impacts of habitat loss on individual fitness
The probability that an individual is able to survive displacement by habitat loss will be affected by a number of factors. The availability and proximity of suitable habitat is a clear prerequisite, although the ability of individuals to relocate and use alternative sites may depend on the site-faithfulness exhibited by the species and whether displaced individuals have had prior experience of these sites. Should alternative sites be limited in quality or extent and already at or near capacity, increased densities may lead to intense competition for available resources (Goss-Custard 1985; Goss-Custard et al. 2002) and thus potentially increased mortality in the population.
Recent work has highlighted how changes in food supply (Camphuysen et al. 1996; Atkinson et al. 2003) and, indeed, densities (Durell et al. 2000, 2001) can impact the survival of waterbirds. However, despite significant modelling work (Durell et al. 2004, 2005; see also Stillman et al. 2000; Stillman 2003), attempts to provide empirical evidence that habitat loss actually does impact the fitness of birds displaced to remaining habitat have been limited (Lambeck 1991; Ganter & Ebbinge 1997; Ganter, Prokosch & Ebbinge 1997). Among other taxa, there is evidence that reductions in habitat quality associated with habitat loss can affect the survival of salmon Oncorhynchus spp. (Magnusson & Hilborn 2003). An experimental study on grey-tailed voles Microtus canicaudus (Miller), however, reported that a 70% reduction in habitat had no adverse effects on adult survival, reproductive rate, juvenile recruitment or population size (Wolff, Schauber & Edge 1997).
The present study found evidence that the loss of Cardiff Bay impacted on both the body condition and survival of the redshank that wintered there formerly. Analysis of biometric data showed that displaced redshank had difficulty maintaining their mass in the first winter post-closure: adults previously only recorded at Cardiff Bay were significantly lighter, controlling for body size and month, than those previously recorded at Rhymney. In The Netherlands, oystercatchers displaced by a loss of mudflats in the Delta region were similarly significantly lighter than those originally ringed at other neighbouring sites (Lambeck 1991). Redshank displaced from Cardiff Bay are likely to have faced intense competition for food, as a result of the increased densities at Rhymney (and other nearby feeding areas), as well as their limited experience of these sites: the redshank that wintered at Cardiff Bay were highly faithful to the site prior to its loss (Burton 2000a; Burton & Armitage 2005).
More significantly, the adult redshank displaced from Cardiff Bay also suffered increased mortality in the winters after barrage closure. The estimated annual survival rate of adult redshank originally caught and ringed at Cardiff Bay fell from 0·846 in the 2 years prior to barrage closure to 0·778 in the 3 following years because of a significant decline in winter survival rates. The local increase in the densities of redshank, in particular at Rhymney, may have impacted survival either directly, through starvation, or indirectly, by increasing the likelihood that birds may have been predated (Goss-Custard 2003), e.g. by sparrowhawks Accipiter nisus (Linnaeus) and peregrines Falco peregrinus (Tunstall). Although it was not possible to evaluate whether the increase in mortality among Cardiff Bay redshank was directly related to poor body condition (as survival rates were calculated from resighting data not recaptures), it does appear that it resulted from the birds’ displacement. First, there was no significant difference before and after barrage closure in the annual survival rate of adult redshank originally caught and ringed at Rhymney. Estimated annual survival for these birds was 0·860, similar to the value for Cardiff Bay adults prior to barrage closure. Secondly, estimated survival of adult redshank at the north Wales control site was actually higher in the post-barrage closure period than beforehand and this did not differ according to race, indicating that the drop in survival of Cardiff Bay birds was not the result of poor weather regionally or a decline in the survival of British redshank.
The survival rate of Cardiff Bay redshank over the winters subsequent to barrage closure may have been underestimated if birds had not been detected once displaced. However, the selected model accounted for a lower resighting rate following barrage closure and there is little evidence that birds moved to sites not surveyed. The majority of colour-marked individuals from the bay moved to Rhymney, just 4 km away; despite extensive searches of sites throughout the wider Severn, none was recorded more than 19 km distant (N. H. K. Burton & M. J. S. Armitage, unpublished data). If birds did move to sites further afield, not covered by surveys, it is perhaps more probable that they would have done this after the first (2000) breeding season following the barrage's closure. Over-summer return rates were found to be constant over the study period, however, and thus it seems that this did not happen. No redshank colour-ringed at Cardiff (in winter) was recorded away from the Severn in winter, despite there be a well-established network for reporting sightings, which by 2000 had resulted in 26 individuals being reported during the breeding season (Burton et al. 2002). There was therefore no evidence that survival was underestimated because of emigration beyond the area surveyed.
Although winter survival rates fell following barrage closure, it should be noted that the annual survival rate estimated for redshank displaced from Cardiff Bay is no lower than some other previous estimates for the species. Großkopf (1959, 1964) (see Boyd 1962) and Ottvall (2004), for example, reported adult survival rates of 71%, 75% and 80%, respectively, and Jackson (1988) rates of 77% and 75% for adult males and females, in studies based on colour-ring sightings in the breeding season. Thompson & Hale (1993), likewise, reported rates of 75% and 72% for males and females in a mark–recapture study of breeding birds, and Insley et al. (1997) a rate of 74% for birds at least three winters old.
It was not possible to distinguish changes in the survival of displaced redshank of the British and Icelandic races because of the small numbers of Icelandic birds that formerly used Cardiff Bay (Burton et al. 2002). However, as the population at Rhymney contained a higher proportion of Icelandic birds prior to barrage closure it might be supposed that Icelandic redshank from Cardiff Bay could have had a competitive advantage over British redshank following their displacement and thus experienced lower mortality. Impacts on the overall local numbers of the two races might have also differed as a result of differences in breeding productivity (Goss-Custard et al. 1995; Durell, Goss-Custard & Clarke 1997).
implications of reduced fitness for the local population
Numbers of redshank at Rhymney had fallen over the 7 years preceding barrage closure. As there had also been a regional decline in the species’ numbers over this period (Maclean et al. 2005), it seems likely that this was not solely the result of local factors and consequently the site may have been below its carrying capacity for the species at the time of the bay's loss. Certainly, the high survival rates of redshank at Rhymney (and at Cardiff Bay pre-closure) suggest that birds were not previously dying because of competition for food.
However, although there may have been some capacity for birds displaced from Cardiff Bay to settle at Rhymney, the poor body condition and reduced survival among these birds indicate that this site could not fully support them. By understanding how habitat loss may affect individual fitness, it is possible to predict impacts on local populations (Goss-Custard 2003). The reduction in annual survival from 0·846 to 0·778 reported here for Cardiff Bay redshank, for example, represents a 44% increase in mortality rate. Without an increase in the recruitment of first-winter birds, such a change is likely to reduce substantially the local population size.