Wyre Forest Adder Census 1999 and Population Trends Through the 1990s

By Sylvia Sheldon

[Sylvia Sheldon has been undertaking an important study of adders in Wyre for many years. The following is extracted from her 1999 census report, with her permission. Ed]

Year Total Number of Sites Surveyed Sites with adders Mature Males Mature females Total Average per site
1990 56 50 185 55 240 4.8
1991 76 61 211 56 267 4.4
1992 78 55 159 33 192 3.5
1993 80 59 186 70 256 4.3
1994 76 50 153 29 182 3.6
1995 76 44 103 14 117 2.6
1996 80 41 112 32 144 3.5
1997 84 44 102 31 133 3
1998 85 42 103 34 137 3.3
1999 67 35 100 20 120 3.4

Table 1, Adder numbers

Year Sites with adders Mature males Mature females (Observed) Mature females (Estimated) Total (Observed) Total (Estimated)
1990 50 185 55 154 240 339
1991 61 211 56 144 267 355
1992 55 159 33 144 192 303
1993 59 186 70 189 256 345
1994 50 153 29 132 182 285
1995 44 103 14 113 117 216
1996 41 112 32 75 144 187
1997 44 102 31 77 133 179
1998 42 103 34 97 137 200
1999 35 100 20 85 120 185

Table la: Adder Numbers Including Estimates of Total

Year First Sighting Air Temp. (C) Grass Temp. (C) First Slough
1999 16 Feb 8.0 9.0 16 Apr
1998 11 Feb 13.6 12.5 17 Apr
1997 15 Feb 8.6 11.0 08 Apr
1996 16 Feb 12.2 15.0 24 Apr
1995 12 Feb 11.4 11.0 10 -Apr
1994 10 -Feb 10.0 15.0 18 Apr
1993 17 Feb 10.7 12.5 16 Apr
1992 23 Feb 13.0 15.0 22 Apr
1991 23 Feb 14.0 17.0 19 Apr
1990 5-Feb 13.0 14.0 08 Apr

Table 3: Dates of Emergence and Sloughing of Adders

Discussion

The 1999 Adder Census confirmed the decline in the adder population during the past decade. Table 1a shows the dramatic drop in numbers, both of adders themselves and the sites where are now present.

The relatively low numbers of female adders recorded in the census was discussed in the 1993 Report, when a study of ten females over a period of 11 seasons from 1983 onwards was completed. The data collected provided valuable information on the variable breeding cycle of each of these adders.

Previous observers had indicated that females in the UK bred in alternate years. This was proven by the results not to be the case in Wyre Forest. The females studied (three survived the full study period) all bred four times. They had "rest" periods to build up fat reserves of between one to four years. The data showed them breeding on average every three years.

In their non-breeding years the females tended to emerge from hibernation later than the breeding females, when the weather was generally warmer and more settled (and therefore more suitable for active hunting). They also tended to disperse from the hibernaculum areas very quickly, so that they were very difficult to observe. This knowledge explained the relatively low numbers of females observed during normal spring counts - the non-breeding females were generally not being seen. By adding the numbers of females from the previous two years to those observed for the current season, a more realistic estimate of the total can be obtained. This method has been used to prepare Table la and Graph 5.

Adder population trends

Graph 5 Adder population trends

1993 was the last time that adder numbers were estimated to exceed 300 (345 estimated). In 1994 there were the first obvious signs of a decline, which continued through 1995 and 1996. The last three years show no signs of recovery towards former levels, This would, in any event, be a slow process given the limitation of the breeding frequency of the females.

Since 1993 there has also been a sharp decline in the number of sites where adders can be found, reaching an all-time low of 35 sites in l999 - since surveys began in 1985 80 used sites have been recorded. However, at no time have all 80 sites been found to be used in one season. A number of sites became unsuitable in the late nineteen-eighties and into the nineties, and new sites were also discovered.

Factors Contributing to the Decline

Housing and Industrial Developmeny

Sites particularly on the forest edge have been completely destroyed by heavy machinery involved in development work.

Forestry Practice

Such as felling, scarifying, and replanting can be destructive to both adders and habitat.

Road Improvement, Drainage and Water Pipeline Maintenance

Spoil deposited on active adder hibernacula has buried many adders alive. They cannot excavate their own holes and are therefore trapped underground.

Conservation Work for Other Species

Has in some cases destroyed adder hibernacula. More liaison is required before embarking upon such projects.

Intensive Pheasant Rearing

These are not a native species, and are known to prey upon adders, especially juveniles. Ins area where intensive pheasant rearing has commenced adders have disappeared.

Human Persicution

This is always evident. Every year adders are found battered to death. The formal protection offered by legislation, which makes it illegal to kill an adder, is of little value in practice.

Dog Walking

This is an additional problem, especially in the adder breeding season which coincides with the seasonal increase in human and dog numbers in the forest.

Accidental Death

Adders have been found to have fallen victim to cars, mountain bikes, and horses.

Availability of Food

This will influence the breeding cycle of females, who will take longer to regain body weight if food is scarce, and the winter survival of both sexes. In Wyre, adders have been observed to feed mainly on bank voles and field voles, but there is no information on variation in their numbers.

Disease

This oes not appear to be an issue, since there is no evidence of it in the existing population.

Climate

It is not known if a series of mild, wet winters has had an impact on adders in hibernation.

Natural Predation

There has been a significant increase in the buzzard population in recent years. Although these are not considered a major threat, they have been known to take adders.

Acknowledgements

I would like to thank Chris Bradley and Jeff Hubball for their census work, and all those who reported reptile sightings, and landowners on whose property the study has been undertaken.

Address for Communications

Ms Sylvia Sheldon, Knowles Mill, Dowles Brook, Wyre Forest, near Bewdley, Worcestershire, DY12 2LX.

Sylvia would be delighted to hear from anyone who would like to undertake census work in the future in new areas, or at sites which she could suggest to them.

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