| CBC sample sizes did not allow monitoring
of House Sparrows until 1976; previously, there had been many
farmland plots with high populations that could not be properly
quantified without better access to farm buildings and housing.
CBC/BBS data indicate a rapid decline in abundance over the
last 25 years, as does the BTO's Garden Bird Feeding Survey
(Siriwardena et
al. 2002, Robinson
et al. 2005b). These results are supported by
many other studies and anecdotal reports, and have generated
great conservation concern (see Summers-Smith
2003). A change in the listing criteria resulted in the
admission of the species, green-listed until 2002, to the
red list. A temporary drop in first-year survival coincided
with the steepest decline, but changes in breeding performance,
especially nest failure rates at the chick stage, have also
helped drive population change (Freeman
& Crick 2002). Possible explanations include a general
reduction in food supply, less grain being spilt during agricultural
operations, tighter hygiene regulations, increases in predation,
and toxic additives to unleaded petrol (Siriwardena
et al. 2002, Robinson
et al. 2005b, Vincent
2005, Summers-Smith
2007, Peach et
al. 2008).
The overall national decline since the 1970s masks much heterogeneity
by region and habitat, and population processes may be relatively
fine-grained: overall, populations in rural areas had declined
by 47% by 2000, and those in urban and suburban areas by about
60% (CBC data: Robinson
et al. 2005b). Within urban areas, House Sparrows
may have disappeared predominantly from more affluent areas,
where changes are more likely to have occurred to habitat
structure (Shaw et
al. 2008). The continued availability of allotments,
gardens and other green spaces in urban areas is crucial to
preventing further decline (Chamberlain
et al. 2007). BBS suggests increases recently
in Scotland and Wales. Overall, brood size has decreased,
raising NRS concern (Leech
& Barimore 2008), but nest success has improved markedly.
Following widespread declines across Europe since the 1980s,
the European status of this species is no longer considered
'secure' (BirdLife International
2004).
|
