Timing of upland vegetation management affects breeding of three species of passerine birds: a case study from southwest England

ABSTRACT

Capsule:

Current permitted timings of vegetation burning and Bracken Pteridium aquilinum control show considerable overlap with the timing of breeding of three widespread UK upland breeding passerine bird species on an upland site in southwest England.

Aims:

To quantify the timing of breeding of Meadow Pipits Anthus pratensis, European Stonechats Saxicola rubicola, and Whinchats Saxicola rubetra and assess overlap with the permitted timing of the two widespread upland land management practices of burning and Bracken control on a moorland site in southwest England.

Methods:

498 nests were monitored across a 4.3 km² area of Dartmoor between 2008 and 2014. The onset and timing of breeding activity, and variation between species and years, were quantified. Overlap between breeding activity and burning and Bracken control was then quantified for several realistic and legally permitted scenarios.

Results:

There was considerable overlap between upland management practices and breeding activity of Stonechats, Meadow Pipits, and Whinchats. In all three permitted burning cessation scenarios (11, 13, and 15 weeks from 1 January) there was overlap with breeding activity of Stonechats and Meadow Pipits. The commencement of Bracken control in July also overlapped with breeding activity, with half of Whinchat nests and more than a quarter of Stonechat and Meadow Pipit nests potentially affected.

Conclusion:

The overlap of timings for breeding and vegetation management is likely to be causing nest failure for a significant number of birds each year. We suggest a change to the permitted burning and Bracken control timings to better accommodate the protection of breeding passerine birds on Dartmoor.

Vegetation management is an important tool for land managers to help achieve agricultural and conservation aims. In upland habitats of the UK, vegetation management has been undertaken for thousands of years to manipulate habitat succession and vegetation structure (Webb 1998, Ausden 2007). This is done to help achieve various agricultural, sporting, and archaeological preservation aims; primarily to create and maintain open habitats for livestock grazing, to maintain high densities of game birds, and, more recently, for conservation and ecosystem services related aims (Butchart et al. 2006, Davies et al. 2016). Methods of upland vegetation management include burning, mowing, herbicide treatment, vegetation removal, grazing and manipulation of water levels (Ausden 2007). Thus, UK uplands today are strongly shaped by vegetation management (Yallop et al. 2006, Chapman et al. 2009).

Two widely applied vegetation management techniques in the UK uplands are burning and the control of Bracken Pteridium aquilinum (Pakeman & Marrs 1992). Burning and both mechanical and chemical Bracken control have been prescribed through Agri-environment Schemes (AES) to restore degraded habitats (Natural England 2008, 2009, 2016), with payment options also available to fund the removal of Bracken and other scrub from scheduled ancient monuments as Bracken rhizomes can damage archaeological features (Crow & Moffat 2005, Natural England 2013).

The regrowth that follows burning creates new growth that is preferentially grazed by livestock. For heather moors, new shoots form a major part of the diet of Red Grouse Lagopus lagopus, and burning can increase Red Grouse breeding success and density (Robertson et al. 2017). Burning maintains early successional habitat and creates habitat mosaics to meet biodiversity aims; for example, for invertebrates such as fritillary butterflies and uplands birds such as the European Golden Plover Pluvialis apricaria (Allred et al. 2011, Bargmann et al. 2015, Bubová et al. 2015, Douglas et al. 2017). However, negative effects of burning on species will also occur, either directly for species that cannot escape the effect of burning, or indirectly for species reliant on the later successional stages of habitats (Powell & Busby 2013). There is evidence that the use of burning as a management tool on uplands has damaged upland landscapes such as blanket bogs, and resulted in negative impacts for carbon storage and water quality (Davies et al. 2016).

Bracken can provide important habitat for species of conservation concern, such as the Pearl-bordered Fritillary Boloria euphrosyne, the European Nightjar Caprimulgus europaeus, and the Whinchat Saxicola rubetra (Burge & Kirkwood 1992, Woodhouse et al. 2005). On the other hand, Bracken is considered invasive on upland heaths and in ericaceous and acidic grassland habitats, as it can dominate large areas to the exclusion of specialist species (Alday et al. 2013). From an agricultural perspective, extensive areas of Bracken reduce the extent of grass and dwarf shrubs available for livestock grazing (Pakeman et al. 2002). In recent decades Bracken has mainly been managed through aerial chemical spraying. The licence to use the main active chemical, asulam, was withdrawn across the European Union (Regulation (EU) No 1045/2011, eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A32011R1045), but spraying is still widespread through both ‘Emergency Authorisations’ for asulam use (Regulation (EC) No 1107/2009, eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex%3A32009R1107) and the use of other chemical agents such as glyphosate, for which there is also an emerging environmental concern (Annett et al. 2014, Bai & Ogbourne 2016, Myers et al. 2016). An alternative widespread Bracken control method is cutting and crushing (or bruising), achieved through manual (e.g. cutting with scythes or bashing with sticks) or mechanical means (e.g. chain harrow by tractor) (Crow & Moffat 2005, Maren et al. 2008). Cutting and crushing is most effective when carried out twice within the first year of control (once in May or June and again in July or August), and annually thereafter (SEARS 2008).

Over recent decades many breeding bird species found partly or exclusively in upland habitats in the UK have shown substantial population declines (Henderson et al. 2004, Sim et al. 2005). For example, since 1970 European Cuckoos Cuculus canorus and Meadow Pipits Anthus pratensis have declined by 53% and 37% respectively and, since 2008, Whinchats, European Stonechats Saxicola rubicola, and Northern Wheatears Oenanthe oenanthe have declined by 14%, 16% and 35%, respectively (Burns et al. 2020). Upland birds rely on some vegetation management to maintain their habitat through the prevention of succession to taller scrub and woodland, but changes in the extent, frequency and technique of upland vegetation management have occurred in recent decades (Douglas et al. 2015, Ross et al. 2012), and management intensity can negatively impact breeding birds (Tharme et al. 2001). Many UK upland areas have seen an increase in the frequency of burning, and the extent of new burns increased significantly between the 1970s and 2000 (Yallop et al. 2006), with further increases across the UK since 2000 (Douglas et al. 2015). Indirect impacts, such as a temporary reduction in the availability of suitable breeding or foraging habitats may affect some species but, in addition, direct impacts, such as the destruction of nests, are likely to increasingly occur where such management becomes more frequent or is practised over larger areas. Given that bird breeding success is strongly linked to recruitment and population size (Fletcher et al. 2010, Pearce-Higgins et al. 2010, Sim et al. 2013), an understanding of the extent and potential impact of any overlaps between land management practices and breeding birds is required.

The timing of vegetation management is regulated through AES prescriptions – with burning regulated by the Heather and Grass, etc. Burning (England) Regulations 2007 and 2021 – and is constrained by commencement and cessation dates: 1 October to 15 April for upland areas (Anon 2007, 2021, Natural England 2015). Although the prescribed dates are designed to avoid overlap with the bird breeding season, whilst still achieving the desired management aims, they do not consider interannual variation in the timing of breeding shown by birds as a plastic response to interannual variations in environmental conditions (Davies & Deviche 2014).

To be effective, Bracken control achieved through cutting and crushing needs to occur earlier in the year compared to spraying, and needs to be repeated twice per year over multiple years to be successful (Stewart et al. 2008, Milligan et al. 2016). As chemical-based Bracken control becomes less widely practised, mechanical control methods are likely to become more widespread, thereby potentially increasing any overlap between vegetation management and bird breeding. Furthermore, a trend to earlier breeding in many bird species in response to climatic change (Dunn & Møller 2014, Parmesan & Yohe 2003), is also likely to increase overlap between bird breeding and burning management.

In this study, we use seven years of field data on ground-nesting birds in Dartmoor National Park, UK, to quantify the onset and timing of bird breeding activity for three co-occurring widespread UK upland birds: Meadow Pipit, European Stonechat (hereafter Stonechat) and Whinchat. We combine these data with information about currently prescribed timing of vegetation management practices to assess the extent of overlap between breeding activity and burning and Bracken control, and provide insight into the proportion of nests potentially negatively affected.

Methods

Study area

The study was conducted between 2008 and 2014 on a 4.3 km² area within Dartmoor National Park, southwest England (50° 31′ 20″N, 3° 51′ 30″W) located 211–457 m above sea level. The habitat is predominantly open, mostly comprising acid grassland and dwarf shrubs with isolated low trees, interspersed by extensive areas of Bracken. Here, and in Dartmoor National Park more widely, landowners and users carry out yearly vegetation burning to patches on a rotational basis to create new grass growth for livestock grazing. In the study area, the Dartmoor National Park Authority recommends that no burning takes place after 31 March (week 13 after 1 January) specifically to reduce overlap with the ground-nesting bird breeding season (DEFRA 2007a, Dartmoor National Park Authority 2011). The burning mostly follows the regulations for timing and burning extents from The Heather and Grass etc. Burning (England) Regulations 2007 and the voluntary Dartmoor code, such that burning generally takes place between 1 October and 31 March, with individual burns not exceeding 10 hectares in size (Anon 2007). Bracken control is undertaken principally to expand open areas for livestock grazing and for the protection of bronze age archaeological sites from Bracken root damage.

Bird breeding activity

The study focused on breeding Meadow Pipits, Stonechats, and Whinchats. All three species are associated with open habitats such as grasslands, heathlands, and upland moorlands, where they nest low on the ground, with Meadow Pipits and Stonechats preferentially nesting in clumps of grass or low shrub, and Whinchats regularly being found associated with Bilberry Vaccinium myrtillus and Bracken (Fuller & Glue 1977, Pearce-Higgins & Grant 2006, Border et al. 2017).

Nests of all breeding pairs on the study site were found by intensive searching for territories and nests, guided by observing breeding behaviours such as birds carrying nest material or food. Monitoring commenced before the start of the breeding seasons and continued until after the completion of the last active nest, to ensure all early and late nests were recorded to provide an accurate as possible estimate of timing of breeding and the numbers of pairs on the study site, and thus the potential impact of vegetation management. Some nests that failed during egg-laying and incubation, as well as a small number of second breeding attempts by Meadow Pipits, may have been missed during the most intensive period (middle) of the breeding seasons due to resource constraints; however, the full population, or very close to this, of breeding pairs were recorded each season.

Each nest was visited between one and eight times whilst active, and after fledging where relevant, to monitor breeding and determine outcome. At each nest visit the breeding stage and contents were recorded following the protocol and coding system of the British Trust for Ornithology (BTO) Nest Record Scheme (NRS). Nest recorders with experience in ageing passerine nestlings determined developmental stage according to the NRS codes, which records the growth of nestlings based on feather development (Crick et al. 2003).

Timing of nest building, hatching, and fledging for each nest was determined from the field observations and allocated a sequential week number, with week one representing 1–7 January and week 52 representing 25–31 December. Hatch week was determined either from finding eggs hatching during a nest visit (which only happened in a few instances), or back-calculated based on the nestling developmental stage when nestlings were first encountered at a visit. Deriving hatch week from the nestling developmental stage was deemed appropriate as feather development follows a predictable day-by-day pattern, and is a reliable indicator of age for passerine nestlings (Jongsomjit et al. 2007, Morales Fernaz et al. 2012). Some developmental delay of feather tracts is possible (e.g. by one to four days in the Barn Swallow Hirundo rustica) under bad weather conditions (Morales Fernaz et al. 2012), which may lead to some nests with an inaccurate estimate of derived hatch week, but this was unlikely for our data as little exceptionally bad weather occurred during the years of the study. Fledging week was determined by comparing the last visit date in which nestlings were recorded alive in the nest, and the visit date at which the nest was found empty. For nests in which hatch week could not be accurately determined, but fledge week could be, hatch week was assigned by subtracting two weeks from the fledge week as all three study species fledge their young at approximately two weeks after hatching.

We determined the full period of breeding activity from nest building through to fledging for each nest using the assigned hatch and fledge weeks, with onset of breeding being four weeks before the hatch week; allowing for one week of nest building, one week of egg laying and two weeks of incubation. The duration of the full breeding season for each species was defined as the period between the week of onset of the first nest and the week of fledging of the last nest. As many species depend on cues or optimal conditions before initiating egg laying or incubation (Visser et al. 2010), we used hatching date as our metric for examining differences between species in breeding activity. To identify the peak of the breeding season, and to compare the timing of breeding between species, we calculated the median hatch week for each species separately, across all years combined. The interquartile range was quantified to extract the main breeding period for each species; defined here as the weeks around the median hatch week during which 50% of all nests hatch.

Vegetation burning

No exact data on burning dates on Dartmoor were available for the purposes of this study, but anecdotal evidence suggests that the timings from the voluntary burning code for Dartmoor are mostly adhered to, although most burning takes place near the very end of the burning period. This is because at this time the vegetation, weather, and soil are deemed to be in optimal condition for burning, i.e. dry vegetation and wet soil. For each species, we calculated the number of weeks during which the breeding period and permitted burning period overlapped. In addition, we calculated the percentage of nests that were active during weeks in which burning was permitted, using three scenarios of cessation of burning: week 11 (the week of 15 March – a hypothetical burning cessation deadline), week 13 (31 March – the Dartmoor National Park Authority recommended cessation date) and week 15 (15 April – the end of the legal burning period following the Heather and Grass etc. Burning (England) Regulations; Anon 2007, 2021). We also quantified the overlap of breeding with burning for different breeding stages (nest building, egg laying, egg incubation, and nestling). Overlap between the timing of vegetation management and bird breeding activity was estimated using timing of breeding data across all study years combined, as by analysing overlap in timing across all years interannual variability is incorporated and we obtain an indication of the full potential period of overlap.

Bracken control

Chemical Bracken control is undertaken from mid-July when fronds are fully unfurled, until fronds senesce in late September (Pakeman et al. 1994, Natural England 2008). Consent dates vary between AES agreements, but on Dartmoor consent is known to have been granted for as early as 1 July (pers. comm. with local landowner). Using data for all years combined, we calculated the overlap between the Bracken control consent period and bird breeding activity under two scenarios, an early but permissible week 26 (starting 1 July) and a later scenario of week 31 (1 August).

Statistical analyses

To test for differences in the median hatch weeks of the three species, Kruskal–Wallis rank sum tests were used with pairwise Mann–Whitney U tests with Bonferroni correction used as post-hoc tests. All data were analysed using R version 3.0.2 (R Core Team 2015).

Results

Bird breeding activity

Between 2008 and 2014, 498 nests were found and monitored: 145 Stonechat, 268 Meadow Pipit, and 85 Whinchat. Between years, the duration of the breeding season from the onset of the first nest to the fledging of the final nest, ranged from 15 to 23 weeks for the Stonechat, with the average breeding season duration being 19 weeks. For the Meadow Pipit, the duration of the breeding season was 12–20 weeks, with an average of 17 weeks. The Whinchat, the only long-distance migrant of the three species studied, had the shortest breeding season with a range of 9–13 weeks between years and an average of 12 weeks (Figure 1). The order of breeding onset between species was consistent across all years, with Stonechats commencing breeding first, followed by Meadow Pipits and then Whinchats (Figure 2). All species showed interannual variation in onset of breeding, up to two weeks for the Meadow Pipit, four weeks for the Stonechat and three weeks for the Whinchat. For all years combined, the median earliest week of onset of breeding was week 12 for the Stonechat, week 14 for the Meadow Pipit and week 18 for the Whinchat.

Figure 1. The number of Whinchat, Meadow Pipit and Stonechat nests with hatched eggs by week, between 2008 and 2014. Weeks are numbered according to week one representing 1 January to 7 January. Vertical line denotes median hatch week.

Figure 2. Earliest week of hatching for eggs in Whinchat, Meadow Pipit and Stonechat nests between 2008 and 2014. Each data point represents the week in which eggs in the first nest hatched for that year and species. Weeks were numbered according to week one representing 1 January to 7 January.

The earliest Stonechat breeding activity occurred in week 10 when one nest (0.7% of all Stonechat nests) was found at the building stage. Meadow Pipit breeding started in week 13, when 4% of nests were at the nest building stage, and Whinchat breeding activity started in week 16, when 2% of nests were at the building stage.

The timing of median egg hatch week was week 18 for the Stonechat, week 21 for the Meadow Pipit and week 23 for the Whinchat (Figure 1). Although there was considerable overlap between species, median hatch weeks differed significantly between species (Kruskal Wallis test, χ² = 64.979, P < 0.001). Stonechat median hatch week was significantly earlier than that for both Meadow Pipit (Mann–Whitney U test with Bonferroni correction, P < 0.001) and Whinchat (P < 0.001). Whinchat median hatch week was the latest and differed significantly from that of Meadow Pipit (P < 0.001). As shown in Figure 1, the timing and duration of the main breeding periods differed between the species. The main breeding period for Stonechats, during which 50% of all nests hatched, was the longest; from week 17 to week 24. For Meadow Pipits this period was from week 19 to 24, and for Whinchats from week 23 to 25.

Burning

Bird breeding activity shows some overlap with permitted burning dates (Figure 3 and Table 1). The Stonechat showed the greatest overlap in all three burning cessation scenarios, whereas the Whinchat was not affected by burning management under any scenario. Under the week 15 burning cessation scenario, which matches current regulations, Stonechat breeding activity overlapped with the permitted burning period by six weeks. In week 15, 6% of nests were at the nest building stage, 13% at laying stage, 41% at incubation stage, and 1% at the nestling stage, with 61% of all nests being active in this burning cessation scenario. Meadow Pipit breeding activity overlapped with a week 15 burning cessation scenario by three weeks, overlapping with 29% of nests; 14% nest building, 11% laying, and 4% incubating.

Figure 3. Durations of breeding season (horizontal bars) and the proportion of all nests active per week (line graphs) for the Whinchat, Meadow Pipit and Stonechat between 2008 and 2014. The first breeding week was defined as the week four weeks prior to egg hatching, when nest building took place. The last breeding week was defined as two weeks after hatching, the last week in which nestlings were in the nest. Weeks are numbered according to week one representing 1 January to 7 January. Vertical bars represent final weeks of vegetation burning and first weeks of Bracken control.

Table 1. Percentage of monitored nests which were active during the final week in which vegetation burning is allowed. Percentages are split by nest stage. Three different final burning week deadlines are shown: week 15 (15 April), week 13 (31 March) and week 11 (15 March).

	Percentage of nests active during final burning week
	Week 15	Week 13	Week 11
Meadow Pipit Anthus pratensis
Building	14	4	0
Laying	11	0	0
Incubation	4	0	0
Nestling	0	0	0
Total potentially affected	29	4	0
Stonechat Saxicola torquata
Building	6	28	2
Laying	13	10	1
Incubation	41	3	0
Nestling	1	0	0
Total potentially affected	61	41	3
Whinchat Saxicola rubetra
Building	0	0	0
Laying	0	0	0
Incubation	0	0	0
Nestling	0	0	0
Total potentially affected	0	0	0

Both the Stonechat and the Meadow Pipit still showed overlap under the earlier week 13 burning cessation scenario; four weeks or 41% of nests for the Stonechat (28% building, 10% laying, and 3% incubating), and one week or 4% of nests (all at the building stage) for the Meadow Pipit. When considering the individual years of data, overlap was present for all years in both species under the week 15 scenario. For the week 13 scenario, overlap was present in six out of seven years for the Stonechat and two out of seven years for the Meadow Pipit. Under the week 11 cessation scenario, Meadow Pipit breeding did not overlap, and overlap for Stonechats was reduced to two weeks or 3% of nests across all years. Under this cessation scenario, overlap for the Stonechat would only occur in years in which the onset of breeding was relatively early.

Bracken control

The final week of breeding activity in the nest, when nests contain nestlings, was week 34 for Stonechats, week 33 for Meadow Pipits and week 31 for Whinchats. Bracken control commencing in week 26 results in nine weeks overlap with Stonechat breeding activity, overlapping with 28% of nests, eight weeks with Meadow Pipit activity, potentially affecting 29% of nests, and six weeks with Whinchat activity, potentially affecting 49% of nests (Table 2). Commencing Bracken control in week 31 results in four weeks overlap with Stonechat activity (potentially affecting 9% of nests), three weeks for Meadow Pipits (3%), and one week for Whinchats (2%).

Table 2. Percentage of monitored nests active during or after the commencement of Bracken control. Two starting week scenarios for Bracken control are shown: week 26 (1 July) and week 31 (1 August).

	Percentage of nests active
	Week 26	Week 31
Meadow Pipit Anthus pratensis	29	3
Stonechat Saxicola torquata	28	9
Whinchat Saxicola rubetra	49	2

Discussion

We found an overlap in the timing of the existing upland vegetation management practices of burning and Bracken control and the active breeding period of Meadow Pipits, Stonechats, and Whinchats on the study site on Dartmoor, southwest England. This strongly suggests vegetation management results in nest losses at all stages of breeding from nest building to fledging, with the extent depending on year, species, and type of management. Of the three species, the Stonechat breeding period overlaps with all burning cessation scenarios and with all Bracken control commencement dates, the Meadow Pipit breeding period overlaps with all except the earliest cessation of burning scenario and Whinchat breeding period overlaps with all Bracken control timing scenarios. The extent of the overlap will likely result in deleterious effects of the management on birds’ nesting attempts and result in the loss of significant numbers of nests in our study area.

For quantifying local population level impacts, information would be needed that quantifies the percentage of the different vegetation types used by each bird species against the percentage of that vegetation area that is managed by burning or Bracken control. Habitat generalists, especially species that are also widespread in lowland areas, will be less impacted (Wilson et al. 2021). Also, relevant to considering local population impacts would be the propensity of these species to re-nest after losing a clutch, and the background rate of nesting attempt success.

Vegetation burning

Studies of the direct impact of vegetation burning on bird nest outcomes are scarce. Where nests are within areas where vegetation is burnt, breeding failure must be almost inevitable, and has been observed by studies of grassland burning practises in the United States, which showed high failure rates from land burning across a range of bird species (Erwin & Stasiak 1979, Kruse & Piehl 1984). Indirect effects of burning on bird populations are more extensively studied. For example, in upland areas in Scotland and northern England managed for Red Grouse shooting, the area of burnt ground was 34% greater in areas managed for shooting compared to unmanaged upland areas, and the density of four bird species, including the Meadow pipit and Whinchat, was lower (Tharme et al. 2001). It is worth emphasizing that burning will not necessarily only impact on the immediate breeding success of a bird for that year, but also through subsequent years in terms of how the successional vegetation and habitat structure affect whether the bird species then chooses to nest there.

Our study showed overlap between legally permitted UK upland burning and the breeding activity of Stonechats and Meadow Pipits under all three burning cessation scenarios. In the last permitted week of burning (15 April), over 60% of Stonechat and nearly 30% of Meadow Pipit nests were active, with a large proportion of nests at the laying and incubation stages (54% of Stonechat and 15% of Meadow Pipit nests). This is reduced to 41% and 4%, respectively, if the cessation of burning follows the Dartmoor National Park Authority recommendation of 31 March. Our earliest cessation scenario (the week of 15 March) would result in no overlap with Meadow Pipit breeding activity, and overlap with Stonechat activity reduced to a maximum of two weeks (nests in building and laying stages only), and only in years in which onset of breeding is early. Our findings of an overlap between burning and breeding for these species is consistent with Wilson et al. 2021, who conducted a similar study at a larger scale. The percentage overlap found in our study is higher, which is at least partly due to a difference in methodology between the studies, with this study considering breeding activity from the nest building stage onward, and Wilson et al. 2021 considering clutch initiation as breeding commencement (one week later by the estimate used in this study). Wilson et al. (2021) found 41% of Stonechat and 2% of Meadow Pipit clutches were initiated by 15 April, and 8% of Stonechat and 0% of Meadow Pipit clutches by 31 March.

Other species are also likely to be negatively affected by the currently permitted timings of burning. In our study area for example, we also found nests of Eurasian Skylark Alauda arvensis and Common Linnet Linaria cannabina at the hatching stage in week 19, meaning both species would have commenced nest-building during the week 15 burning cessation scenario.

Bracken control

The timing of Bracken control overlapped with breeding activity when commenced in July, when there are six to nine weeks of the breeding season remaining for the three bird species in our study. More than a quarter of Stonechat and Meadow Pipit nests, and half of Whinchat nests, are potentially affected by current timings of Bracken control. Although the breeding season is not fully completed for any of the three species by 1 August, this later start date would substantially reduce overlap, as less than 10% of nests would be active. Other ground-nesting species are also likely to be affected by Bracken control; for example, in our study area, we found Tree Pipits Anthus trivialis fledging as late as mid-July (week 29).

Vegetation management

To quantify the overlap between vegetation management and bird breeding periods, we calculated the percentage of nests active during the management window. In the case of burning, all active nests in burnt areas would be destroyed. However, for mechanical Bracken control, patches of non-Bracken vegetation might be left untouched, allowing birds breeding in patches with low Bracken cover within wider Bracken-dominated areas to breed successfully, despite management taking place in their vicinity. This is more likely to be the case for Stonechats and Meadow Pipits than for Whinchats, as Whinchats are known to show a breeding preference for Bracken-dominated areas (Stillman & Brown 1994, Pearce-Higgins & Grant 2006). Therefore, decision-making regarding permitted timings in areas where Bracken control is desired should take into account the presence of bird species with an association with Bracken-rich habitats.

We are not aware of any published data on nest loss resulting from Bracken control, but impacts from mechanical control are likely to be similar to the management practice of mowing, which shows substantial impacts on nest success and adult survival. For example, early season mowing negatively affects the breeding success of Whinchats (Müller et al. 2005, Grüebler et al. 2012, Strebel et al. 2015), Corncrakes Crex crex and some waders (Vickery et al. 2001), and nest losses also occur from arable crop harvesting (Brickle & Harper 2002, Perkins et al. 2013). Losses can be reduced by changing the timing of management practices. Grüebler et al. (2012) showed that Whinchat nest survival in Swiss alpine meadows increased from less than 10% to over 70% when mowing was carried out later in the season, and Perkins et al. (2013) found that delaying mowing to 1 August could increase Corn Bunting Emberiza calandra breeding productivity by 20%.

Our study focuses on the period of breeding centred on the nest, but there may also be negative impacts resulting from an overlap between Bracken control and the post-fledging period. For Whinchats, previous work shows that at eight days after fledging, half of Whinchat fledglings respond to threat by remaining still rather than attempting to escape (Tome & Denac 2012), and the same study found evidence of fledglings being killed directly by mowing. Therefore, whilst a start date of Bracken control of 1 August affects a small proportion of active nests, an area can still contain family groups of fledglings which may not survive mechanical Bracken control activities.

Timing of breeding

The onset of breeding, the main breeding period and the duration of the breeding season varied between the three species, consistent with previous studies (Fuller & Glue 1977, Joys & Crick 2004). Interannual variability in onset of breeding in our study was two to four weeks depending on species. Timing of bird breeding is well known to vary between years – for example, in response to climatic conditions – and shows temporal change (Crick & Sparks 1999, Hoover & Schelsky 2020, Wesołowski et al. 2021); thus the extent of overlap with vegetation management will similarly vary between years, although by pooling years in our study we cover the full range under current breeding timings of these species. If the current trend to earlier breeding continues, the extent of overlap with burning will increase (and lessen with bracken control) if the currently prescribed management dates remain unchanged (see also Wilson et al. 2021). Management timings therefore need to track and respond to observed changes in the timing of breeding of upland species wherever possible.

Timing of breeding also varies with geographical factors such as elevation and latitude (Sanz 1998, Mainwaring et al. 2012); therefore, potential impacts of land management are likely to differ between locations not only due to differences in land management practice but also due to variation in timing of breeding. However, the peaks in breeding activity we recorded are similar across England (Joys & Crick 2004) and, therefore, the land management dates we suggest should be directly applicable to other upland areas in the UK.

Study limitations

Due to a largely voluntary nature of the fieldwork, nest survey effort varied, with less effort in some weeks and years, and in addition, nesting attempts that failed early (e.g. during egg-laying) were more likely to be missed. This may have resulted in small inaccuracies in the estimates of the percentage of nests active under the different vegetation management scenarios. However, monitoring always comprehensively covered early and late nests for all species in each season so we deem it highly unlikely that estimates of weeks of overlap between breeding and management are affected by these limitations.

Both Meadow Pipits and Stonechats are double-brooded species, and Whinchats can also double-brood in some years. As the birds in our study were not marked, they could not be followed individually through the season; for this reason, each nest was considered to be independent. Therefore, subsequent breeding attempts by the same pairs could not be considered separately. It should be noted, however, that for early-breeding pairs which lose a nest early in the season, for example due to burning, re-nesting may occur soon after and thus may not constrict the opportunity to successfully produce two broods. Future studies of daily egg and nestling survival would enable modelling of the probability of re-nesting (Maggs et al. 2015), which could be used to explore annual productivity, and hence potentially improve precision of estimates of nest losses from vegetation management.

It should be noted that nest loss was not directly studied. The number of nests negatively affected by vegetation management will vary according to differences in breeding densities and in the extent of land managed. To evaluate impacts on populations on the Dartmoor study site, knowledge of habitat specific estimates of population densities, as well as dates and spatial extents of burning and bracken control, are needed. This includes collecting data to assess to what extent the burning has occurred in the habitats favoured or avoided by the focal species, and direct monitoring of nest losses in the burnt and Bracken treated areas.

Recommendations

We found that the land management practices of both burning and Bracken control, applied to Dartmoor for the purposes of facilitating grazing enhancement, may have negative impacts on some of its upland breeding birds. We thus recommend that wherever possible the prescribed dates for these management practices are set to take into account the full active breeding period of the upland breeding bird community. To minimize direct impacts, we recommend that burning is avoided in late March. A burning cessation date of 15 March would prevent overlap with Whinchat and Meadow Pipit breeding activity and minimize overlap with Stonechat breeding activity. We also recommend that Bracken control is delayed to after 1 August. Due to the ecology of Bracken, effectively managing Bracken growth whilst completely avoiding overlap with bird breeding activity is challenging, especially in the first year of mechanical control. We therefore recommend that if Bracken control needs to be carried out before August, sites are surveyed to record local breeding hotspots and the presence of likely affected species, which can then be avoided.

Acknowledgements

We are grateful to Joshua Marshall and Lowell Mills for their help with nest finding and nest monitoring. We would like to thank the Dartmoor National Park Authority and the study area landowners for facilitating the work.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research was funded by the University of Exeter (CRT) and also supported by Devon Birds.