Causes of change

There is good evidence to indicate that the most likely cause of declines in Skylark is agricultural intensification, specifically the change from spring to autumn sowing of cereals, which reduces the number of breeding attempts possible and may also reduce overwinter survival due to loss of winter stubbles.

Further information on causes of change

Demographic trends presented here show that an increase in clutch and brood sizes and a decrease in the daily failure rate of nests at the chick stage until the 1990s, since partially reversed; the number of fledglings per breeding attempt has not changed. Chamberlain & Crick (1999) and Siriwardena et al. (2000b) found that breeding success per nesting attempt increased during the steepest period of decline, suggesting that these demographic changes have not contributed to the causes of population decline. The available data do not allow tests for effects of survival. Conversely, it is easy to test for effects on breeding success, especially locally and with respect to contemporary as opposed to historical land use. This creates a big imbalance in the amounts of evidence available.

Agricultural intensification has been put forward as the ultimate cause of Skylark declines. The relevant changes in agriculture have been decreases in preferred crops (spring cereals and cereal stubble) and an increase in unfavourable habitats (winter cereals, oilseed rape and intensively managed or grazed grass) (Chamberlain & Siriwardena 2000). There is good evidence that the most likely cause of the decline is the change from spring to autumn sowing of cereals. This practice restricts opportunities for late-season nesting attempts, because the crop is by then too tall. Chamberlain et al. (2000a) used habitat data from CBC surveys to show that the occurrence of autumn-sown, winter cereals increased from 33% to 78% between 1965 and 1995. Evans et al. (1995) and Wilson et al. (1997) all found that Skylarks deserted areas of autumn-sown crops as soon the sward reached a critical height, which occurred before the end of the breeding season. Jenny (1990), Chamberlain et al. (1999, 2000a, 2000b) and Donald & Vickery (2000) all recorded low and seasonally declining densities of Skylarks in cereals and suggested that this was at least partly due to the effects of changing vegetation structure. As well as preventing nesting, crop development also influences the positioning of the nests and hence their productivity: as the crop develops the birds are forced to nest closer to tramlines with a consequent increase in nest predation rate (Donald & Vickery 2000, Morris & Gilroy 2008). Skylark plots in crop fields have been found to support higher densities of breeding pairs (Schmidt et al. 2017) and may have the potential to help reverse population declines, but further research is needed on this subject.

Analyses by Chamberlain & Crick (1999) provided detailed evidence from both regional and habitat-based analyses that the greatest declines in Skylark numbers were associated with agricultural habitat, although their evidence suggests that different patterns of decline were unlikely to be due to differences in breeding success per attempt between habitats. However, Siriwardena et al. (2001) showed that the population trend can be explained by national changes in crop areas, together with a cold winter in 1981/82.

There is also some evidence that the increase in autumn sowing may depress overwinter survival by reducing the area of stubbles (Wilson et al. 1997, Donald & Vickery 2000, 2001). Donald & Vickery (2001) used data from BTO and RSPB studies to show that, in winter, cereal stubbles were strongly selected by Skylarks, probably owing to the presence of spilt grain and regenerating weeds, and go on to state that the area of stubbles has declined greatly in recent years. Gillings et al. (2005) identified better population performance in areas with extensive winter stubble, presumably because overwinter survival is relatively high. In a study in France, autumn sown fields were avoided by Skylarks in winter, and a steep depletion in seed abundance was observed in other fields between December and March, with Skylarks showing much stronger flocking behaviour in late winter when seed availability was lowest, suggesting this may be a key period during the winter (Powolny et al. 2018). Note, however, that definitive evidence about Skylark survival rates in the UK and what may have influenced them is not available because the species is rarely ringed and ring-recovery sample sizes are extremely small.

Use of pesticides and associated declines in weed populations and weed-seed abundance have been suggested as another factor in the decline of Skylarks (Wilson 2001). Wilson et al. (1997) found higher densities of Skylarks in organic systems. Chamberlain & Crick (1999) suggest that the use of toxic pesticides mediated through effects on food supplies may be responsible for declines in invertebrate food, due to non-target insects being killed by insecticide and insect food-plants being killed by herbicide. However, since this would in theory affect breeding success, it doesn't seem to have been a problem. Donald et al. (2001) state that, although recent agricultural changes have affected diet and possibly body condition of nestlings, these effects are unlikely to have been an important factor in recent population declines. There may also be implications for overwinter survival, as herbicides reduce weeds, and hence seeds for the winter, making stubbles and uncropped land less valuable as a food resource. However, the increases in pesticide use have happened at the same time as the switch to autumn sowing, so is hard to detect this as a specific effect.

There is some evidence to suggest that high densities of raptors may reduce the abundance of local Skylark populations (Amar et al. 2008b). Chamberlain & Crick (1999) state that recovery of Sparrowhawk numbers has been most evident in the most intensively farmed areas, and that this is correlated with the declines in Skylark numbers across habitats and regions. However, this apparent link cannot be taken as evidence of a causal relationship as there have been many other broad-scale changes in the countryside that are at least as well correlated with Skylark changes. They state that it is doubtful whether predation alone could account for the decreases in Skylark numbers.

Information about conservation actions

Skylark is among the most well-studied farmland species, and the decline is believed to have been caused by agricultural intensification, in particular the change from spring to autumn sowing of crops, which reduces the number of breeding attempts and also the availability of stubbles during the winter. Actions or policies which provide or encourage a mosaic habitat including some spring sown crops and/or some overwinter stubble would therefore benefit Skylarks. If overwinter survival is a significant driver of the declines, actions which provide overwinter food may also help (e.g. set-aside or cover crops).

A substantial amount of work has been undertaken to research options which enable the autumn sowing of crops whilst still enabling Skylarks to raise more than one brood. In particular, these have focused on the provision of 'Skylark plots' (small gaps deliberately left within crops) (Schmidt et al. 2017; Donald & Morris 2005), which are now an option in agri-environment schemes. Plots can be created either at the time of sowing (by turning off the seed drill) or at a later date (by spraying herbicide). The former is a better option for Skylark, as the plots have greater vegetation cover and higher invertebrate abundance, and should be preferred. If plots are created by spraying this should be done no later than December (Dillon et al. 2009).

The provision of grass buffer strips around fields with cereal crops is another agri-environment option which may benefit Skylarks: a study in Sweden found more beetles and spiders in fields with buffer strips and also more Skylarks, but suggested that they may need to be implemented at a landscape scale to be most effective (Josefsson et al. 2013).

Where Skylarks are not nesting in cereal crops, delaying mowing or reducing the number of cuts on silage fields, and reducing stock densities could help improve breeding productivity (Donald et al. 2002).

Policies which reduce the use of pesticides may also benefit Skylarks in the UK: A study modelling population trends predicted that declines would occur or would be worsened in all scenarios including the use of insecticides (Topping et al. 2005).