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New Forest breeding Raven survey 2020-21

Co-ordinated by Russell Wynn (Wild New Forest)


Note that a text-only version of this report can be downloaded below in PDF format.

NF_Raven_survey(20-21)
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Executive Summary

  • Ravens returned to Hampshire as a breeding species in 2003 and have increased rapidly, with the New Forest appearing to be a particular stronghold for the species.

  • An informal survey of breeding Ravens was therefore conducted in the New Forest National Park in 2020-21, to quantify recent population growth and provide a baseline for future change.

  • A total of 23 Raven territories were identified, with evidence for ongoing recruitment of immature birds.

  • Of 16 confirmed or probable nests (where there was high confidence in the nest location), four were on electricity pylons and the remainder were in large non-native conifers.

  • Nests and territory centres were typically spaced 3-5 km apart, with breeding ranges in the order of 35-50 km2 – there are some indications that the population may already be approaching the local carrying capacity.

  • Productivity was consistent with national data, averaging about three fledged young per pair.

  • The results indicate that as many as 100 Ravens may be present in the New Forest during the post-fledging period in May and June, and that this may exert additional pressure on vulnerable ground-nesting waders such as Curlew and Lapwing.

  • Future changes to forest landscapes, e.g. underground burial of pylons and removal of non-native conifers, may negatively impact breeding Ravens.


A typical view of a Raven on territory in the New Forest, taken in Feb 2020


Introduction

Ravens Corvus corax have successfully reoccupied much of their former range in lowland England over the last three decades, primarily due to reduced persecution and increased availability of roadkill and other anthropogenic food sources. Ravens returned as a breeding species to Hampshire in 2003 and increased rapidly, with the New Forest being a particular stronghold, although a lack of dedicated monitoring has meant there is high uncertainty as to the actual number breeding in the National Park.


Although Ravens now have a positive conservation status in England, their success has brought them into conflict with livestock farmers and there are increasing concerns about their impact on vulnerable ground-nesting birds such as waders.


In early 2020, Wild New Forest initiated an informal volunteer-based survey of breeding Ravens in the New Forest National Park. The aim was to provide a baseline against which further population and distribution changes could be measured, and to better understand their use of man-made structures and non-native conifers as nesting sites. The survey was conducted over two years (2020-21).


Methods

Ravens are early breeders, with territories (re)occupied during the winter and BTO data indicating an average laying date in early March. Clutches generally comprise four to six eggs and the young fledge in early May, with an average of three fledged young per breeding attempt. Consequently, the New Forest survey focussed on the period from 01 January to 30 June.


Access restrictions associated with COVID-19 came into force in March 2020 and at first it looked as though the survey would need to be postponed. However, Wild New Forest were commissioned at short notice by Forestry England to conduct a survey of breeding waders in the New Forest interior, to assess potential changes in habitat use associated with reduced recreational pressure during lockdown. This survey covered over 100 km2 of suitable breeding habitat across the forest and provided a great opportunity to locate breeding Ravens, which were particularly audible due to reduced anthropogenic noise.


As Ravens are conspicuous birds and relatively easy to identify, they provide a good opportunity for volunteer observers to provide useful records from their local area. A request was therefore issued via Wild New Forest social media channels for any New Forest Raven records in the January to June period. Observers were asked to record date/time, location, number of birds, and any pertinent behaviour, e.g. tumbling, nest-building, carrying prey. In addition, several observers volunteered to search for and then monitor known nests in their local area.


Raven feather found beneath an active nest in May 2021 - the nest is just visible in the tall conifer in the background


Finally, Hampshire Ornithological Society (HOS) provided a summary of historical New Forest breeding records, as well as contemporaneous Raven records from the survey period; these included data drawn from online sources such as Birdtrack and Going Birding, but perhaps surprisingly the only records of confirmed breeding were those already submitted by the author of this report!


Breeding evidence was categorised as follows:

  • Confirmed breeding related to an occupied nest containing an incubating bird or chicks.

  • Probable breeding related to birds carrying nesting material or food towards suitable habitat and/or displaying defensive behaviour

  • Possible breeding related to multiple records of birds in suitable breeding habitat, including territorial behaviour such as display and vocalisations, but where there was no direct evidence of breeding (some of these may relate to non-breeding immature birds that are nevertheless occupying territories).

Results

From analysis of all available data, comprising about 1000 records, a total of at least 23 Raven territories were identified in the New Forest over the two years of survey, of which nine were confirmed nesting and seven were probably nesting (see map below). Four of the confirmed nests were on pylons around the northern and eastern margins of the forest, and the remainder were in tall conifers such as Scots Pine and Douglas Fir; all remaining records of probable or possible breeding related to birds associated with tall conifers. Based on data provided by HOS, many of these are traditional nest sites that have been used for many years. There are a further six locations that are historic breeding sites but with no evidence of breeding in 2020/21, or where there are scattered recent records but no evidence for territorial behaviour.

Map of the New Forest National Park showing locations of the 23 Raven territories mapped during this study. Coloured circles mark the position of nests or territory centres and are shown here at 2 km resolution to avoid disclosing specific locations. Note the regular spacing of territories, with the only ‘obvious’ gaps being in the southwest and southeast of the New Forest.


Fledged young were observed at or close to several sites from the first week of May onwards (earliest 02 May), with 12 reports of two to four fledged young over the two years, corresponding to an average of about three fledged young per nest, consistent with national BTO data.


A recently-fledged juvenile Raven in May 2021 - note the relatively fresh flight feathers with a brownish cast, and the yellow gape


Ravens were observed acting aggressively towards a range of aerial predators, including Buzzard, Red Kite and Goshawk. Both breeding and non-breeding Ravens were observed being mobbed by breeding Curlews and Lapwings on numerous occasions, and a report was received of a Raven possibly predating a Lapwing nest at one site. Ravens were seen feeding on dead Canada Geese at the coast on multiple occasions, and on a Fallow Deer carcass, roadkill Grey Squirrels, and food waste in black bin bags awaiting collection in the New Forest interior.


Raven mobbing a Buzzard over a nest site in April 2021


The closest nest spacing was about 3 km, and it was notable that almost all identified territory centres were spaced 3-5 km apart. There were several observations of territorial birds driving away other Ravens within 1-2 km of known nest sites. In addition, a distinctive bird with a damaged (see image below) provided an opportunity to assess foraging range and was seen on multiple occasions carrying food back to the nest from open farmland and paddocks up to 3 km away. In another area, a large herd of cattle attracted a mixed flock of corvids that included off-duty Ravens from a territory up to 3.5 km away. Overall, these observations suggest an approximate New Forest territory size of 35-50 km2.


This distinctive Raven with a damaged leg was regularly seen foraging in paddocks and fields up to 3 km from the nest tree in a conifer plantation


Ravens don’t breed until they are three or four years old and commence wing moult in April and May – this is much earlier than breeding birds, allowing them to be easily identified in flight. It was therefore unsurprising that several non-breeding immature pairs and roving flocks were seen during the survey. Roving flock sizes were typically in the order of four to ten birds, much smaller than the large concentrations seen in recent winters on nearby downland sites, which presumably mostly comprise immature birds attracted to agricultural food sources.


Discussion

Data collected during this survey indicate that there were at least 23 Raven territories within the New Forest National Park in 2020-21, including several apparent immature pairs not yet recruited into the breeding population. There was evidence at one site for an immature pair on territory in 2020 going on to breed successfully in 2021, supporting the idea that recruitment is ongoing.


This nest in a Scots Pine in May 2021 belonged to a pair of Ravens making their first nesting attempt, which resulted in at least two juveniles successfully fledging


Ravens are highly mobile, even when nesting, so interpreting the large number of scattered records in the breeding season away from established territories is challenging. It is possible that one or two pairs may have been missed on private land or at traditional breeding locations that are hard to access, but it is considered unlikely that there are more than 25-30 pairs currently nesting in the New Forest.


Given that existing territory sizes are mostly in the range of 35-50 km2 and that the New Forest National Park covers 566 km2, there would appear to be limited space for new recruits; it is therefore possible that the breeding Raven population will soon reach a plateau, similar to the New Forest Goshawk population that has experienced a similarly dramatic increase over the last two decades but which now appears to have stabilised at 40-45 pairs, suggesting it has reached the local carrying capacity.


Ravens are omnivorous and highly adaptable, and there is evidence that they are benefitting from anthropogenic food sources, including the remains of culled deer, roadkill mammals, and food waste. During breeding wader survey work conducted by Wild New Forest in spring 2020 and 2021, there were regular observations of Ravens being mobbed by breeding Curlews and Lapwings, both during the incubation and chick-rearing phases. Of all the potential avian predators in the New Forest, Ravens generated the most extreme mobbing response by breeding Curlews, with two observations of up to seven Curlews from multiple territories noisily working together to drive Raven family groups away from breeding areas. Based on the population estimate derived from this survey, and an average of three fledged young per nest, it is possible that as many as 100 Ravens are present in the New Forest in May and June when the eggs and chicks of breeding waders are most vulnerable to predation; despite a lack of direct evidence, it seems inevitable that the recent increase in Ravens will be contributing to the observed low productivity of breeding Curlew, Lapwing and Redshank in the New Forest interior.


Raven feeding on carrion at Oxey Marsh in April 2020 (Marcus Ward)


Breeding Ravens have no doubt benefited from ready access to suitable breeding sites in the form of electricity pylons and non-native conifers. In the future, proposals to reroute pylons underground and a planned transition away from non-native conifers as part of a long-term management strategy on the Crown lands are likely to negatively impact this species. The baseline data generated in this study will hopefully allow these predicted future changes in status to be identified and measured.


Raven in flight over the New Forest at sunset in May 2020


Acknowledgements

I am grateful to all members of the Wild New Forest community who provided records as part of this survey, particularly those who identified and/or monitored nest sites. I am also grateful to all HOS members who submitted Raven records, and to Keith Betton and Marcus Ward for extracting historical and contemporary New Forest records from the HOS database. Wild New Forest survey work on the Crown lands is conducted under Forestry England permit.

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