The Breeding Ecology of Spotted Flycatchers– an RSPB study.
In 2002, a Spotted Flycatcher survey carried out by Bedsbirds emails group subscribers and members of the Bedfordshire Bird Club identified around 94 pairs. The RSPB were looking for suitable areas for a pilot study into the breeding ecology of Spotted Flycatchers. The Society approached the club to see if the data collected in 2002 could be used for this project. The 2002 dataset was supplied to the RSPB who identified a suitable study area in northeast Bedfordshire. The results of this study by Will Kirby, Katrina Black, Sarah Pratt and Richard Bradbury are summarised below.
The project was based largely in northeast Bedfordshire and set out to trial research methods and inform planned future autecological study of the species. The project focussed on adult territory and nest monitoring, chick growth and survival, invertebrate prey and habitat assessment. Standard research methods were found to be largely applicable in terms of territory and nest monitoring, although locating territories without prior knowledge proved difficult due to the cryptic behaviour of the species. Colour ringing of nestlings is feasible and re-sighting is relatively simple, due to perching behaviour. Adult colour ringing should be possible and would be highly advantageous. Malaise trapping was found to adequately sample their favoured prey (diptera), but is time consuming in terms of trap erection and subsequent sample analysis. Focal area counts of insect activity were quick, simple and repeatable in a variety of habitats, but could not be correlated to simultaneous malaise trap results. Novel measures of habitat heterogeneity proved workable and results suggest they may provide a key insight into habitat requirements. Birds begin nesting shortly after arrival on territory, and results from this study show early season nests to be more successful than later ones. There was a suggestion that nests on buildings were more successful than those on trees.
Information gathered by members of Bedfordshire Bird Club in 2002 provided the basis of study site selection. As part of a survey, members were asked to record sightings of spotted flycatchers in their local area, churchyards were suggested as good potential sites.
With birds located from over 80 sites in Bedfordshire in 2002, around 25 of the sites in the Northeast of the county were chosen for re-survey in 2003. Other sites were added where cold-searching potentially suitable areas found birds, or where information was received following requests to various interest groups. In all, 14 of the 39 sites searched in Bedfordshire were occupied and followed throughout the season, along with a similar number of unoccupied sites to be used for habitat comparison purposes. Reports of a small number of birds in wholly woodland habitats were received, but we were unable to locate any despite numerous visits, hence the study focused largely on churchyard and rural garden sites.
This study has proved that the techniques investigated were generally suitable for further study of the spotted flycatcher. Indeed, even within the scope of the pilot, some interesting results have become apparent. A few data sets collected have yet to be fully analysed; these include forage watches, nestling faecal samples and video recordings from two nests. It is anticipated that further examination of these will be carried out on these as part of future study.
An extremely wide variety of sites were used for nest placement, ranging from holes or platforms in walls and trees, through creepers on walls and trees to open nests on boughs and disused nests of other species. Height of nests ranged from 1.3 to 10m. Given that virtually all potential territories would contain at least some of the above, it seems extremely unlikely that the spotted flycatcher is nest-site limited. There is a suggestion (albeit non-significant) from the results obtained that those nesting on trees are more likely to fail than those nesting on or in buildings. If this is the case, the most likely explanation is that predation risk is increased for tree nesters. It is known that numbers of some potential nest predators (eg corvids, grey squirrel) are increasing and it is likely that these may predate tree nests more readily than those on buildings, due to ease of access. Although there is no evidence from nest record data that nest success rates are declining per se, further investigation of the data to include nest site may be worthwhile. The greater success rate of higher nests in this study would also suggest predation (from ground based predators) as the causal factor, although again there is no evidence from nest records, without further investigation, that predation incidents are increasing.
In terms of habitat, there is a difference between occupied and un-occupied churchyards in that sites with greater heterogeneity are more likely to contain breeding spotted flycatchers. This could be due to a simple preference for specific foraging habitat types, or it may be linked to increased invertebrate abundance within a more varied habitat. It is known that farmland has become more homogenous over recent decades with a move away from mixed farming, loss of hedgerows and crop specialisation. Trends in woodland, parks and gardens are less well documented but it would certainly seem unlikely that heterogeneity has increased. The suggestion that big (mature) trees may be an important factor influencing territory selection could provide possible mechanisms for decline in parts of the UK where Dutch Elm Disease has greatly reduced the number of these since the early 1970’s. Twenty million trees have been lost nationwide (Aboricultural Information Exchange) and this loss has been a suggested factor in the decline of a number of other species, including mention in the BAP for tree sparrow.
A healthy and available supply of invertebrate food is an obvious requirement for successful breeding of this obligate insectivore. They have a marked preference for larger invertebrates and favour diptera as prey, particularly when feeding chicks (Davies 1977). Although there is evidence of widespread declines in invertebrate abundance on farmland (eg Benton et al 2002), trends in woodland and garden habitats are not well understood. Malaise trap samples from this study suggest that the proportion of large and small diptera in our samples are not very different from those obtained by Davies in the mid 1970’s, however overall numbers have not been compared, and indeed annual and site differences would render such comparisons inconclusive. There was little substantive evidence from this study that lack of food was impeding breeding success. Breeding started rapidly after arrival of adult birds on territory, suggesting that adult condition was not a limiting factor. There were only three known cases of brood reduction out of the 20 nests that went on to fledge at least one chick, another indication that food shortage was not particularly apparent.
Of the eleven failed nests (Cambs and Beds), three were known second nesting attempts after successful first broods, all three of these failures were due to abandonment (two at egg stage and one at nestling stage). All coincided with periods of bad weather. The other eight failed nests are assumed predated; although it is possible that predation occurred subsequent to death of the chicks. Despite the observed failures, a calculated nest success rate of over 50% is high for an open nesting passerine species, and would suggest that, given the ability to re-lay, pairs should manage to raise at least one brood.
From this study and other sources (e.g. Snow and Perrins 1998) a proportion of breeding pairs can manage to raise two, even occasionally three, broods in a season. Summers-Smith (1952) estimated from early nest record data that around 20% were double brooded. The amount of pairs achieving this level of production is likely to have a large effect on future populations. If most pairs (as suggested from this study) have only one successful brood fledging an average of 3.1 chicks, the annual survival rate would have to be high (for a small trans-Saharan migrant passerine) for the population to be maintained. There are many possible causes that could lead to a reduction in the number of breeding attempts per season, including shortening of the breeding season due to climate change and shortage of food leading to reduced adult fitness. From current available information, it is not possible to say whether there has been any change in the number of pairs fledging more than one brood, but elucidating this is considered a primary concern for future research.
One outstanding question that needs to be addressed alongside the detailed breeding ecology of the spotted flycatcher, before effective conservation measures can be prescribed, is to establish the primary habitat of the species, which may vary across the UK. We know that it breeds in woodland, farmland and both rural and urban parks and gardens, but it is not apparent from the current literature which, if any of these habitats is of primary importance. Each habitat is however fundamentally different, and it is unlikely that any future prescriptions could be developed that would span all habitats. It is equally likely that if the recent declines are connected with problems on the breeding grounds, rather than in wintering areas or on migration, there are likely to be different factors involved in the different habitats. A multi-faceted approach to future research is deemed essential.