There is considerable evidence to link major declines in Skylark populations in Britain to agricultural intensification. A factor that has been especially implicated in the population decline is the switch from spring to winter-sown cereals, the latter crop being generally less favoured by nesting Skylarks as it develops a dense vegetation structure early in the breeding season and is not left as a stubble over-winter. Cereals are by far the commonest nesting habitat used by farmland Skylarks, BTO data indicating that 40% of the farmland population in England and Wales and 34% in Scotland nest in cereals. This does not necessarily reflect a preference for cereals over other crops. Rather, it demonstrates that cereals occupy by far the largest area of habitat types used by Skylarks in lowland Britain.

Skylark - photo by Derek Belsey & Cliff Reddick

The Breeding Skylark Survey was set up by the BTO in order to determine the population size and habitat preferences of breeding Skylarks by surveying singing males in over 600 randomly selected 1-km squares throughout Britain. In farmland, set-aside was consistently the most preferred habitat, whether considering different regions of the UK, or considering habitat preferences at different times of the breeding season. The preference of Skylarks (and indeed a number of other species) for set-aside is well known and is probably due both to favourable vegetation structure for nesting and a relatively high abundance of food compared to most conventionally farmed land. Apart from set-aside, there was little difference in density between habitats in the early half (March to mid-May) of the breeding season. In the later half of the breeding season (mid-May to July), density declined on winter cereals, which showed significantly lower density than a number of habitats at this time, including spring cereals and legumes. Therefore, the relative preference for different crops seems to vary according to the stage of the breeding season. The following two scientific papers contain further details from the Breeding Skylark Survey: Chamberlain et al. 1999 and Browne et al. 2000.

A more intensive study of breeding Skylarks was carried out on lowland farmland in England to investigate seasonal variations in habitat preference further, including the collection of data on crop height. Within lowland farmland, there were significant effects of crop height on Skylark occupancy, with crops of greater than 30cm in height being occupied at relatively low rates. Winter cereals reached this height significantly earlier in the breeding season than a number of other crops, including spring cereals and legumes. The effects of vegetation height on Skylark abundance support the hypothesis that increases in winter cereal, and simultaneous loss of spring cereal, have had an adverse effect on Skylark populations by reducing the number of breeding attempts made per year. Winter cereals have become much more common in Britain since the 1960s and have tended to replace the preferred habitat of spring cereals. However, there is also evidence that the actual quality of winter cereals as a nesting habitat has declined due to increases in fertilizer inputs and growth regulators and changes in cereal varieties. These changes have tended to increase growth rates and hence sward density in winter cereal crops. However, there may be other factors (e.g. loss of stubbles) that could also have contributed to the population decline. See Wilson et al. 1997 for further details.

Large numbers of Skylarks occur on farmland in winter, including UK breeders and perhaps also some birds from the continent. The Winter Skylark Survey, and subsequently the Winter Farmland Bird Survey both examined habitat preferences of Skylarks in winter. During winter Skylarks have a mixed diet of vegetation (e.g. cereal leaves) and seeds, but it is the latter that are energetically most valuable. Their preferred habitats therefore include any weedy, seed-rich habitats such as stubble fields. However, detailed work by BTO and others has shown that many stubble fields are actually quite poor in seed resources (Vickery et al. 2005), so it is not enough necessarily to simply leave a stubble field over the winter – the type of management of the preceding crop (e.g. the number of herbicide applications) is also critical. See Gillings & Fuller 2001 for further details of the Winter Skylark Survey.

The extent of the British Skylark population associated with agricultural land suggests that sympathetic changes in farming practice are likely to provide the best mechanism for improving the status of this species. The inclusion of options, such as spring cereal or fallow land (an equivalent to set-aside), and retention of over-winter stubbles in agri-environment schemes is likely to benefit Skylarks by providing suitable nesting habitat throughout the breeding season and suitable winter foraging habitat. In addition, reductions in the intensity with which cereals are managed, such as reduced pesticide and fertiliser input under approaches such as precision farming, and the creation of 'Skylark Scrapes' in cereals (2-4m square unsown patches within the field centre: see www.saffie.info), are also likely to increase the suitability of winter cereals for nesting Skylarks. However, it should be stressed that it is as yet unclear exactly how much habitat such as this is required in order to bring about a recovery in the Skylark population. Work is ongoing to answer this question. One piece of work used the habitat data collected by volunteers in the Winter Farmland Bird Survey to classify Breeding Bird Survey squares according to how much stubble they contained during winter. We were then able to demonstrate that declines in breeding Skylark numbers were steepest where no stubble was present in winter. Critically, when 10ha or more stubble was present in a 1-km square, the recent decline in Skylark numbers appeared to level off (click here for more results). This is our first indication of the likely area of habitat that a farmland bird may need to recover. Ongoing work will assess the optimal spatial arangement of seed resources created, for example, under agri-environment schemes at the landscape scale (see Siriwardena & Stevens 2004).

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