5 Ecology of gulls in urban areas
This chapter provides a review of our current knowledge of the ecology of urban gulls in Scotland, with additional information from the rest of the UK if it is of relevance to the Scottish situation. We first consider the available evidence for the origins of the gulls that inhabit towns and cities. We then consider some possible reasons for their attraction to urban areas and evaluate the scientific evidence to support the possible hypotheses.
Published studies of urban gulls have concentrated on surveys of abundance and distribution of breeding birds ( e.g. Monaghan & Coulson 1977, Raven & Coulson 1997, Mitchell et al. 2004) or are otherwise reports commissioned locally aimed at quantifying or suggesting measures to alleviate perceived problems. Studies on the ecology of gulls specifically in urban areas are scarce and those from other natural habitats must be interpreted with caution as to whether their results are applicable to the urban environment.
5.1 Exchange between urban areas and the wider countryside
The gulls that use urban areas are, as far as is known, not of populations that remain distinct from those using non-urban areas. For example, ringing recoveries have shown that young Herring Gulls reared on the Isle of May (a 'natural' site in the outer Firth of Forth, Scotland) have been found breeding in urban areas of north-east England (Monaghan & Coulson 1977, Monaghan 1979). Analyses of ringing recovery data from Britain and Ireland for four of the gull species considered in this report (not reported for Great Black-backed Gull) suggest that breeding dispersal (the movement of adults between breeding locations) and natal dispersal (the movement of young birds from their natal colony to the colony in which they first breed) distances are typically in the tens of kilometres, and movements of over 100 km have been recorded (Paradis et al. 1998, Wernham et al. 2002). Hence transfers between non-urban and urban sites are, in theory, readily feasible.
A recent study suggests that levels of exchange of Lesser Black-backed Gull recruits between urban and 'traditional' colonies around the Severn Estuary are low, however, based on sightings of colour-ringed individuals (Rock 2003, Rock in prep); this suggested result could be confounded by different numbers of colour ringed birds marked at the different colonies and also by differential search effort between the two types of colony however. Indeed, general ringing recoveries from elsewhere in England and Wales suggest that the interchange of recruits from different types of colonies continues to be widespread, for Lesser Black-backed Gulls at least (Rock in prep). Furthermore, there are likely to be seasonal differences whereby some local populations or individuals will use towns as nesting sites while others may use them as winter foraging areas. One of the very few published studies on urban gulls in winter showed that Herring Gulls were present in much reduced numbers during September to November compared to during the breeding season (March to August) during the single year reported (Gibbins 1991 in north-east England), suggesting that many of the birds moved away from the urban environment outside the breeding season.
Based on the surveys described in Chapter 4, numbers of the five species of gull breeding in Scotland's urban areas, and also considered in this report, have apparently continued to increase up to at least the most recent survey in 1998-2002. In contrast, the overall population trends in Scotland for Black-headed Gull, Herring Gull and Great Black-backed Gull show declines between 1986-88 and 1998-2002, while the Lesser Black-backed Gull has shown only a very modest increase during the same time period, although increases in England, including the north of England, have been great (Mitchell et al. 2004). The Common Gull is the only species with a population that has apparently increased overall in Scotland, as well as in urban areas. Although there are biases associated with the different survey methods that have been used to count gulls through time (Chapter 4), there is no reason believe that there have been differences in the efficiency of monitoring gulls in urban and non-urban environments, and thus the divergence in the trends in numbers of gulls breeding in urban and non-urban areas is believed to be real. The expansion of breeding gull numbers in urban areas does not appear always to be driven by expansion of the populations of the species as a whole (although this may have been the case up until the late 1970s; J. Coulson pers. comm.) but, rather, more complex causes are suggested to explain current increases.
Some of the most plausible hypotheses for the recent expansion of numbers in urban areas might be as follows (and these need not be mutually exclusive):
- Urban sites have become more attractive to immature gulls recruiting into the breeding population;
- Urban sites have become more attractive to adult gulls that have bred previously in 'natural' sites;
- Non-urban or 'natural' sites (typically coastal cliffs and islands) have become less attractive to nesting gulls;
- Survival rates of breeding adult gulls are higher in urban areas;
- Gulls breed more successfully in urban areas and their surviving offspring recruit into local breeding colonies or those at other similar urban sites.
In the sections that follow, we examine current knowledge of the ecology of urban and non-urban gulls for evidence in support of one or more of the above explanations.
5.2 Breeding success
In north-east England in the 1970s, the breeding success of roof-nesting Herring Gulls was significantly higher (1.2 - 1.6 fledged chicks per pair) than those breeding in 'natural' areas over the same time period (typical reported estimates of 0.6 -1.2 fledged chicks per pair) (Monaghan 1979). The latter study considered reduction in the intra-specific predation on chicks and reduced territorial aggression in structurally isolated sites for roof nesting gulls (except for denser colonies on flat roofs) to be principal factors leading to greater breeding success. In 1990, in Sunderland, the recorded breeding success of roof-nesting Herring Gulls at or above levels in the 1970s, averaging 1.86 fledged chicks per pair (Gibbins 1991). Generally high levels of breeding success are also reported for urban nesting Lesser Black-backed Gulls in the Bristol area (Rock in prep). It has also been suggested, although not demonstrated analytically, that gulls are able to nest earlier in urban areas than in 'traditional' colonies in response to generally warmer ambient temperatures (Rock in prep), although there is little evidence for this earlier breeding in Dumfries compared to gulls breeding at natural sites (J. Coulson pers. comm.). There is no data available currently on the breeding success of gulls in urban areas of Scotland that would allow a comparison with the productivity of pairs at more natural breeding sites.
In North America, studies of urban nesting Herring Gulls and Ring-billed Gulls in northern Ohio reported lower hatching and fledging success by urban nesting pairs than those in more traditional colonies in the Great Lakes area (Belant et al. 1998) in contrast to the limited observations from Britain (Monaghan 1979, Gibbins 1991).
5.3 Survival rates
Although estimates of survival rates for adult Herring Gulls and Lesser Black-backed Gulls breeding in non-urban areas in Britain are available (Chabrzyk & Coulson 1976, Coulson & Butterfield 1986, Wanless et al. 1996), no studies of the survival rates of gulls breeding in urban areas are available for comparison.
The recruitment mechanisms of gulls, whereby birds breeding for the first time select where to nest, are poorly understood. Because all the species under consideration here are colonial breeders, young gulls tend to be attracted to areas that have already been colonized and to areas where breeding by conspecifics is successful. A study of Kittiwakes at a non-urban site in Brittany demonstrated a significant positive correlation between breeding success in a colony, and also within parts of a colony, and subsequent local recruitment (Danchin et al. 1998); new breeders tended to recruit into areas where breeding success had been relatively high in the previous year, and also some failed breeders moved to more productive areas. Kittiwakes, in common with most of the species of gull that breed in urban areas of Scotland, normally breed first at the age of four years, and it is probable that they spend some of their time as immatures assessing the reproductive success of their conspecifics. A study of individually marked Herring Gulls recruiting into the breeding population showed that potential recruits often first returned to their natal colony but, before actually breeding, many moved and eventually nested elsewhere, suggesting that young birds evaluate their natal colony before 'deciding' whether to stay or breed elsewhere (Vercruijsse 1999).
Assuming that high breeding success (Section 5.2) is typical for roof-nesting gulls, and that recruitment can be encouraged by performance-based conspecific attraction, it could be that urban nest sites on roofs may be more attractive to recruiting gulls than more natural nest sites. In contrast to this view, some studies in North America have suggested that roofs are suboptimal habitats that have only been colonised as a result of numbers in more suitable areas expanding such that natural habitats have become saturated (Belant 1997). This has also been suggested as a cause for the initial development of some urban colonies in the UK, whereby recruits from saturated natural colonies moved onto roofs during a period of general increase in the gull population (Monaghan & Coulson 1977). The subsequent increase in gull numbers in urban areas of Scotland over a period when non-urban colonies have declined, particularly those Herring Gulls (Chapter 3), implies that this is not the likely cause of current increases in urban gulls in Scotland however.
A corollary to the above could be that more natural or non-urban colonies have become less attractive to recruiting gulls, thus leading to a greater proportion of the total population using urban nest sites. From the 1960s through to the 1980s, breeding gulls were culled at a number of natural colonies ( e.g. Thomas 1972, Coulson 1991). Although some culls have continued, these have mostly been on a reduced scale. Extensive culling and clutch destruction at colonies in the Forth of Forth, Northumberland and Lancashire are speculated to have contributed to the spread of urban breeding gulls in Scotland (Raven & Coulson 1997). Potential recruits may judge a colony where breeding success is being suppressed as unsuitable and chose to nest elsewhere. Any adults that survive a cull may also chose to move to new breeding sites. Although adult gulls can show marked fidelity to nesting areas once selected, even in the presence of disturbance ( e.g. Southern et al. 1985, for Herring Gulls in North America), this behaviour may vary between species ( e.g. Wanless et al. 1996). As well as differences between species, the likelihood of established adults abandoning a breeding colony to move elsewhere may also vary between sites but there are too few studies to allow rigorous comparisons to be made. The greater rates of increase and spread of Lesser Black-backed Gulls breeding in urban areas compared to Herring Gulls during the 1980s and 90s (Raven & Coulson 1997, Chapter 3) may be at least partly attributable to a difference in tenacity to breeding sites.
5.5 Food and foraging behaviour
Whatever the mechanism by which urban sites could potentially be more attractive to some nesting gulls, the ultimate causes are likely to involve a combination of availability of food and security of nest sites. All five of the suggested biological reasons for the apparent divergence in trends between urban and non-urban gull colonies given above, are influenced to some degree by the availability of food. Urban areas potentially offer additional sources food, such as garbage (both litter and from nearby refuse tips), scraps from shops (including fast-food outlets), waste from fish docks, and also food put out by some people specifically to feed the gulls ( e.g. Raven & Coulson 1997, pers. comms with Scottish Local Authority representatives, see Chapter 3). Street lighting within urban areas also gives the potential for gulls, which are normally diurnal feeders, to forage at night ( e.g. Rock 2003). Furthermore, gulls nesting within urban areas can potentially travel to more 'traditional' food sources, for example within the inter-tidal zone, on agricultural land and also at sea, provided these are within an acceptable foraging distance from nesting areas for the species concerned. No published studies explicitly quantify differences in the diets of gulls in urban and non-urban areas in Britain. Studies of Ring-billed Gulls and Herring Gulls in North America suggest that anthropogenic food from garbage tips can form a higher proportion of the diet for birds in urban areas than elsewhere, but also that the relative importance of this food source can vary between species (Brousseau et al. 1996, Belant et al. 1998). The relative importance of garbage taken from organised tips or as 'litter' discarded within towns is unknown.
Gulls nesting in urban areas need not necessarily feed within them, and similarly, those seen foraging within urban areas need not necessarily be birds nesting there. In Sunderland, the numbers of Herring Gulls frequenting the town centre in summer exceeded, by up to three-fold, the expected number based on immediate local breeding numbers (Gibbins 1991). Furthermore, infrequent sightings of colour-ringed individuals suggested that many birds in the town centre were transients and used it infrequently even during the breeding season (Gibbins 1991). An analysis of remains in pellets regurgitated by Lesser Black-backed Gulls nesting in Dumfries during May - July, suggested that the surrounding agricultural land provided more important foraging areas than the town itself (Coulson & Coulson 2003); although processed foods (the largest components of edible urban garbage) are not generally identifiable within pellets due to their high digestibility, observations of foraging gulls provided supportive evidence that only a relatively small proportion of those breeding in the town actually foraged there. During the period when gulls first colonised urban areas in the UK (1960-1980), observations suggested that few actually fed within the urban environment (J. Coulson, pers. comm.), and unpublished direct observations relating to gulls in Dumfries and in towns in north-east England suggest that even now, less than 5% of breeding gulls actually forage in the streets of urban areas (J. Coulson, pers. comm.). Observations of 300 individually colour-ringed and dyed Herring Gulls ringed at Burniston refuse tip, c.3km from Scarborough, found only two of the birds breeding within Scarborough but many breeding at natural cliff sites up to 35km away from the tip (J. Coulson pers. comm.). Other observations of colour-marked Herring Gulls nesting in urban areas of Tyneside and Wearside have also indicated that they rarely visited landfills in the area (J. Coulson pers. comm.), indicating that at least in some cases, such landfills are not a major food source for the urban gull population.
The foraging ranges of gulls vary between species, and based on observations of birds breeding in non-urban areas and feeding at sea, Lesser Black-backed Gulls can range the greatest distances from breeding colonies, followed by Herring Gulls, Common Gulls and Black-headed Gulls in order of decreasing normal ranging distance (Camphuysen 1995, Garthe 1997, Noordhuis & Spaans 1992). At sea, Lesser Black-backed Gulls have been recorded over 135 km from breeding colonies in the summer (Garthe 1997), although the individuals observed were not necessarily current breeders. Observations of individuals foraging from breeding colonies suggest that gulls typically forage up to 30 km distant (Pearson 1968, Mudge & Ferns 1982, McCleery & Sibly 1986), and the ranking of ranging ability observed at sea probably applies across all habitats. Where Herring Gull and Lesser Black-backed Gull diets were similar, their foraging areas tended to differ spatially, Lesser Black-backed Gulls travelling further (readily up to 30 km) whereas a lesser proportion of Herring Gulls tended to range to those distances (Mudge & Ferns 1982). The availability of food close to suitable nest sites is likely to place a greater limitation on the choice of breeding sites for those species with a lesser ability to range. In this case, of the species considered, Lesser Black-backed Gulls might be the species most able to take advantage of buildings as secure nest sites when food sources are either dispersed or unreliable close to the nesting area.
5.6 Nest site selection and predation pressure
Of the two most abundant species nesting in urban areas, the Herring Gull and the Lesser Black-backed Gull, in natural sites, the latter tends to nest in more vegetated, often flatter areas (Harris 1964, Hosey & Goodridge 1980, Calladine 1997). With supportive evidence from attendance rates of adults at nest sites, it has been suggested that the more vegetated sites are selected by Lesser Black-backed Gulls to offer greater opportunities for concealment from predators during longer periods of parental absence (Calladine 1997). If predation of young is a less significant risk amongst roof-nesting gulls in general, as observed in northeast England in the 1970s (Monaghan 1979), then nesting in urban sites may permit gulls to forage more widely and therefore be less susceptible to variations or reductions in immediately local food supplies. A reduction in the availability of food within acceptable foraging distances of breeding colonies has been implied as causal reasons for some recent declines in a natural colony (Skomer Island in Wales; Perrins & Smith 2000). Therefore, as well as the supplementary food sources potentially available to gulls in urban areas, urban sites might prove more attractive if young are at a lesser risk from predation permitting adults to forage more widely where food resources could be dispersed or unpredictable.
The principal reason for reduced predation of young gulls in urban colonies was suggested as the greater isolation of nest sites, with these typically resting between chimney pots, leading to a much reduced instance of intra-specific predation, or cannibalism, of the chicks (Monaghan 1979). In situations where gulls nested on flat roofs, nests were not as isolated, and breeding success was lower and more typical of natural sites (Monaghan 1979). Contrary to this, in Sunderland in 1990, breeding success on flat roofs was reported as the highest of all roof types (Gibbins 1991). It may be that differences in the overall densities of breeding gulls between these study areas also influenced the rates of intra-specific predation and hence breeding success but these densities are not reported to allow direct comparison. Nor are there any comparable data available for urban gulls in Scotland.
The preferred nest sites in urban areas reported during the 1970s were on chimneys, typically between double rows of chimney pots, on small roofs over dormer windows or such similar isolated platforms (Cramp 1971, Monaghan & Coulson 1977, Monaghan 1979). Lesser Black-backed Gulls, in particular, have taken increasing advantage of more extensive flat roofs ( e.g. Raven & Coulson 1997, Coulson & Coulson 1999-2003, Rock in prep.) that in some ways mimic their preference for flat nesting areas in traditional colonies ( e.g. Calladine 1997). The development and expansion of industrial units with flat roofs is likely to have increased the availability of suitable nest sites for Lesser Black-backed Gulls in many areas.
5.7 General causes of changes in gull populations
For much of the 20 th Century, most populations of breeding gulls in Britain increased, with increases attributed widely to reduced exploitation and persecution, increased protection and increased food availability ( e.g. Lloyd et al. 1991). The most recent extensive surveys (1998-2002; Mitchell et al. 2004) show that Black-headed and Herring Gulls have declined during the last two decades, and that some local or recent declines in Lesser Black-backed and Common Gulls have also occurred (Section 4.2).
The reasons that have been suggested for these declines, where known or strongly suspected, have generally been reversals of those conditions thought to have led to the earlier increases. During the 1960s, 70s and 80s, Herring Gulls in particular, and Lesser Black-backed Gulls to a lesser extent, were subject to organised culls (see Section 6.5). Reduced food availability, through changes in refuse disposal and fishery discard management, has also been implicated in the declines for some colonies.
In addition to these general reversals in human behaviour, two other factors have also been implied as potentially contributing to some declines in breeding gulls: increased levels of mammalian predation and ingestion of toxins. Predation by mink has been suggested as the principal cause of decline for a number of colonies of Black-headed, Lesser Black-backed and Common Gulls in the west of Scotland (Craik 1997, Craik & Campbell 2000) and increases in other mammalian predators, such as Red Fox (Tapper 1992) may also have impacted on gull numbers on the UK mainland (Ratcliffe 2004). These latter changes may have impacted more on natural colonies than those in urban areas.
Botulism, a condition whereby birds are poisoned by toxins produced by the bacterium Clostridium botulinum, is often associated with rubbish tips during warm weather, and has been recorded amongst gulls at a number of colonies (Ratcliffe 2002). Botulism poisoning is considered to have influenced some local colonies of Herring Gulls (Madden & Newton 2002) but evidence of influence at the wider population level (national or regional) is lacking. It is also likely that many reports of botulism poisoning have not been fully diagnosed, such that other toxins may have caused the observed symptoms, such as wing drooping and an inability to stand (J. Coulson, pers. comm.). In addition, a recent outbreak of botulism amongst Common Gulls in the Lake District suggests that landfills are not the only source of this toxin, as this gull species rarely utilises landfills as a food source (J. Coulson, pers. comm.).
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