Skylark Density Research Articles

Is early grass harvest for silage an overlooked reason for skylark Alauda arvensis population decline in Sweden?

In 2015 we studied the relative importance of silage and cereal crops as breeding habitat for the skylark Alauda arvensis. In 2014 we had found that skylarks failed to breed successfully in grass silage fields. We compared skylark density between crops in one organic and one conventional farm, c. 100 ha each, at Kvismaren valley in south Sweden (approx. 59.2°N; 15.4°E). The densities were similar in all crops: 83 territories/km2 in silage and 75 (spring sown) or 80 (autumn sown) in cereal. However, silage harvest in early June and July destroyed, as in the previous year, almost all nests, effectively preventing skylarks from reproducing successfully in that habitat. Cereal fields are not harvested until after the young are fledged. Approximately 45% of the farmland area in Sweden is grass for silage, normally harvested in late May or early June. We suggest that this early harvest is an important and overlooked driver for population decline of birds nesting in such fields.

Habitat management effects on Prealpine grassland bird communities

Currently, land abandonment constitutes a serious threat for many species tied to semi-natural grasslands. In southern Europe, the conservation of many grassland bird communities largely depends on low-intensity mountain farming. In this study, we analysed the effects of the main farming practices promoting grassland maintenance, i.e. grazing and mowing, on species richness of a Prealpine grassland bird community and on the breeding densities of the two commonest grassland species, the skylark (Alauda arvensis Linnaeus, 1758) and the tree pipit (Anthus trivialis Linnaeus, 1758). The two study sites were located between 1100 and 2000 m above sea level in the Italian central Prealps. Grassland species richness was positively affected by grazing, and increased with increasing distance from the nearest woodland. Skylark density was positively influenced by mowing, grassland cover extent and distance from woodland, with the latter effect occurring only at a single study site. Finally, tree pipit density was positively affected by grazing. Our findings suggest that, in order to maintain open grasslands below the natural treeline and enhance the biodiversity of grassland bird communities, grazing should be preferred over mowing. Moreover, woodland edge effects may have a negative impact on mountain grassland bird communities. Advancing woodlands and abandonment of low-intensity farming and grazing practices may therefore constitute a serious threat for open-habitat specialist bird species of the Alpine region.

Grass buffer strips benefit invertebrate and breeding skylark numbers in a heterogeneous agricultural landscape

The loss of non-crop habitat is often suggested to be a key driver of biodiversity decline on arable land. Grass buffer strips on cereal field edges, to reduce erosion and agro-chemical runoff into surface water, could be useful to mitigate this diversity loss as they are often assumed to provide refuge and food for invertebrates, small mammals and birds. Evidence for this idea is, however, scarce and it remains unclear whether densely vegetated buffer strips benefit biodiversity in structurally complex landscapes of Northern Europe. Here, we examined whether buffer strips affected breeding skylark Alauda avensis numbers and its main food supply (i.e. beetles Coleoptera and spiders Arachnida) on cereal fields in a heterogeneous agricultural landscape of south-central Sweden. We also examined whether buffer strip effects on skylark density depended on seasonal sward height differences between sowing regimes (spring- vs. autumn-sown) as they presumably influence seasonal invertebrate accessibility. Fields with buffer strips supported on average 0.51±0.26 more skylark territories per hectare up to 100m into the field and boosted invertebrate activity densities compared to fields without buffer strips. These effects were most apparent early in spring, but persisted throughout the sampling period, and were similar among spring and autumn sown fields. Thus, our results provide evidence to suggest that buffer strips target multiple environmental objectives on cereal fields in heterogeneous farmland. Future research should work to identify buffer strip management practices that further increase their value to biodiversity at the local scale, and investigate how they affect farmland biodiversity in different landscape types at larger spatial scales for more efficient implementation across Europe.

Bird use of establishment-stage Miscanthus biomass crops during the breeding season in England

Capsule Summer bird densities in establishment-stage (2–3-year-old) Miscanthus were the same as in winter wheat fields for many species, but were higher for Lapwing, and for Reed Bunting and Blackbird late in the summer. Aims To compare abundances of farmland birds in establishment-stage Miscanthus biomass crops with those in crop types they are likely to replace (winter wheat and grassland) during the breeding season. In particular, to investigate whether such a rapid-growing, dense crop has lower abundances of field-nesting species. Methods Bird surveys were conducted in 51 Miscanthus fields and an equal number of grass or winter wheat control fields between May and July, in Lincolnshire and southwest England. Results Species richness was similar in Miscanthus to that in winter wheat and grass. Skylark densities were similar in Miscanthus and wheat, while Lapwing densities were higher in Miscanthus and grass than wheat. Miscanthus contained very high densities of Reed Buntings and Blackbirds late in the summer. Conclusion Densities of most bird species, including field-nesting species, were not lower in establishment-stage Miscanthus than in the crops it most commonly replaced. The Miscanthus surveyed was relatively weedy and patchy; further studies as crops age and knowledge of Miscanthus husbandry increases would be valuable.

The effect of prescribed burning on Skylark density in Watarase Marsh

The effect of prescribed burning on Skylark density in Watarase Marsh

Response of ground-nesting farmland birds to agricultural intensification across Europe: Landscape and field level management factors

European farmland bird populations have decreased dramatically in recent decades and agricultural intensification has been identified as the main cause contributing to these declines. Identifying which specific intensification pressures are driving those population trends seems vital for bird conservation in European farmland.We investigated the response of ground-nesting farmland birds to the multivariate process of agricultural intensification in six European countries covering a bio-geographical and intensification gradient. Supported by PCA analysis, two groups of factors, related to field management and landscape modification, were considered, seeking to discriminate the relative importance of the effects of these main intensification components.Variance partition analysis showed that landscape factors accounted for most of the variation of ground-nesting farmland bird individual and breeding pair densities, as well as Skylark (i.e. our single model species) individual densities. In the case of Skylark breeders, field factors were found to be more important to explain their density. Our results suggest that in general farmland bird densities as well as Skylark densities are higher in simple landscapes dominated by agriculture, but with smaller fields and more different crops on the farms. In addition, high yields were negatively related to bird densities.We conclude that while management actions aimed at farmland bird conservation taken at landscape level may exert a strong positive effect on overall bird densities, those taken at field level are also relevant, particularly for breeders and, therefore, may potentially influence the persistence of these species’ populations.

Habitat associations of Eurasian Skylarks Alauda arvensis breeding on Irish farmland and implications for agri-environment planning

Capsule Skylarks breeding in Ireland prefer extensive grassland habitats and almost completely avoid tillage habitats.Aims To describe the distribution and habitat use of breeding Skylarks in Ireland, particularly in lowland agricultural habitats, and to use this information to inform conservation measures for this species.Methods Countryside Bird Survey (CBS) and Farmland Bird Project (FBP) data were examined to determine large-scale (national) distribution and habitat selection, in addition to smaller-scale (farm- and field-level) habitat use. The CBS is a national breeding bird monitoring scheme involving 397 1-km squares. The FBP collected detailed bird and habitat data from 122 farms.Results CBS and FBP data both showed significant regional differences in breeding Skylark densities, with the highest relative abundances in the northwest and west. Dry grassland/grass moor habitats supported the highest densities of breeding Skylarks in the CBS, which were significantly higher than in improved grassland or tillage. At the farm-level, Skylark numbers were positively related to wetland habitats but negatively associated with trees in field boundaries, dense ground vegetation and overall density of farm boundaries. At the field-scale, larger fields and unimproved grasslands were preferred.Conclusion Agri-environment measures tailored to region-specific requirements and to the relatively local habitat preferences of target species are required if population declines of species of conservation concern, including Skylarks, are to be reversed.

Autumn-sowing of cereals reduces breeding bird numbers in a heterogeneous agricultural landscape

The switch from spring-sown to autumn-sown cereals and the loss of habitat heterogeneity are often suggested to be key drivers of breeding bird decline on arable farmland. Yet, both factors are interlinked and it remains uncertain whether autumn-sown cereals reduce breeding bird numbers also in the structurally complex arable farmland of northern Europe. We tested whether autumn-sowing of cereals at both local and landscape scales affected the breeding bird community in a heterogeneous agricultural landscape of south-central Sweden. Rotation between sowing types was used as a semi-experiment based on 34 spring- vs. 41 autumn-sown cereal plots centred on infield non-crop islands of similar structure, size and surroundings. Species richness and territory abundance of ground-foraging species were significantly lower in autumn- than in spring-sown cereal plots both in the crop fields and the infield non-crop islands during the breeding season. No such effect was observed among foliage gleaning birds. Species richness in spring-sown cereal plots was less the more autumn-sown crops in the surrounding landscape within a 500 m radius. Average skylark densities did not differ between autumn- and spring-sown cereal plots because habitat preferences changed; densities declined in autumn-sown cereals during the growing season whereas they increased on spring-sown fields which had shorter swards throughout the breeding season. Our results indicate that negative effects of autumn-sown crops on breeding bird numbers spill over into both neighbouring non-crop and crop habitats. We conclude that agri-environmental schemes should place more emphasis on facilitating the value of the cropped area of fields as a foraging and nesting habitat. The retention of various non-crop habitats alone may not provide sufficient food close to nest sites for farmland birds that rely on crop fields for foraging.

Density and reproductive success of Skylarks Alauda arvensis on organic farms—an experiment with unsown Skylark plots on autumn sown cereals

Skylark plots (unsown plots; 10–25 m2 per ha) in autumn cereal fields have had positive effects on Skylark density and reproductive success on conventional farms in Britain. We tested if the same effect could be found in organic farms in Sweden by comparing paired fields with and without unsown plots in similar settings. We found that the plots had no effect. Skylark density was only associated (negatively) with field size and not with occurrence of Skylark plots, vegetation structure, yield or weed harrowing. Number of clutches per territory was affected by yield (negative) and vegetation cover (positive) but not by Skylark plots. Number of fledglings per clutch showed negative association with yield and visibility through the sward. The amount of weed was larger near Skylark plots from the preceding year than in other parts of the field. The yield (and probably vegetation density) was lower on these fields than in conventional autumn sown fields, but still negative effects of the crop (yield levels) were found.

Agricultural land use and Skylark Alauda arvensis: a case study linking a habitat association model to spatially explicit change scenarios

The development of forward scenarios is a useful method of envisaging the environmental implications of potential changes in land use, as a tool for policy development. In this paper, a spatially explicit case study is used to provide insight into the environmental impacts of Common Agricultural Policy reform on Skylark Alauda arvensis, a species which is widespread on arable farmland, breeds in crops and has declined in recent decades. A generalized linear mixed model was used to estimate Skylark breeding population densities in different crops, using survey data collected from farms in the east of England, supplemented by the literature. Model outputs were then used to predict Skylark densities in an East Anglian Joint Character Area dominated by arable cropping. Predicted densities were mapped at field level using GIS, based on actual cropping derived from Integrated Administration and Control System data collected for the administration of subsidy payments. Three future scenarios were then created, based on expert opinion of potential changes in cropping over the next 5 years, and potential changes in Skylark density mapped on the basis of the predicted changes in cropping patterns. Overall, Skylark densities were predicted to decrease on average by 11–14% under ‘market‐led’ (increasing wheat and oilseed rape, reduced set‐aside) and ‘energy crop’ (5% area under short rotation coppice) scenarios, but remained virtually unchanged under an ‘environment‐led’ (diverse cropping) scenario. The ‘market‐led’ scenario is closest to short‐term agricultural trajectories, but wider cultivation of biomass energy crops as modelled under the ‘energy crop’ scenario could occur in the medium term if energy policies are favourable. Appropriate mitigation strategies therefore need to be implemented if a continued decline in the Skylark population on lowland arable farmland is to be averted. The results provide a readily accessible visualization of the potential impacts of land‐use change for policy‐makers; similar techniques could be applied to visualize effects of changes arising through other drivers, including climate change.

Breeding Skylarks Alauda arvensis on organic set-asides—effects of time of cutting, vegetation structure and landscape composition

We assessed the quality of organic set-asides as habitat for breeding Skylarks. We evaluated (i) density in relation to vegetation structure and landscape composition, (ii) reproductive success on set-asides cut at normal time (1 June) and with delayed cutting (15 June), and (iii) nest predation and destruction by using artificial nests. Skylark density was negatively associated with vegetation height and cover, and it was three times higher on fields without forest than with more than 50% forest in the surrounding landscape. Of the artificial nests, 56% were destroyed by cutting that also increased predation on surviving nests. Delayed cutting resulted in higher reproductive success; mean number of fledged broods per territory increased from 0.42 to 0.52. The negative effects of early cutting seemed to be largest on fields with sparse vegetation. Based on fledging dates we suggest that 20 June is a reasonable date for first cutting in south-central Sweden. Reproductive success on organic set-asides could probably be further improved if less dense vegetation (or unsown patches or strips) is combined with a delayed cutting.

Density and productivity of breeding Skylarks Alauda arvensis in relation to crop type on agricultural lands in western France

Capsule Small field size and the maintenance of set-aside and lucerne are important to ensure high breeding pair densities and productivity. Aims To investigate the effects of crop types and their attributes on density and productivity of breeding Skylark. Methods At each of four selected study sites in western France, territory density, vegetation height, vegetation cover and field size was estimated by field and attempts were made to find nests. Crop types included winter and spring cereals, oilseed rape, sunflower, maize, grass, lucerne, set-aside, and bare ground. Results About 80% of Skylark territories included more than one crop type. Birds preferred small fields and territory density decreased with increasing field size. Density was highest in crops with low vegetation height and cover. Set-aside, lucerne and grass supported highest territory density. Fledging productivity was highest in set-aside and lucerne, and was zero on bare ground. Skylark density decreased throughout the breeding season (-26% in 1999 and -29% in 2000), suggesting an instability in territory distribution or activity in intensive farmland. Conclusions Farming systems that decrease field size and increase set-aside and lucerne instead of oilseed rape, maize and sunflower will benefit Skylark and other declining farmland species.

Effects of habitat type and management on the abundance of skylarks in the breeding season

Summary 1. There is increasing evidence to link major declines in skylark populations in Britain to agricultural intensification. However, whether causal mechanisms identified through localized studies can be generalized to the national scale remains unknown. The abundance of breeding skylarks was determined by surveying singing males in over 600 randomly selected 1‐km squares throughout Britain, in which skylarks recorded were assigned to homogeneous habitat patches. A more intensive survey of skylarks was carried out on lowland farmland sites in England. Singing males were assigned to specific crop types, and data on crop height and field boundary features were recorded. 2. Skylark occupancy (presence/absence) and density where birds were present (i.e. omitting zero counts) were analysed in relation to habitat type, habitat diversity and time of year, using generalized linear modelling. 3. Set‐aside, moorland and winter cereals had high rates of skylark occupancy at the national scale. Set‐aside had consistently high rates of occupancy and high densities across the breeding season at different spatial scales. 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 significantly on winter cereals, which showed significantly lower density than a number of habitats at this time, including spring cereals, legumes and moorland. 4. Within lowland farmland, there were significant effects of crop height on skylark occupancy, with crops of greater than 30 cm 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. 5. Skylark density increased with increasing habitat diversity across the whole sample of 1‐km squares and in lowland 1‐km squares in England. However, within the lowland farmland plots in England, skylark density showed a significant decrease with increasing habitat diversity. These conflicting results suggest that crop type rather than habitat diversity per se is important. 6. 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. These results support findings from smaller scale studies showing the generality of these habitat effects at different spatial scales. 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), in agri‐environment schemes is likely to benefit skylarks breeding on farmland by providing suitable nesting habitat throughout the breeding season. In addition, reductions in the intensity with which cereals are managed, such as reduced pesticide and fertilizer input under approaches such as precision farming, and the creation of sparser patches of cereal sward, are also likely to increase the suitability of winter cereals for nesting skylarks.

Coarse and fine scale habitat associations of breeding Skylarks Alauda arvensis in the UK

The occurrence of breeding Skylarks, in terms of both density and presence/absence, was assessed from Breeding Bird Survey data in relation to habitat in a random sample of about 1500 1-km squares throughout the UK in two consecutive years, with particular emphasis on the farmland habitat. Upland grassland and other seminatural grassland habitats held the highest mean densities of Skylarks. Within farmland, arable land held the highest and pasture land the lowest densities, but there was a seasonal decline in arable density. Regression analyses showed that Skylark occurrence was most consistently related to the area of arable farmland (positive effects) and the area of woodland (negative effects). Skylark density increased with increasing habitat diversity in upland squares only. Within farmland, boundary characteristics and the presence of grazers had the most consistent negative effects. The diversity of field types was positively related to Skylark density, which gives some support to the hypothesis that a decrease in the diversity of field types as a result of simplification of farm rotations in recent decades may have led to the decline of the Skylark in the UK, but further conclusions about the effects of specific crop types were not possible at this scale of analysis.

Breeding skylarks Alauda arvensis on Environmentally Sensitive Area arable reversion grass in southern England: survey‐based and experimental determination of density

1. In the South Downs and South Wessex Downs (southern England), designated an Environmentally Sensitive Area (ESA) in 1987 and 1993, respectively, farmers may undertake to conserve existing areas of downland turf (chalk grassland rich in plants and invertebrates) and to create new areas of grassland out of previously cultivated land (arable reversion), in exchange for annual payments. These areas are subject to strict management prescriptions. 2. Singing skylarks were surveyed on 40 km2 of downland turf, arable reversion, non‐ESA grass and arable land between 1994 and 1996 (April–July) in the two ESAs. The emphasis was on two types of arable reversion: permanent grassland sown with agricultural grass species, and chalk downland sown with traditional chalk grass species. 3. The surveys showed that singing skylark density was greater on chalk downland arable reversion than on permanent grassland reversion, in swards above 15 cm in height and in fields without boundary scrub. In general, permanent grassland reversion did not support more skylarks than winter wheat or grass not entered in the ESA schemes; highest densities were in undersown spring barley. Singing skylark density decreased after mowing in May–June in the South Wessex Downs ESA. 4. In 1995 and 1996, a fully randomized experiment was carried out on 12 fields (each c. 5 ha) at Plumpton, Sussex, to verify the survey findings. Six fields were grazed to maintain a sward below 10 cm in height (short‐grazed), and the others grazed to maintain a sward between 15 and 25 cm (long‐grazed) during the skylark breeding season (April–July). 5. On average, singing skylarks were six times as abundant, and non‐singing ones twice as abundant, in experimental long‐grazed fields as in short‐grazed fields. The total number and number of taxa of chick‐food invertebrates were about twice as large on average, and the number of grass seed heads recorded in July was two to 15 times as great, in long‐grazed as in short‐grazed fields. 6. Grazing‐exclusion cages within a randomized block experiment at New Erringham, Sussex, were sampled for invertebrates in mid‐May 1995 & 1996. Ungrazed cages (sward up to 40 cm tall) supported over five times as many invertebrates and over three times as many invertebrate taxa as grazed areas (sward <2 cm tall). 7. These results contributed to revised management prescriptions for the South Downs (1997) and South Wessex Downs (1998) ESAs. Prescriptions to encourage taller swards and to prevent mowing during the nesting season in the South Wessex Downs, and removal of the 10‐cm limit on sward height, allowance of fertilizer on some arable reversion grassland, and an arable tier encouraging undersown spring cereals and overwinter stubbles in the South Downs, will benefit grassland birds.

Territory Distribution and Breeding Success of Skylarks Alauda arvensis on Organic and Intensive Farmland in Southern England

Skylark numbers declined by 51% between 1968 and 1995 on UK lowland farmland; a loss of approximately 3 million breeding birds. This study examined whether distribution and breeding success of skylarks varied with the cropping of organically and intensively managed fields in southern England in accordance with the hypothesis that changes in agricultural land-use and intensity of management have contributed to this decline. Density was lowest on fields surrounded by tall boundary structures or unsuitable habitat, and those with tall, dense vegetation cover. After controlling for these effects, set-aside and organically-cropped fields supported significantly higher skylark densities throughout the breeding season than intensively cropped fields or grazed pasture. Nests were usually built in crops between 20 and 50 cm tall. In fast-growing broad-leaved crops (e.g. oilseed rape, legumes), skylarks held territories, but no nesting activity was observed. Rapid crop growth probably allows too little time for nesting to begin. Breeding success was higher on set-aside than on intensively managed cereals. Predation caused most nest failures, but did not vary in frequency with crop type. Silage cutting and trampling caused many failures on grass fields, and all cases of apparent brood starvation occurred in cereal fields. These breeding success data, together with published estimates of survival rates, suggest that skylark pairs must make 2-3 nesting attempts per season in order for populations to be self-sustaining. A single crop type rarely provides a suitable vegetation structure for nesting throughout the breeding season. Skylarks therefore require structurally diverse crop mosaics in order to make multiple nesting attempts without territory enlargement or abandonment. Mixed farms are more likely to fulfil these requirements than those dominated by winter cereals and broad-leaved crops. These results are consistent with the hypothesis that loss of mixed farming and rotational cropping, and concomitant increases in autumn sowing of crops, agrochemical inputs, multiple silage cuts and grazing intensities since the 1950s have reduced the breeding productivity and population density of skylarks on lowland farmland in southern England. The following recommendations are made for changes in farming systems that would assist the conservation of breeding skylark populations on lowland farmland. Organic farming systems, set-aside and habitat management for gamebirds are all likely to improve nesting and feeding conditions for skylarks. More generally, breeding skylark populations are only likely to increase on farms that reduce agrochemical inputs, reduce grazing intensity and frequency of silage cutting, and increase the structural diversity of field vegetation by adopting mixed rotations of winter and spring cereals, root crops and grass. Traditional mixed farming systems of this kind are now rarely economically desirable. Only agricultural policy reforms motivated in part by environmental concerns rather than solely by production control are likely to direct subsidy support to reduced-intensity. mixed farming enterprises of this kind, and thus help to restore populations of breeding skylarks on lowland farmland.

Skylark ( Alauda arvensis) utilisation of micro-habitats in spring barley fields

Within-field habitat utilisation by skylarks Alauda arvensis was studied in spring barley crops by means of radio-tracking and visual sighting. Arthropod food abundance and dropping densities were monitored within three different micro-habitats. The effect of maintaining open areas within a cereal field on local skylark densities was examined on two experimental fields. Radio-tracked skylarks showed a strong preference for the tramline micro-habitat. There was a positive correlation between crop height and the proportion of birds located in tramlines. This preference was supported by visual observation of ascending and descending birds as well as the mean density of droppings found in the three habitat types. Tramlines and unsown plots were utilised significantly more than expected and the crop was avoided. The density of arthropod food items was highest in crop and lowest in tramlines and unsown plots. This indicates that the skylarks' preference for tramlines and unsown plots was probably due to unhindered ground locomotion, facilitating detection of prey items, rather than prey density. Artificially maintained open areas had a positive effect on the density of breeding pairs and territorial males, and on the duration of the breeding period. These results suggest a positive effect of the open areas within field crops on the number of re-nesting attempts. The management implications of the results are discussed.