 2 × number of active nests from the total number of individuals.
In Spain, floaters were a negligible component of the studied population during the breeding season (Baglione et al. 2002a). Small flocks of floaters were observed only twice within the study area in 5 years (nine and 13 individuals, respectively). Almost all individuals that do not own a territory either lived on their natal territory or immigrated to a new territory, where they associated with the resident birds. The total number of potential competitors for territory vacancies was therefore obtained by counting the extra-birds in the territories. To do so, we have recorded the group size of every territory since the beginning of our study. For 64–79% of territories we assessed group size every year by observing the territories from a favourable spot for at least 3 h (for details see Baglione et al. 2002a). In the remaining territories we obtained group size when we visited them for routine data collection (egg-laying sequence, nestlings’ growth, fledgling dispersal). Because group members usually form a cohesive group, those short visits to the territories (15–30 min) provided reliable information on group size.
The population sex ratio was known both in Spain (Baglione et al. 2002a) and in Italy, where it was derived from a large sample of adult crows (n = 121) that had been caught as part of provincial administration's pest management programmes (see Acquarone, Cucco & Malacarne 2001). The sex ratio of non-breeders was derived from the population sex ratio, allowing assessment of habitat saturation for each sex separately in both populations (Table 1).
Table 1. Territory turnover, percentage of individuals competing for territory vacancies in the population and habitat saturation (number of competitors per breeding vacancy) in Spain (2000–2002) and Italy (2001 and 2002)
| ||Year||Turnover (%) (no of individuals)||Percentage of individuals competing for vacancies||Sex ratio population??://||Habitat saturation (HO)1|
|Spain||2000||20·0 (10)||28·6 (7)||39·8|| ||4·7||1·3|
|2001||20·0 (15)||45·5 (11)||41·1||1 : 0·67||5·0||0·9|
|2002||18·2 (11)||16·7 (6)||38·5|| ||4·8||2·3|
|Italy||2001||22·2 (9)||37·5 (8)||55·8||1 : 0·61||8·1||2·0|
|2002||30·8 (13)||37·5 (16)||58·4|| ||6·5||2·2|
To measure the proportion of breeding vacancies available each year, we caught free-flying crows in 1999 and 2000 in Spain (n = 61) and in 2001 and 2002 in Italy (n = 41; see Baglione et al. 2002a for details on trapping system). Crows were banded with wing tags and an individual code of coloured rings and blood samples were collected. P2/P8 DNA-based identification (Griffiths et al. 1998) provided the sex of the carrion crows (for details on blood samples storage and DNA extraction see Baglione et al. 2002a,c).
In Italy, during the first days after banding, we observed crows with binoculars and spotting scopes until we saw them providing food to the nestlings. We therefore regarded them as breeders of that particular territory. In Spain, captured crows were assigned more easily to a specific territory, due to the virtual lack of floaters and to the scattered distribution of the territories (Marcos & Baglione 2003), and breeding status was assigned according to a parentage analysis based on eight polymorphic microsatallite loci (see Baglione et al. 2002c for details). In both populations we considered that a breeding vacancy arose when a breeder disappeared from a territory before the commencement of the breeding season. In Spain, in groups where reproduction was shared, we considered that a vacancy arose when the alpha-breeder disappeared. As almost all vacancies were occupied later by new territorial crows in both sites (100% in Italy, n = 15; 87% in Spain, n = 15), the proportion of breeding vacancies approximated the actual turnover of individuals in the breeding territories. Therefore for simplicity, we will refer hereafter to this datum as ‘territory turnover’.
The fate of the breeders that disappeared from the territories was unknown in most cases. Two individuals, one in Italy and one in Spain, were found dead. One Spanish male immigrated to another territory, probably after usurpation of his own territory, and one breeding female in Italy joined the non-breeding flock after her mate had disappeared. The lack of resightings during our regular surveys of the study areas, roosts and communal feeding grounds suggests that most of the missing individuals had died.
variability of territory quality
We defined the variability of habitat quality as among-territories coefficient of variation (CV) of the number of fledglings produced in a number of territories over the study period (according to Stacey & Ligon 1991). We measured the productivity of 19 territories in Italy and 20 territories in Spain over 3 (1988, 1989, 1991) and 5 years (1995–99), respectively, considering only those territories that could be recognized unequivocally throughout the study period (Baglione et al. 2002c). The difference between the CVs was tested according to Sokal & Braumann (1980). To ensure that the result was not influenced by the different duration of the sampling periods at the two study sites, we also calculated the Spanish CV on the productivity of territories in periods of three consecutive years (1995–97, 1996–98 and 1997–99) and compared the results.
When helpers have a positive effect on reproductive success, the CV of productivity of the territories can be a biased index of the variation of habitat quality because the difference between territories might be overestimated. Better territories produce more young that will become helpers, which will enhance further productivity and therefore also the difference with less successful territories. This potential bias may inflate the calculation of the CV only in Spain, where crows are cooperative, but this is conservative with regard to the conclusions of this paper (see Discussion).