Minsmere Nature Reserve Bittern Breeding Census 1976

MINSMERE

NATURE

RESERVE CENSUS

BITTERN

BREEDING

1976 R . BERRY

History F R O M 1959 and possibly before, bitterns (Botaurus stellaris) have bred at Minsmere Nature Reserve. It appears that the breeding population has increased over the years from six pairs in 1959 to 10 pairs in the late 1960s and from 1970 to the present the population has fluctuated between twelve to fourteen pairs.

It would also appear from records made by H. E. Axell, the Warden at Minsmere for 16 years, that some of the breeding areas used have been traditional sites for many years, such as the 'Decoy Pools' site first mentioned in H. E. Axell's Reserve Log Book in 1960, also two pairs which have been breeding in an area known as the North Marsh since 1969.

The only feeding flight mentioned by H. E. Axell in the Reserve Log Book, of a bird going outside the reedbed to feed was in 1961; it concerns the 'Decoy Pools' bird, the extract reads as follows, 'Parent bird bringing food has been landing very close to nest after hunting in ditches in Ropes field ca f mile away.'

DĂźring 1975 and 1976 a large number of feeding flights were observed, so it does appear therefore that feeding flights of birds going outside the reedbed have increased in recent years. A possible reason for this is discussed later.

Number of Pairs Thirteen pairs of bitterns bred at Minsmere in 1976, this figure includes one pair which bred off the main marsh in an area known as 'Meadow Marsh'. This is around 800 m. from the main reedbeds, and because of this the least data were collected and at one time during May it was thought that this pair had left the area because booming ceased.

The main difhculty during the census was the very short booming bursts from the various sites tbroughout. Despite this, simultaneous booming was recorded from the most difficult areas, these being the sites that were close together, and without the evidence of simultaneous booming in one case two pairs might have been plotted when in fact there were three pairs in the area.

The most interesting information gained from the census was the distance some birds flew on feeding flights during June. One bird from the North Marsh area which is below Dunwich Common,

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made regulär flights to some grass meadows to the south, a distance of 1,500-1,800 m. or just over one mile.

T h e main reedbed, an area known as the 'Island Mere', where nine pairs of bitterns bred, Covers a total area of some 6 2 | ha.

Within this are fourteen areas of open water totalling around 5 | ha.

T h e second area of reedbed, where three pairs of bitterns bred, is known as the 'North Marsh' and is a very much smaller and drier reedbed. It Covers a total of 24 ha., with only f ha. of open water in a series of small open pools and several drainage ditches.

Regulär visits were made to all areas of the main marsh from the third week in February to mid-July. All bittern sightings from February up to the end of May were plotted onto a scale map of 4 cm. = 100 m . ; these were to show the usage made of the reed marsh by all pairs before feeding flights commenced. Boomings were plotted in the same way but more accuracy was used as it was intended to mark if possible the extent of each booming territory, thus to show that birds did not boom from just one position in the reedbed.

All feeding flights were again plotted in the same way, a special note being made of the distance travelled and the type of area used, e.g. outside the reed marsh. T h e feeding flights on Map 3 represent only a small sample of the actual flights that were recorded.

Booming T o help overcome this known problem of locating the exact position of booming males out in the reedbed, as much elevation as possible was sought; one such vantage point was a tall Alder tree along the north-west side of the main reedbed, another was on top of a hill overlooking the Island Mere, an area known as Whin Hill.

T h e first two bitterns were heard booming on February 20th in the area south of the main mere. From this date booming became more frequent from other sites, but the most noticeable aspect was the lack of any continuous booming from any of the thirteen birds involved. It soon became apparent that the early morning was the most active period, only on eight days was booming recorded after 09.00 and it did not recommence until an hour before dusk. On one occasion seven simultaneous boomings were plotted, despite the fact that none of the birds gave more than two or three booms. T h e simultaneous booming made it possible to separate three pairs each 50-100 m. apart, these would otherwise have been very difficult to plot with accuracy.

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T h e temperature appeared to play a very important part in the booming activity or lack of it. There were two periods when booming ceased altogether, one from March 5th-8th when the average maximum day temperature reached only 3°C and the night —1°C. T h e other period was from March 15th to April Ist when the maximum day temperature was 7°C and the minimum 3°C. It is by no means certain that the drop in temperature was the direct cause of the cessation of booming activity; however, many other species of birds cease to sing when there is a sudden drop in temperature or adverse weather conditions as was the case in this instance.

Many visits were made to all areas of the reedbed and each booming recorded onto a map (see Map 1), the object being to plot with as much accuracy as possible the area covered by each male. Each booming site was plotted by estimating the distance in metres from a known point on the map. It was soon established that all twelve males in the main reedbed boomed over large areas, with some having booming sites as much as 200 m. apart.

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One bird ceased booming during the third week in March and no other activity came from this site. Just after, however, a bird was recorded booming in the area known as Meadow Marsh some 800 m. to the west, and continued to be recorded on a number of visits to the area up tili May 20th.

T h e last bird to be heard booming was in the main mere area on May 29th. Between February 20th and May 29th well over 200 booming sites were recorded, of these only 173 were plotted onto a map from the twelve males within the main marsh. This was because in many cases the recorders could not give the exact location of the booming bird within the reedbed.

Bittern Sightings Prior to Feeding Young T h e sightings of birds from February 20th to May 30th were also plotted, a special note being made of their point of ascent and descent. As can be seen (Map 2) many fit in well with the booming sites. It was, however, apparent that some birds were Aying and feeding within another bird's territory, and moreover that birds

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from the North Marsh were Aying over long distances within the reedbed areas even before any young had hatched. This was the first indication of a possible food shortage within this area, further data to suggest this being the feeding flights later in the season.

Sightings within the main marsh were over much shorter distances, this area having many narrow ditches and shallow dykes, along with an open water area of some 5f ha. The combination of more reed edge, i.e. open water, and the sightings over much shorter distances could further indicate that this area is a richer feeding ground and is ideal for bitterns not only before but also after the young have hatched when a far greater food supply is required.

Feeding Flights The most important information to come from the observations made during feeding flights was the considerable distances travelled by five females to and from the nest (see Map 3). The longest distance was covered by one of the females from the North Marsh, this bird fed over one mile from the nest site on an area of grazing meadows which has many old ditches and dykes across it. This was the same area used by two other females, also from

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the North Marsh. The average distance travelled by these birds was 1,400 m., and they were away from the nest site between 50-70 minutes at a time. On July 6th one of these females was watched during the day rising from her nest site and made ten flights to and from the feed area covering a total distance of around eighteen miles in nine hours.

It was interesting to note that from February 20th to May 30th these same birds also travelled the longest distances to feed.

This, together with the information gathered throughout June when young were being fed indicated yet again that there was a possible shortage of food or a lack of open water areas in the North Marsh.

Two females from the west end of the main reedbed also spent a long time travelling to and from their nest sites to an area of grazing meadow 800 m. away. However, it was interesting to see that these flights did not take place until the young had hatched and this could therefore suggest that there was an ample food supply in the ditches close by before hatching, but that it is insufficient when the young have hatched.

Very little movement was recorded from the remaining seven pairs in the main reedbed throughout June and early July. The feeding flights when recorded were over short distances and within the reedbed, though on a few occasions a female was observed going to the edge of the reeds to feed.

Summary To sum up the information gathered on feeding flights, it was thought that flve pairs had to seek a main or supplementary food supply some distance from the nest site, and that seven pairs were able to obtain sufficient within the reedbed where there were much larger areas of shallow open water with many ditches.

Because of the lack of any activity from two nest sites throughout June and July it was thought that both had failed sometime during the incubation period. The pair which bred in the Meadow Marsh were seen on several occasions Aying to and from the reedbed to the edge of some grazing meadows, a distance of around 300 m. However, due to the limited number of visits made to this site it is not known if this was a regulär habit. This reedbed, parts of the main marsh and large areas of the North Marsh are of poor quality with very little open water; it was from these areas that the females flew long distances to feed.

One possibility could be that the population has grown so that much of the top grade feed areas are taken up by long established pairs within the reedbed. Furthermore these areas have become smaller and overgrown by the encroaching reed and sedge that

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has spread over the open water and ditches which at one time criss-crossed the main reedbed and were many years ago the main drainage ditches for what was then grazing meadows.

Another possible explanation for these long flights could be that during the spring thousands of frogs and toads come down into the reedbeds to spawn, afterwards moving out, followed in May by the young toads to the damp grazing meadows south and west of the main reedbed. It could be that this is the food supply which attracts the female bitterns and is utilised if there is a shortage of easily obtainable food in the immediate nest area both before and after incubation.

Acknowledgements I would like to thank all the voluntary watchers who made this breeding census possible during the summer of 1976, and to the full-time staff at Minsmere.

References H. E. Axell, 1959-1974.

L o g Book.

Unpublished records f r o m the R.S.P.B. Reserve

R. Berry, Minsmere Nature Reserve.