Alessandra Gagliardi

From mass of body elements to fish biomass: a direct method to quantify food intake of fish eating birds

The winter diet of great crested grebe (Podiceps cristatus) was studied in the major lakes of Insubria region, N Italy, in 2000–2003 by analysing stomach and oesophagus contents of birds found dead. Winter diet of cormorant (Phalacrocorax carbo) was studied using pellet analysis. Diet was expressed in terms of numerical abundance, frequency and biomass of prey. A detailed methodology is described of how to prepare a reference collection of otoliths, pharyngeal bones and chewing pads (key-elements), essential for species-specific identification of fish remains in stomachs and pellets, and to estimate ingested fish biomass. Correlations between mass (mg) of key-elements and mass of corresponding fish were significant, and exponential regression models were used to calculate biomass of each single prey item directly from the mass of its key-elements. Advantages of this method are compared with the more traditional two step approach in which size of elements is used to estimate fish length, followed by calculating fresh mass from fish length. Cyprinids formed the largest part of the diet for both predators. Great crested grebes selected small prey, feeding mainly on bleak (Alburnus alburnus alborella), while diet composition of cormorants varied significantly in relation to prey availability.

The Extent of Great Crested Grebe Predation on Bleak in Lake Como, Italy

Abstract Food intake (fish biomass), during the winters of 1997-98 and 1998-99, by Great Crested Grebes (Podiceps cristatus) on a 47 km2 area of the western basin of Lake Como, North Italy, was estimated by analyzing recognizable remains of the fish prey in the stomach contents of birds found dead. Using the size of otoliths and pharyngeal bones, we showed that grebes mainly ate the younger age-classes (2-yr. old and younger) of Bleak (Alburnus alburnus), 80% of all fish prey. Monthly rates of Bleak consumption by wintering grebes were estimated by multiplying daily food intake, extrapolated from the relationship between food biomass intake and water temperature in the different winter months, with the proportion of Bleak in the diet and with the number of wintering grebes present each month. Great Crested Grebes were most numerous in the study area in January-February and there were no significant differences in numbers between the two winters. Size of Bleak eaten increased from November to February and the biomass of Bleak consumed was highest at the end of winter, when water temperature was lowest. Overall in the western basin, grebes consumed between 640 kg (conservative estimate) and 1,000 kg (maximum estimate) of Bleak per winter, which corresponded to 10% to 20% of the annual commercial Bleak harvest in that area. We suggest that grebe predation on Bleak might increase the mortality rates in the younger age-classes of the fish, and thus contribute to a decrease in the stock, but that economic impact of grebe predation on gross annual income from the local fishery is negligible.

Behavioral Responses of Wintering Great Crested Grebes to Dissuasion Experiments: Implications for Management

Abstract Great Crested Grebes (Podiceps cristatus) wintering on prealpine lakes of north Italy are considered to cause economic damage to fish, reducing the stock of native Bleak (Alburnus alburnus). We experimentally investigated the effectiveness of three deterring actions (gas-cannon, crackers and shooting) in four study sites at Lake Como, by comparing the time-budget and space-use of wintering grebes in absence and presence of disturbance. All dissuasion methods affected grebe behavior and position on the lake, but some resulted more effective than others. The gas-cannon detonations had little effect, but, crackers and shooting caused significant changes in grebe behavior. Grebes spent less time feeding, resting and preening, but more time moving/swimming during dissuasion than in the control situation. Moreover, birds also moved away from the areas with bleak shoals, resulting in a significant decrease in grebe numbers with either dissuasion methods. Although dissuasion reduced feeding of grebes in areas with shoals of Bleak (Alburnus alburnus), and probably reduced grebe predation on immature fish at localized areas, it involves intensive effort, in terms of man-days.

A Floating Platform: a Solution to Collecting Pellets When Cormorants Roost Over Water

Abstract Wintering roosts of Great Cormorants (Phalacrocorax carbo) on Lake Como, northern Italy, are often inaccessible. We describe an easy-to-build, cheap and efficient technique to collect pellets at inaccessible roosts, which enabled us to study the diet of cormorants without causing disturbance, or killing birds. A floating platform was built using iron bars, sea-plywood panels and plastic tanks which was placed on the lake’s surface, to collect pellets ejected by cormorants using a roost on the eastern shore of the western basin of Lake Como. During two winter periods (February 2001-March 2001 and December 2001-March 2002) 106 samples were retrieved from the platform, measuring 17.5 m2.

Habitat Selection by Alpine Rock Ptarmigan Lagopus muta helvetica in the Italian Alps

Abstract We analysed space use strategies in two populations of Alpine Rock Ptarmigan Lagopus muta helvetica considering three topographical variables (elevation, slope and aspect) to investigate whether populations living at range margins can adapt to suboptimal conditions by exploiting the available habitat differently. Selection for topographic features differed between populations, suggesting that Rock Ptarmigan adapt their habitat selection to local availability. Therefore, we suggest considering a wide range of habitat parameters when planning conservation strategies for species living at the margins of their range.

Where is the pulse to have the finger on? A retrospective analysis of two decades of Alpine Galliforms (Aves: Galliformes) census and game bag data in Italy

Information on the abundance of the Italian populations of black grouse (Lyrurus tetrix), Alpine rock ptarmigan (Lagopus muta helvetica) and Alpine rock partridge (Alectoris graeca saxatilis) rely only on extrapolations of local data to the national scale, since there is no national standardized survey. Consequently, their status is virtually unknown. We performed a first-ever assessment of a medium-term (1996–2014) population trend of these species using and comparing post-breeding count and bag data at hunting district scale. These data were collected from various authorities in charge of wildlife management and allowed us to test the influence of hunting policies on the estimated trends. Rock partridge showed a stable trend with numbers fluctuating between years, while there was evidence of a severe decline for rock ptarmigan. No general conclusion could be drawn for the black grouse, as we detected lack of consistency of count and bag data. Counts were greatly overdispersed as a result of an uneven count effort among hunting districts. Adding the game management authority as model covariate resulted in more robust trend estimations, suggesting a significant effect of different policies that emerged also as similar hunting pressure across species within authorities. Hunting effort variation over the time was instead negligible. Species-specific game management bias is discussed. Our results highlight the need for a survey scheme or guidelines to be applied uniformly at a national scale.

Forest and biodiversity: a multiscale analysis of forest bird community in Varese district

Forests represent more than 45% of the Varese district area. Data of forest bird community composition are correlated with the appearance of woods. With a multiscale approach (1 km2 units and territories) some landscape metrics are analysed, in order to evaluate the influence of environmental variables on bird species.

Eco-ethologicol characterisation of alien species: Vinous-throated Parrotbill, Paradoxornis webbianus, and Ashy-throated Parrotbill, Paradoxornis alphonsianus, with radiotracking

The study of the Parrotbill in Province of Varese during spring and autumn 2006 is based on 7 birds marked with radio-tags of 0.43 g. The dimension of home range during the breeding season is significantly smaller than in autumn (7 ha). The analysis of vegetation in the home range showed a preference for wetland and swamp bush by these birds.