OBSERVATIONS OF THE WINTER PRE-ROOSTING BEHAVIOUR OF VARIOUS CORVIDS USING BUNKERS HILL WOOD.

Susanna Grunsell and Brian Johnson
(This article is based on work undertaken for the Birmingham University Certificate in Ornithology)

Introduction.

The Corvid roost at Bunkers Hill Wood has for many years being considered to be one of the larger Corvid roosts in the West Midlands. The wood was until 1996 a private shooting area, but was subsequently purchased by the Woodland Trust and opened up to public access. This study on the pre-roosting behaviour of the birds using the roost was carried out to confirm the use of the pre-roosting and roosting areas. The study was carried out over seventeen separate visits during the winter from November 2001 to March 2002 when the roost would normally break up as the birds move into the breeding season.

Methodology

The study site was visited on several occasions prior to the start of the project to determine the best site to observe the Corvid activity around the wood. Eventually agreement was reached on the observation point being 200 metres from the south east end of the wood. This was selected because of the increased visibility of the wood, and its height relative to the surrounding area.

To enable the information to be recorded quickly we produced a standard recording form that allowed us to record the activity while observing the birds. The standard form broke down the various activities such as flying, circling, calling, gliding or actually landing on the ground or in a tree. The form also enabled the flight path to be recorded when the activity occurred.

The information from the seventeen visits that was gathered and then used to identify possible patterns and trends, and to assess the behaviour between the various Corvid species. Analyses of the recorded data were carried out to identify any particular patterns and trends.

Results

The results of the study indicated a reduction both in the number of birds and the size of the flocks using the roost during the last decade. The maximum flock size of 750 Jackdaws Corvus monedula was observed on only one occasion. The maximum flock size of the Carrion Crow Corvus corone was only 60 birds. During our visits there were no recorded observations of Rooks Corvus frugilegus using the roost.

The variation in the number of birds observed in each of the visits ranged from a maximum of 1132 birds observed on the 29th December to a minimum of 108 birds observed on the 14th December. Temperature was at 7 ºC and 6 ºC respectively and weather conditions were similar for both dates.

There are definite flight paths used by the different species to fly to and from the pre-roosting area or return to the roost in the evening. The Carrion Crows flew in a southerly direction to approach or leave the wood 146 times in comparison to the Jackdaw that flew along the same flight path 170 times. When both species flew in a northerly direction the number of observations for the Carrion Crow was 73 and for the Jackdaw was 42. Other directions were used to fly into or over the wood, the Carrion Crows flew into the wood from the east on 20 occasions and from the west on 26 occasions. The Jackdaws flew in from the east on four occasions and from the west on nine occasions. To a much greater degree the north south flight path was used by both species to return to the roosting area. As the breeding season approached behaviours changed, flock sizes decreased, and the Jackdaws returned to the wood to roost.

The flock sizes varied from visit to visit and there was a decline in the larger flock sizes over the five months of visits. The size of the flocks for the Jackdaws started in November with only two flocks, one of three and the second flock was forty. The flock sizes rose steadily throughout December and January when the highest numbers were recorded, one flock of 750 at the beginning of December and the second largest of 550 at the end of December, which also coincided with the largest number of flocks. The number of flocks began to reduce in February and March, with the flock sizes also reducing in February and March to a maximum of 40 birds in any one flock. These flock sizes varied considerably from that of the Carrion Crows where the flock size remained consistently lower throughout the period reaching a maximum of 60 birds on only one occasion. The number of flocks also fluctuated on a visit to visit basis with the maximum number observed on one visit being Jackdaws on 28th December.

The summary of the behaviours observed showed that flying was the predominant activity. The data also suggested that circling increased in both species in March, and during December and January calling was the main activity. Gliding was not a common occurrence in either species. Jackdaws were not seen to land, other than exceptionally, during observations. Carrion Crows were seen to land in the field with other birds throughout the winter period, and this was seen to increase suddenly at the end of February.

Further investigation of the wood as a roost and the surrounding area would be beneficial to determine the relationship between Bunkers Hill Wood and the other local roosts which are being used by the Corvids. These could have an affect on the numbers of birds using the roost, our number of observations of Jackdaws flying over the wood and continuing south towards other possible roosts were significant.