The Decline of Common Birds Exemplified by the Western Jackdaw Warns on Strong Environmental Degradation

Abstract

Bird populations associated with agricultural ecosystems have declined markedly in Europe during the last quarter of the 20th century due to land-use intensification. This has meant that some very common species, in some cases even species considered as pests, are now threatened or subject to management programs to ensure their conservation. Considered pests of crops and predators of small game species, corvids are among the most persecuted common farmland birds. The consideration that these birds are pests lacks any scientific evaluation and is justified by the subjective impression that they are abundant. Here, using estimates of absolute and relative abundances of both the total and the breeding population, we show how jackdaws Corvus monedula have shown a marked negative population trend in central Spain during the last 40 years. Decline involves the loss of multiple colonies, the apparent absence of the species as a breeder in riverside forests, and an overall numerical decrease of about 75% (from 35,000 to 9000 individuals) according to counts in communal roosts. The population decline seems to be more pronounced in areas where land use has been intensified, probably in response to the reduction in the availability of once-abundant food (i.e., invertebrates and weed seeds) but also due to more direct effects such as intoxication and medium to long-term accumulation of agricultural pollutants which may have also affected reproduction and survival. Intensive hunting over decades has undoubtedly contributed to this decline and should therefore be made forbidden urgently. Generally, it seems that high-intensity agricultural management more drastically affects smaller and less adaptable common species, which are expected to decline before and at a higher extent and magnitude than jackdaws. Given that global population estimates based on direct counts of individuals are readily achievable through simultaneous counts in communal roosts, the jackdaw can serve as a model for assessing temporal trends potentially linked to large-scale anthropogenic modifications of open and agricultural environments.

Competition and generalization impede cultural formation in wild jackdaws

“…Where animals will face many new but related informational challenges, the ability to draw on already acquired information will be a potent tool. In the case of jackdaws, a species prominent in anthropogenically altered habitats [ 48 ], regular foraging from anthropogenic refuse will be facilitated by the ability to generalize similar safe stimuli. In this study, we only varied the colour of the stimulus, with all other characteristics identical, which may not be entirely realistic, or at least does not fully explore the scope of potential stimuli.…”

“…Jackdaws are highly social, colony-breeding corvids [ 68 , 69 ] with a generalist, omnivorous diet, which often includes anthropogenic foods [ 48 ]. They display high levels of neophobia towards novel foods and objects [ 33 , 36 ], form large flocks and forage socially [ 34 , 35 ], providing ample opportunities to learn from conspecifics [ 36 ].…”

The impacts of free‐roaming cats cannot be generalized and their role in rodent management should not be overlooked

“…Humans are altering ecosystems in many different ways that, overall, pose a much more serious threat to biodiversity than cats, such as habitat destruction for building human settlements and infrastructures, intensive agriculture, pollution, climate change, hunting, fishing, and wildlife trade (Blanco et al, 2022; Camacho et al, 2022; Ceballos et al, 2020; Coimbra et al, 2020; IPBES, 2022; Lewison et al, 2004; Newbold et al, 2020).…”

Long-Term Winter Population Trends of Corvids in Relation to Urbanization and Climate at Northern Latitudes

“…[ 9 , 10 , 11 , 12 , 13 , 14 , 15 ]) benefit from urbanization all over the world. However, common species are often-neglected study objects despite their important ecological roles [ 16 , 17 ]. This is still true even after a heavy decrease in some common urban species, such as the House Sparrow ( Passer domesticus) [ 7 ].…”

“…For example, low levels of predation [ 25 ], good availability of (artificial) food resources [ 26 , 27 , 28 , 29 ] as well as an availability artificial nest sites [ 13 , 30 , 31 , 32 , 33 ] may benefit corvids. However, some studies have indicated that hunting activities [ 17 , 34 ] and persecution [ 12 , 14 , 35 ] may have negative influences on corvids. The ability to use novel anthropogenic resources such as food, safe nest and roost sites [ 36 ], as well as a high tolerance towards of conspecifics may allow some species, such as corvids, to thrive in urban environments [ 37 ].…”