The House Sparrow Passer Domesticus Case Study

INTRODUCTION
The House Sparrow Passer domesticus has been associated to the humans since Bronze Age (Ericson et al. 1997) and, nowadays, it is the bird with the wider distribution area of the world (Cramp & Perrins 1994, Del Hoyo et al. 2009, BirdLife International 2016). This is a consequence of an anthropocoric process by releases in non-native areas (Erskine 2006, Sætre et al. 2012, Liebl et al. 2015). However, this species is on decline, especially during the last quarter of the XXth century (Summers-Smith 2003) and the beginning of the XXIst (Gil-Delgado et al. 2002, Raven et al. 2003, Hayhow et al. 2014) in Central and Occidental Europe. These declines have occurred in both, urban and farmland areas (Crick et al. 2002, Summers-Smith 2000,
In Great Britain, this decreasing tendency seems alarming, since in the last 35 years it has been noticed a decline of the 65% in their populations (Hayhow et al. 2014). For instance, the House Sparrow in London had a decline of 71% in the period 1994-2002 (Raven et al. 2003). In Spain, this species is still abundant (Carrascal & Palomino 2008), even though it presents an annual decline of 0.6 % (SEO/BirdLife 2010). However, the tendencies of the populations in the Valencian Community seem to follow the observed ones at the United Kingdom. Chronologically, the first signs of its decline came from the orange groves of Sagunto, as between 1975-2001 the population decreased more than 90% (Gil-Delgado et al. 2002). Later, in the parks of the city of Valencia, it was notified a population decline of 70% (1998-2008), equivalent to an average annual decline of 15% (Murgui & Macias 2010). In addition, global studies in the Valencian Community have detected an interannual decrease of 3.3% (1998-2015), with a decline of 22.3% those 17 years (Vera & Escandell
The man dependent species are more likely to have negative trends when the environments where they obtain the greatest benefits are modified (Siriwardena et al. 1998). Several hypotheses to explain the declines in urban areas have been proposed, among which are: predation by domestic cats Felis silvestris catus or Common Sparrowhawks Accipiter nisus (Woods et al. 2003, Bell et al. 2010); competition for food resources with pigeons in their domestic variety or competition for nesting sites in the case of Black Starlings Sturnus unicolor (Weitzel 1988, Murgui 2014). The decline has also been attributed to other causes as food reduction by the better cleanliness and increasing human socioeconomic status in towns and cities (Shaw et al. 2008); electromagnetic radiation (Balmori & Hallberg 2007); diminution of food resources and invertebrate availability by the increase of the development in green areas (McKinney 2002, Vincent 2005); poor quality trophic resources in urban areas (Herrera-Dueñas et al. 2015); pollution and vehicle traffic (Summers-Smith 2007, Meillère et al. 2015) and reduction of nest site availability (Wotton et al. 2002), among others. The most accepted hypothesis is that this important decline is not due to a single factor, but a combination of several of them. However, studies are still not clear in determining the most important factor that