Skip booklet index and go to page content

COSEWIC Assessment and Status Report on the Red Knot in Canada

Limiting Factors and Threats

Deterioration of Food Resources During Spring Migration

The principal known threat to knot populations (rufa, Florida/SE US and Maranhão, Brazil populations of roselaari) migrating northwards through eastern North America is the dwindling supply of horseshoe crab eggs at their final spring stopover in Delaware Bay; with rufa particularly at risk because of its much longer migration. Several studies have confirmed that horseshoe crab eggs are the major component of the diet of knots and other shorebirds during northward migration in Delaware Bay (Castro and Myers 1993; Botton et al. 1994; Tsipoura and Burger 1999; Haramis et al. 2002, 2005). This once superabundant food supply was literally decimated as a result of over-fishing of horseshoe crabs: mid-Atlantic state landings of horseshoe crabs rose from less than 1 million pounds before 1993 to a peak of over 6 million pounds in 1998 (falling to between 2 and 3 million pounds by 2002-2003; Morrison et al. 2004). As the number of breeding crabs decreased, egg densities in the upper 5 cm of sand on beaches in New Jersey fell from a mean of 33 373 ± 18 906 SD eggs m-2 in the period 1985-1991 to a mean of 3 026 ± 1 675 SD from 1996-2005 (ANOVA F1,12 = 28.77, p = 0.0002; Niles et al. 2005, data from Loveland and Botton (unpublished data) for 1985 - 1999, NJ Endangered and Nongame Species Program (unpublished data) for 2000 - 2005), a reduction of some 90%. Studies by Stillman et al. (2003) and Hernandez (2005) have shown that the latter densities are too low for efficient foraging by knots and that the birds may not be able to meet their energetic requirements during their stopover. The result is that the birds are unable to attain adequate departure masses before the flight to their Arctic breeding grounds, at least in some years (Baker et al. 2004). Failure to attain the required stores before migration can have severe fitness consequences. 

Baker et al. (2004) showed that the proportion of knots reaching adequate departure masses in Delaware Bay decreased significantly by 70% during the period 1997-2002, and lower nutrient storage and possibly reduced sizes of intestine and liver during refuelling led to severe consequences for adult survival and recruitment of young in 2000-2002. Adult survival fell from 85% during 1994 - 1998 to 56% in 1999-2001, and decreasing proportions of immature birds were found in flocks of knots in Tierra del Fuego between 1995 and 2001. Population modelling showed these factors predicted a population trajectory closely matching the observed declines of birds on the wintering grounds during the aerial censuses (Morrison et al. 2004), and furthermore predicted that if the survival was to remain at the reduced level (56%), the population was likely to be at risk of extinction as early as 2010 (Baker et al. 2004; see Fluctuations and trends above). Although the population appeared to stabilize from 2002-2004 on the wintering grounds, the large decrease in 2005 (confirmed in 2006) suggests the population is back near the “extinction trajectory”. Other studies with shorebirds, including knots, have shown that birds in poorer condition during migration appear to have lower survival rates (Pfister et al. 1998; Morrison 2005). These studies have provided an authoritative scientific basis for understanding the impact of reduced food supplies at the final stopover area.

Although protective measures have been introduced for knots in Delaware Bay, including cessation of the horseshoe crab harvest and protection from disturbance, initial results suggest that there has not been a noticeable recovery in the crab populations and hence egg availability (L.J. Niles, unpubl. data.). Given that horseshoe crabs do not attain sexual maturity until 8 - 9 years of age, it would appear that a recovery of the food resource would be unlikely in the near future, and could be difficult (Scheffer et al. 2005). It would therefore be anticipated that rufa knots, and the Florida/SE US and Maranhão, Brazil populations of roselaari, will continue to experience difficulty in Delaware Bay for some time. The much longer, time-constrained migration of the Tierra del Fuego knots compared to those wintering in Florida or northern Brazil appears to put them more at risk from the reduced food availability, and several studies have shown lower annual survival of the southern population (Harrington et al. 1988; Baker et al. 2004; Atkinson 2005).

Drastic reductions in crab numbers have also been reported in other areas (e.g., Cape Cod, Widener and Barlow 1999; see ASMFC 1998 for discussion). The widespread reductions in horseshoe crabs suggest that alternate suitable spring staging areas are much less available than before. Furthermore, there is some doubt that alternative food resources may be able to replace the loss of horseshoe crab eggs. Knots (rufa and other populations) generally feed on bivalves and other intertidal invertebrates during migration and winter (Bent 1927; Gonzalez et al. 1996; Harrington 2001; Harrington and Winn 2001; Truitt et al. 2001; Sitters 2005), but preliminary studies by Escudero and Niles (2001) have suggested that invertebrates in many Atlantic coast habitats may not, unlike horseshoe crab eggs, supply the energetic needs of knots on spring migration, a suggestion supported by observations of night feeding (Sitters 2001; Sitters et al. 2001).

One of the main threats to C. c. islandica populations appears to be the overharvesting of shellfish on the Dutch Wadden Sea. Here mechanical dredging has resulted in declines in availability of food for the birds, a situation somewhat analogous to that occurring in Delaware Bay for knots on migration in North America (van Gils et al. 2006).


Habitat Loss and Degradation

Extensive wetland losses in the USA have included the disappearance of almost half of the marshes extant in 1900 along the Atlantic and Gulf coasts (Dahl 1990, GLCF 2005). Loss rates in the North and mid-Atlantic regions were high up to 1978 (Ducks Unlimited 2005), though have declined dramatically since protective legislation has been passed (Dahl and Johnson 1991; Dahl 2000; Ducks Unlimited 2005). Nevertheless, it is clear that a large portion of the previously available habitat has been altered or destroyed. This again suggests that knots faced with a decimated food resource in Delaware Bay now have many fewer alternative habitats that they can use, making population recovery potentially much more difficult.

Spartina invasion of major migration habitats in WA state may have affected the Pacific coast population of roselaari knots, though Spartina has been cleared in recent years at a key stopover site (Grays Harbor).

Habitat degradation in wintering areas in the Dutch Wadden Sea may also threaten wintering populations of islandica (see above).


Disturbance

Numerous studies have shown that repeated disturbance can negatively affect shorebirds, disrupting behaviour patterns and affecting their energy balance (e.g., Davidson and Rothwell 1993; Gill et al. 2001; West et al. 2002). Although disturbance was initially a significant problem for shorebirds in Delaware Bay during spring migration (Burger et al. 1995; Sitters 2001), closure of major sections of the New Jersey shore since 2003 to human use at the peak of migration has successfully reduced disturbance (Burger et al. 2004; Niles et al. 2005).

In other parts of the range, disturbance can be a significant factor. Disturbance of roosting and feeding flocks by humans and dogs has been reported from Florida, Georgia, North Carolina, South Carolina, Virginia, and Massachusetts (Niles et al. 2005). On the wintering grounds in Tierra del Fuego, roosting flocks at Rio Grande are frequently disturbed by walkers, runners, fishers, dogs, all-terrain vehicles, and motor cycles (Niles et al. 2005; RIGM pers. obs.). In Argentina, disturbance of knots on migration has been reported from Rio Gallegos, Peninsula Valdes, San Antonio Oeste, and Bahia Samborombon (Niles et al. 2005).


Severe Weather Events During Migration

There has recently been a significant increase in the number and strength of hurricanes globally, including those occurring in the North Atlantic region (Webster et al. 2005) at times and in areas used by knots (RIGM unpubl. data). Whether knots have actually been affected is not known, but the increasing number of severe weather events during their southward migration across the North Atlantic certainly represents an increased risk, which is likely to increase with predictions of global warming and increasing ocean temperatures. Hurricanes, which could affect knots, have also recently been recorded in the southern Atlantic Ocean (TRMM 2005).


Oil Pollution and Other Developments in North and South America

Extensive oil developments, with onshore and offshore wells, occur near major wintering areas of rufa knots in both the Chilean and Argentinean sectors of Tierra del Fuego, and represent a considerable potential for disaster (R.I.G. Morrison and R.K. Ross unpublished data). Two oil spills from shipping have been recorded near the Strait of Magellan First Narrows (Niles et al. 2005) and small amounts of oil have been noted on knots captured during banding operations in Bahia Lomas (A. Dey and L.J. Niles unpubl. data). Over the past 8-10 years, oil operations have been decreasing in Chile near Bahia Lomas and increasing on the Atlantic coast of Tierra del Fuego. Petroleum exploration, mangrove clearance, and iron ore and gold mining, which can result in oil and mercury pollution and habitat loss, are important threats on the north-central coast of Brazil and could affect the Maranhão/Brazil population of roselaari (Niles et al. 2005).

The important migration stopover area at San Antonio Oeste, Argentina, also faces potential pollution from a soda ash factory (which could release up to 250 000 tons or more of calcium chloride per year, affecting intertidal invertebrate food supplies) and from port activities (e.g., pollution from shipping).

In North America, important estuarine areas such as Delaware Bay and the Gulf of St. Lawrence are at risk from pollution and shipping incidents. The Mingan Islands, in the St. Lawrence, are particularly at risk because large ships carrying titanium and iron navigate through the archipelago to the Havre-St-Pierre harbour throughout the year (Y. Aubry pers. comm. 2007). Some additional large scale developments that could have a major impact on important shorebird migration areas include tidal power in the Bay of Fundy, plans for which have recently been resurrected (CBC 2005). Also, barging has been proposed in connection with diamond mining developments near Attawapiskat on the west coast of James Bay, which could affect the river mouth habitats (W. Crins pers. comm. 2007).

Developments in California (e.g., San Francisco Bay) and Mexico and along the migration route of the Pacific coast population of roselaari could potentially affect wintering and migrating birds, respectively. 


Climate Change: Arctic Breeding Grounds

The Arctic is one of the regions most likely to be affected by climate change (ACIA 2004). Meltofte et al. (2005) have provided a detailed review of potential effects of climate change in the Arctic on shorebirds; major concerns include changes in habitat, especially long-term reductions in High Arctic habitats, and uncoupling of phenology of food resources and breeding events. As the High Arctic zone is expected to shift northwards, Red Knots, as High Arctic breeders, are likely to be among the species most affected. This would be particularly the case for populations breeding towards the southern part of the High Arctic zone, such as rufa breeding in the central Canadian Arctic.


Climate Change: Sea Level Rise and Loss of Coastal Habitat

Potential losses of intertidal habitats owing to sea level rise was projected to range between 20% and 70% during the next century at five major sites in the USA, including Delaware Bay (60%; Galbraith et al. 2002). While detailed effects are difficult to predict (IPCC 2001), the authors concluded that the scale of the losses cast serious doubts on the ability of the sites to continue supporting current numbers of shorebirds, indicating increased future stress on knot populations.


Disease and Parasites

Piersma (1997, 2003) pointed out that long-distance shorebird migrants, such as knots, occupy relatively parasite-free salt water habitats, possibly reflecting a trade-off between the requirements of an energetically and physiologically demanding lifestyle and a reduced need for a highly developed immune system. Shorebirds inhabiting freshwater (parasite and disease rich) habitats tend to have higher parasite loads (Figuerola 1999; Mendes et al. 2005). The occurrence of parasites or disease amongst long-distance shorebird migrants including knots has been recorded in Brazil (Baker et al. 1998; Araújo et al. 2003, 2004; see Niles et al. 2005; Baker 2005), Uruguay (Niles et al. 2005), Florida (Woodward et al. 1977; Forrester and Humphrey 1981), and Delaware (Southeastern Cooperative Wildlife Disease Study 2002; Niles et al. 2005). The poor condition of Red Knots in Delaware Bay and northern Brazil in recent years (Baker et al. 2004, 2005a) suggests that they may be at an elevated risk of disease or parasitic infection, since birds in poor condition as indicated by low body mass are more prone to infection by parasites and pathogens (Booth et al. 1993).


Predation

Shorebirds have enjoyed what Butler et al. (2003) termed something of a “predator vacuum” over the past 30 years, arising from greatly depleted raptor populations caused by persecution and pesticide poisoning. Whether increasing predation from raptors has affected knots is unclear, but any recovery of the population will take place in a situation with increased numbers of avian predators (see predation section). Human hunting of shorebirds including knots may occur in some areas, including Caribbean islands and north-central Brazil, though this practice is thought to have decreased greatly in the latter area over the past decade (Serrano pers. comm. in Niles et al. 2005).