Bullfinch Research and Conservation

by Gavin M Siriwardena

Populations

The Bullfinch is, fundamentally, a bird of scrub and deciduous woodland, but it occurs, if inconspicuously, across farmland, parks and gardens where dense bushes and trees are to be found. Nevertheless, the Common Birds Census (CBC) has identified declines in the abundance of Bullfinches in both farmland and woodland in Britain (see graph below; Siriwardena et al. 2001a) and the species now has its own Biodiversity Action Plan. Although the species prefers wooded areas, nearly 40% of the population occurs in each of these two broad habitats (Gregory & Baillie 1998), reflecting the dominance of agricultural land in the British landscape. The Bullfinch, therefore, has usually been considered to be a member of a group of related, seed-eating species that have undergone severe declines on farmland. However, it is unlike many declining granivorous species in making much less direct use of crops or their associated weeds at any time of year. It is also unique in having been regarded as a pest species in Britain, because of a liking for the buds of fruit trees in orchards which has led to legally licensed culling in areas such as Kent and Worcestershire.

Mechanisms

The demographic mechanism underlying the Bullfinch's decline are, as yet, uncertain. BTO researchers have used the Trust's data archives to conduct in-depth analyses considering the following potentially important factors:

annual adult survival rates annual first-year survival rates daily failure rates of nests during the egg stage daily failure rates of nests during the nestling stage clutch size brood size hatching success numbers of breeding attempts per pair per year survival rates in the post-fledging period (between leaving the nest and reaching independence, in practice the date on which the average fully-grown juvenile is ringed).

These analyses show that adult survival, first-year survival, nestling period daily nest failure rate and egg period daily nest failure rate could all provide a mechanism, although changes in the latter may be the best single explanation for the population trend during the principal period of decline, i.e. 1977-1982 (Siriwardena et al. 2001a). It is likely that changes in brood size and clutch size have not been important. Also, the population trend can be explained without invoking variation in numbers of breeding attempts or post-fledging survival rates, although such variation cannot be ruled out (Siriwardena et al. 2001a). Other work has found up to two-fold inter-annual variations in local Bullfinch productivity, due primarily to changes in the amounts of late breeding, suggesting a strong influence on abundance of the proportion of the breeding population that are able to make several breeding attempts (Newton 1999).

The decline in Bullfinch abundance on farmland has been larger than that in woodland (65% versus 28%), but the population trends were, otherwise, not dissimilar, suggesting that a common demographic mechanism could be responsible (Siriwardena et al. 2001a). However, egg period nest failure rates have been higher in the preferred habitat, woodland, than in farmland and have fallen over time in farmland, where a larger decline has occurred, thus suggesting that this parameter does not provide such a common, causal link with abundance.

Causes

The uncertainty over the mechanism for the Bullfinch's decline makes the determination of the ecological causes difficult. However, despite evidence for a negative effect of agricultural intensification on Bullfinch presence, little evidence exists clearly linking any demographic rate to environmental change and the aspects of agricultural land-use that have been tested have had little effect on nest failure rates (Siriwardena et al. 2000ab, 2001b). Predation appears to have had no significant impact on abundance (Thomson et al. 1999), a conclusion supported by the absence of clear changes in survival. The latter also suggests that control measures have not caused the decline. It is perhaps most likely that subtle reductions in habitat quality are responsible for the decline of the Bullfinch. Specifically, a loss of complexity and density in the structure of hedgerows and woodland understorey vegetation could have multiple effects including reducing food availability and thus the survival of adults, first-years or nestlings and reducing the available cover for nests such that the risk of nest predation grows.

The latest information on Bullfinch populations, as provided by the BTO's surveys can be seen here: at www.bto.org/birdtrends.

References

Gregory R.D., Baillie S.R. 1998. Large-scale habitat use of declining British birds: implications for conservation. J. Appl. Ecol. 35: 785-799.

Newton I. 1999. An alternative approach to the measurement of seasonal trends in bird breeding success: a case study of the bullfinch Pyrrhula pyrrhula. J. Anim. Ecol. 68: 698-707.

Siriwardena G.M., Baillie S.R., Crick H.Q.P., Wilson J.D. 2000a. Agricultural land-use and the spatial distribution of granivorous lowland farmland birds. Ecography 23: 702-719.

Siriwardena G.M., Baillie S.R., Crick H.Q.P., Wilson J.D. 2001b. Changes in agricultural land-use and the breeding performance of granivorous farmland passerines. Agric. Ecosyst. Env. 84: 191-206.

Siriwardena G.M., Crick H.Q.P., Baillie S.R., Wilson J.D. 2000b. Agricultural habitat-type and the breeding performance of granivorous farmland birds. Bird Study 47: 66-81.

Siriwardena G.M., Freeman, S.N., Crick H.Q.P. 2001a. The decline of the Bullfinch Pyrrhula pyrrhula in Britain: is the mechanism known? Acta Orn. 36(2): 000-000.

Thomson D.L., Green R.E., Gregory R.D., Baillie S.R. 1998. The widespread declines of songbirds in rural Britain do not correlate with the spread of the avian predators. Proc. Roy. Soc. Lond. B 265: 2057-2062.

This advice page was prepared as part of work funded by a partnership of the BTO and Joint Nature Conservation Committee (on behalf of Natural England, Scottish Natural Heritage, the Countryside Council for Wales, and also on behalf of the Environment & Heritage Service in Northern Ireland).

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