Information and Advisory Note Number 133                             Back to menu

NATURAL HERITAGE TRENDS Species diversity: breeding bird species

BREEDING SEABIRDS

1. Introduction

1.1 Scotland's breeding seabird populations are internationally important, encompassing half of the world's great skuas and North Atlantic gannets, over one third of Europe's Manx shearwaters and at least 10% of the European breeding populations of ten other species¹ (Lloyd et al, 1991). Together, seabird numbers exceed four million birds, in hundreds of colonies around the coast. There have been only two comprehensive seabird surveys in Britain; in 1969-70 and 1985-87. A third survey, 'Seabird 2000', is now underway, and is due to be completed in 2002. These data are augmented by annual surveys within a sample of seabird colonies (Thompson et al., 1999).

1.2 Although the following figures describe changes in seabird abundance throughout Scotland, it should be noted that trends sometimes vary markedly between Scottish regions, for example, between populations in the North Sea and Irish Sea/Atlantic Ocean.

 

2. Trends

2.1 Of 18 seabird species, 11 showed a marked increase in their breeding population (i.e. by at least 10%), and four a marked decline, between 1969-70 and 1985-87

[Figure 1]. Note, however, that these figures apply to coastal populations only; two species, the black-headed and common gull, breed predominantly inland, where trends may have differed.



Figure 1. Seabird species showing changes of at least 10% in their Scottish breeding populations between 1969-70 and 1985-87.
t Percentage change between 1969-70 and 1984-85. * Changes in coastal colonies only; the majority of the population of these species
breeds inland.

2.2 Several of those showing marked increases (great skua, Arctic tern and gannet) have especially large populations in Scotland.

2.3 The greatest proportional decline was that of the roseate tern [Figure 2], whose numbers in Scotland fell from 134 pairs in 1969-70 to eight in 1998.

¹Excluding the storm and Leach's petrel, for which acceptable Scottish population estimates do not exist. Note, however, that 66-75% of the world's storm petrel population is thought to breed in Britain and Ireland, with sizable colonies in Scotland

2.4 The main factors influencing population trends in Scotland's seabirds are as follows.

• Food availability. This has a major influence on breeding performance, and is in turn affected by commercial fisheries and climatic fluctuations. Two-thirds of seabirds in the North Sea in summer are thought to feed to some extent on fishery waste, and the abundance of commercial fishing discards has been linked to population increases in some species. Conversely, commercial fisheries, particularly for sandeels, can have a substantial, negative impact on food availability.

• Predation. Rats, feral cats, ferret and American mink can have a severe impact on breeding and adult survival.

• Drowning. Nets, particular monofilament drift nets, were considered the main cause of unnatural deaths among auks in the 1980s.

• Pollution. Chronic oil pollution from illegal discharges has had a greater impact than occasional accidental spills. Pesticide residues and other toxic chemicals have been implicated in population crashes.

• Culling. Now controlled through legislation, egg collection, and the hunting for food, feathers and sport, has historically had a major impact on populations

2.5 Population trends are driven by a wide range of pressures, and may differ markedly between colonies of the same species. Species trends for Scotland, from 1969-70 to 1985-87, were as follows.

2.5.1 Increases of greater than 50%

• Apparent increases in Arctic skua and great skua populations partly reflect a change in survey methodology, both species having been under-recorded in 1969-70.

• Apparent increases in Arctic tern numbers partly reflect census difficulties, and mask substantial declines during the 1980s. In 1980 about 85% of the British and Irish population bred in Orkney and Shetland, where, by 1989, numbers had fallen by 42% and 55% respectively (Avery et al., 1993). In Shetland, these declines coincided with very low breeding success, associated with a lack of sandeel prey.

• The guillemot, fulmar and lesser black-backed gull showed increases of greater than 50% between 1969-70 and 1985-87. East coast populations of guillemots continued to rise substantially during 1986-99 (Upton et al., 2000).

2.5.2 Increases of 10-50%

• Apparent increases in common tern numbers partly reflect improved survey coverage during the 1980s.

• The gannet has shown a recovery from historical persecution. Between 1969-70 and 1984-85 its Scottish population increased by about 36%, and by a further 27% between 1984-85 and 1994-95 (Murray and Wanless, 1997).

• Although common gull numbers increased at coastal colonies, inland colonies, which hold the majority of the Scottish population, were not included in the two seabird surveys.

• Shag numbers increased by 10-50% during 1969-70 to 1985-87. However, in Shetland, which held about 20% of the Scottish population in 1985-87, numbers fell by more than 50% between 1986 and 1999. East coast populations also showed a dramatic drop in numbers following a winter 'wreck' in 1993/94.

• Little tern numbers increased by 19% between 1969-70 and 1985-87.

2.5.3 Changes of less than 10%

• The Scottish kittiwake showed little change between 1969-70 and 1985-87. However, colonies in Shetland, which held about 14% of the Scottish population in 1985-87, showed a 50% decline during 1981-97. This was attributed to poor breeding success, due to low availability of sandeels, and to increased predation by great skuas (Heubeck et al., 1999).

• Great black-backed gull and sandwich tern showed relatively minor changes, of apparently less than 10%, between 1969-70 and 1995-97.

2.5.4 Declines of 10-50%

• The cormorant and herring gull declined by 10-50%.
2.5.5 Declines of 50% or more

• Black-headed gull numbers declined by more than 50% at coastal colonies. However, like the common gull, the bulk of the black-headed gull population breeds inland, and was not covered by the two seabird surveys.

• The number of breeding roseate terns (a Biodiversity Action Plan priority species) fell by 83% between 1969-70 and 1985-87, and by 65% between 1987 and 1998 [Figure 2]. Its decline in Scotland has been attributed, in part, to emigration to Irish colonies, predation in its breeding colonies and to adult mortality associated with human persecution in its West African wintering grounds. However, its overall population in Britain and Ireland increased during the late 1990s.



Figure 2. The number of pairs of roseate terns recorded in Scotland between 1987 and 1998.

2.6 Reliable national trend data for Manx shearwater, storm petrel, Leach's petrel, razorbill, puffin and black guillemot were not available. In the case of razorbill and puffin this was due to a change in the methods used during the two survey periods. Nonetheless, the figures suggest that there may have been an increase in number of razorbills, and little change in the puffin population.

3. Source

3.1 This profile has been developed using Lloyd era/. (1991), and information provided by M.A. Ogilvie and K.R. Thompson.

BREEDING LAND BIRDS

4. Introduction

4.1 Being extremely mobile and, in many cases, short-lived and highly fecund, birds respond quickly to variation in habitat quality, through changes in breeding output, survival or dispersal. Since most species are also relatively easy to identify and count, geographically widespread, abundant and diurnal, birds are often used as indicators of environmental change. The abundance of 139 terrestrial breeding birds has, for example, been selected by the government as a 'headline indicator' of the quality of life in the UK (DETR, 1999), and shows that many common bird species are in decline in the UK.

5. Trends

5.1 Geographic range

5.1.1 Changes in geographic range are described below in terms of the number of 10 x 10 km grid squares in which birds were detected during two national surveys; in 1968-72 and 1988-91. Of 124 widespread terrestrial and freshwater breeding bird species surveyed², the ranges of 38 appeared to have decreased by at least 10% between surveys, 24 expanded their range by at least 10%, and 62 changed by less than 10% [Figure 3]. In woodland, upland and lowland wetlands the proportion of species showing a range expansion was similar to that showing a contraction; about 20-30%. In contrast, almost two-thirds of farmland bird species declined by at least 10% of their former range; only 5% showed an expansion.

5.2 Abundance

5.2.1 Between 1994 and 1998, 16 out of 53 terrestrial and freshwater breeding birds showed a statistically significant increase in abundance, five showed a significant decrease and 32 showed no significant change [Figure 4]. All of these species were recorded on at least 30 survey sites in Scotland. Three additional species, recorded from fewer than 30 sites, showed significant increases.

5.2.2 Among fifteen species showing statistically significant changes both in Scotland and in the UK as a whole, 10 showed a greater increase in Scotland and three showed a greater decline. The two remaining species showed increases in Scotland, but declines in the UK.

² Species recorded in at least 30 10 x 10 km grid squares in Scotland during 1968-72, excluding seabirds. Of the 124, 31 were associated with the uplands, 15 with lowland wetlands, 36 with woodlands, and 20 with farmland (the remainder falling into other categories).








5.2.3 Regional differences were particularly marked in the case of the linnet, a Biodiversity Action Plan (BAP) Priority species which increased by 60% in Scotland during 1994-98, while declining by 10% in the UK as a whole [Figure 5]. A similar, but more marked discrepancy was shown by another BAP Priority species, the bullfinch, which was detected on slightly fewer than 30 Scottish sites. Note, however, that many of these changes are likely to reflect short-term, localised fluctuations in environmental conditions rather than long-term trends.



Figure 5. Species present on at least 30 Scottish survey sites, and showing statistically significant changes in abundance between 1994 and 1998.

6. INTRODUCED SPECIES

6.1 Eleven bird species in Britain have self-sustaining breeding populations derived mainly or entirely from introduced stock³. These include pest species such as the Canada goose and ring-necked parakeet.

6.2 Of greater global concern is the threat posed by North American ruddy ducks, now well-established in Britain [Figure 6]. Some of the British population winter alongside Spanish populations of their close relative, the white-headed duck. The latter is now threatened with global extinction, partly through hybridisation with its North American congener (BirdLife International, 2000).



Figure 6. Peak counts of ruddy duck in Scotland since 1980

³ Excluding re-introduced species such as the capercaillie