WHERE HAVE ALL THE BEARDED TITS GONE? Autumn 2001

The colonisation in 1973 and the subsequent build up of the breeding population of bearded tits was one of the most dramatic and exciting events at Leighton Moss. By 2000 the population had reached an all time peak of ca. 65 pairs with 275 fully-fledged juveniles being ringed that year. This year we will be lucky to have six pairs and to date we have ringed only 18 fully-fledged juveniles!

So what went wrong? The bearded tits problems started with the wettest autumn for 200 hundred years, this resulted in the most prolonged period of high water levels from late October through to late December since records were first kept in 1964. During these two months the water level was often 30 - 35 cms above normal resulting, in the almost complete flooding of the reed litter, a favoured feeding area for bearded tits in winter.

Our first inkling that the high water levels were producing problems for bearded tits was that many were seen from late October on, in the reed, which had the shallowest water. This was mainly the reed at the edges of the reedbed, and was very restricted in area. This move of feeding area was best illustrated by the changes observed in one of our ringing rides. The 'spring' ride has five 18 metre mist nets reaching from almost the waters edge right back to the willows. Under normal water levels ca 95% of the bearded tits occurring along this ride are caught in the two nets nearest the open water, ca 5 % were caught in the next two nets and only the occasional one in the net adjacent to the willows.  However during the high water levels 95 % were caught in the two nets next to the willows and covering the driest part of the reedbed.

Weight is a good indicator of the bird's condition. In normal winters bearded tit weights gradually increase towards mid winter, presumably because fat is laid down to tide the birds over lean spells. The table below shows the October and November 2000 weights compared with the average weights for the past four years.  This years weights were rather low, averaging around a gram lower than normal and no real increase in weight was recorded over the period.

Because of the continuing high water levels ringing activities were rather difficult, our catches however were well down on previous years despite starting the autumn with the largest number of birds since our study started in 1992. This suggested that there was probably a greater mortality than usual.

The weather turned very cold on December 25th with 8cms of snow on the 26th..  Bearded tits suddenly became very visible. On several occasions a walk around the hides would produce sightings of three or four flocks of up to 15 in each flock. feeding on the top of the reed heads, a lovely sight in the cold winter sunshine. They would perch on the seed heads, pulling out reed seeds, but this made them very vulnerable to predation by sparrowhawks and in one two hour watch two birds were seen to be taken by this efficient predator. We suspended our ringing activities over the cold spell as we felt it was unwise to put the birds under more stress, but we did get two small catches just before and just after the cold spell. The weights of these birds are shown below compared with the previous years averages. The samples are small but they averaged three grams lower than normal which is almost 20% down on the average of previous years, the birds were obviously suffering.

Further evidence that they were experiencing problems were that as soon as the snow fell small flocks moved out of the reedbed and could be seen silently creeping among the tussock sedge areas on the path to and past the Lower Hide. On occasions they were trying to find food in the small strip of dead vegetation no more than three metres wide between the path and the field. I have never seen bearded tits in this area during normal winter weather.

So it appears it was a combination of factors, which produced the undoubted high mortality. Firstly prolonged high water levels for three months, denying the bird's access to the reed litter, an important feeding area. The crop of reed seed also appeared to be poor, perhaps because of the wet weather in September and October not allowing it to set well. Birds could not accumulate fat in readiness for any cold spell. This came in December, when ice and snow covered any remaining reed litter forcing the birds to feed on the exposed seed heads, where they became very vulnerable to predation, or into peripheral normally unused habitats where food was probably very scarce.

Similar declines have been reported before. When I visited Lake Takern in southern Sweden they told me that in one year at just one end of this extensive reed-bed they ringed almost a thousand bearded tits in one autumn. The next autumn, following an unusually cold and snowy winter they ringed fewer than 50! The total British population declined to under 5 pairs after the very cold and snowy 1947 winter. Since colonisation the Leighton population fluctuated between a high of 40 pairs in 1980 & 81 to a low following a hard winter of 12 in 1987, bouncing back to the all time high of 65 in 20001. So in all theses cases they quickly bounce back with the possibility of producing 3 broods of 5-7 young per brood.

Because of our colour-ringing programme I was able to identify the birds making up the surviving breeding population. Of 6 males 4 were adults and 2 first year birds. Of 5 females 3 were adults and 2 first winter birds. Again the sample is small but it strongly suggests that the more experienced adults survived better.

John Wilson