Breeding Bitterns at Minsmere 1977

Breeding Bitterns at Minsmere 1977 ROB

BRRRY

Introduetion In c o n t r a s t with the high bittern, Botaurus stellaris population of 1976 w h e n t h i r t e e n pairs were r e c o r d e d breeding, n u m b e r s in 1977 d e c l i n e d dramatically with only seven pairs p r e s e n t , their success o r failure being u n k n o w n for obvious reasons.

A f t e r t h e c e n s u s of 1976, the main emphasis in 1977 was on establishing first the n u m b e r of b r e e d i n g pairs and secondly locating t h e i m p o r t a n t f e e d i n g a r e a s t h r o u g h o u t the season.

T h e r e a s o n f o r the 1977 decline may have b e e n that it was simply a n a t u r a l fluctuation in the p o p u l a t i o n , or it could have b e e n associated with the general decline that a p p e a r s to be t a k i n g place with the bittern p o p u l a t i o n s in East Anglia.

H o w e v e r o n e can only wait to see what transpires during the Coming s e a s o n (1978).

Nest Site Distribution A s can be seen f r o m m a p 1 the reserve lost a pair of bitterns b r e e d i n g in the N o r t h M a r s h , f o u r f r o m the M a i n Marsh and also o n e f r o m t h e M e a d o w M a r s h which is 800 metres west of the M a i n M a r s h . T h e losses occurred in both wet a n d dry r e e d b e d sites. In all a r e a s w h e r e t h e r e was a loss of a b r e e d i n g pair, b o o m i n g s a n d sightings were n e v e r r e c o r d e d during the c e n s u s p e r i o d (19th J a n u a r y to 31st A u g u s t ) , indicating that no birds e v e r a t t e m p t e d to establish a territory at any time during the b r e e d i n g s e a s o n .

O f t h e two pairs b r e e d i n g in the N o r t h M a r s h below the D u n w i c h C o m m o n (see m a p 1), o n e pair m a d e regulär flights f r o m t h e r e e d b e d to the grazing m e a d o w s s o m e 1800 m e t r e s s o u t h , but t h e s e w e r e m a d e only in the m o r n i n g and were not so f r e q u e n t as t h o s e r e c o r d e d in 1976, usually consisting of just o n e flight out a n d back. Twice the point of descent was i n s p e c t e d but no trace of the adult bird, or a nest was ever f o u n d , possibly b e c a u s e the y o u n g may have been moving a r o u n d t h e r e e d b e d having left t h e nest site.

Booming T h e first b o o m i n g r e c o r d e d was on 25th F e b r u a r y at 08.00, five d a y s later t h a n t h e first r e c o r d e d b o o m i n g in 1976, the second

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b o o m i n g in 1977 came f r o m the same bird a week later on 4th March.

T h e second established booming site, and what was to be the second pair was not recorded until 24th March. In fact b e c a u s e of the paucity of booming it was not until mid April that it was established the reserve had at least seven booming sites (see m a p 1). Although low t e m p e r a t u r e s could have been t h e cause of the lack of booming activity in February and M a r c h , this was obviously not the cause in April or May when t h e t e m p e r a t u r e s remained well above freezing point. O n e w o n d e r s if the reason could just have been the lack of c o m p e t i t i o n in the vicinity of each recorded booming area, as i n d e e d later in this paper lack of competition is shown to have possibly a f f e c t e d feeding behaviour also.

Bittern Sightings and Feeding Flights D Ăź r i n g 1977 sightings of bitterns throughout the Minsmere L e v e l were r e c o r d e d from 19th January to 31st August, each o n e being e n t e r e d into a log book. O n c e again much emphasis was placed on the importance of establishing accurately the p o i n t of ascent and descent. Of the 180 sightings recorded only 123 w e r e sufficiently accurate to be considered and from these a s a m p l e were plotted o n t o a map (see m a p 2), again this year the intention was to establish the usage of the reedbed and wet grass m e a d o w s to the south of the reserve by the bitterns.

A s can be seen f r o m m a p 2 one pair of bitterns used the wet m e a d o w s with their associated small ditches and old drainage d y k e s f r e q u e n t l y f r o m 2nd J u n e to early in July. T h e s e flights habitually took place in the morning and consisted of a single tiight o u t and back. Without any d o u b t many flights went u n r e c o r d e d d u e to the lack of time available for this work.

T h e s e flights were identical to those followed by a pair of b i t t e r n s which bred within this area of marsh during 1976.

D ß r i n g the same period a pair of bitterns at the south-east c o r n e r of the N o r t h Marsh also made flights to a small wet r e e d b e d at the southern end of the area known as the Scrape, h o w e v e r unlike 1976 these flights were only made sporadically. It is quite possible that this could be explained by the a b s e n c e of the third pair of bitterns (see m a p 1) f r o m the N o r t h Marsh in 1977, which gave this particular pair and the pair f u r t h e r to the north-west, a far greater expanse of open w a t e r and ditch systems in which to ränge over.

T h i s is substantiated by the fact that there were many

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sightings of short distance flights from the pair at the south-east corner of the North Marsh over the bank north into the vacant territory of the absent pair (see maps 1 and 2).

Bittern sightings within the Main Marsh throughout the period from January to August followed almost the same pattern as that of 1976, that is apart from a pair which held a territory at the west end of the Main Marsh, these did not leave the reedbed area during the period from May-July as they did in 1976. Again a possible explanation could have been the absence of a pair which bred 150 metres further west in 1976, thus again giving this pair a much larger territory to utilise.

During 1977 fewer sightings of bitterns were recorded over the Minsmere Level. This could have meant that there was a much greater foraging area available within the reedbeds because of the decrease in the population during 1977 or that the high density of bitterns breeding on the reserve in 1976 forced many of them to seek a food supply further afield when young were in the nest. There was in fact a density of one pair per 6.65 hectares of reedbed at Minsmere in 1976, however in 1977 this figure reduced to one pair per 12.36 hectares.

Summary It was established that a decline in the bittern population had taken place at Minsmere, with only seven pairs breeding in 1977, c o m p a r e d with thirteen pairs in 1976 and the areas from which these losses had occurred were located.

Booming is then discussed and is shown to have commenced later and occurred much less frequently than in 1976 and it is suggested that a possible reason for this could be the lack of competition due to fewer pairs being present in the reedbeds.

T h e incidence of bittern sightings and feeding flights was then investigated. O n e pair from the North Marsh made regulär feeding flights when young would have been in the nest exactly as occurred in 1976, however another pair in the North Marsh which had also made regulär feeding flights in 1976, m a d e a much reduced number in 1977. A further pair in the Main Marsh made no feeding flights at all in 1977, in contrast to their behaviour in 1976. A possible explanation for the general lack of activity and hence sightings is again suggested as being the reduced population of 1977 and consequent lack of competition between pairs.

Rob Berry, R.S.P.B.

Minsmere