Food Habits

Main Foods Taken

Invertebrates, especially bivalves (Zwarts and Blomert 1992, Dekinga and Piersma 1993, Gonzalez et al. 1996), small snails (Harrington et al. 1986, Alerstam et al. 1992), and crustaceans (BAH), during nonbreeding season; terrestrial invertebrates on breeding grounds (Portenko 1981).

Microhabitat For Foraging

In coastal areas, mainly intertidal sand flats and beaches, peat banks, or specialized formations such as the hard-packed restinga on the Argentine coast. Hunts on falling tides over tidal sand flats in bays in Massachusetts in areas still having high water content (Schneider and Harrington 1981, BAH). Also “grazes” for epifaunal bivalves at migration staging areas in Argentina (Gonzalez et al. 1996), Massachusetts (Harrington 1983), and the United Kingdom (Prater 1972). Feeds on eggs of horseshoe crabs by pecking on tidal flats and beaches in Delaware Bay (Botton et al. 1994, Tsipoura and Burger 1999). Frequents shallow lagoons in s. Brazil, where water movement from wind seiches exposes small snails on expansive mudflats (Harrington et al. 1986). In Suriname and French Guiana, favors patchily distributed hard clay banks on eroding coastlines over the more widespread soft-mud habitats (Spaans 1978). Nonbreeders on Waddenzee coast forage on falling tides on tidal sand flats (Piersma et al. 1993a). Probes for Emerita (amphipod crustaceans) and Donax (bivalves) in the wave-wash zone of high wave-energy beaches in s. Brazil, west coast of Florida, and Gulf of Venezuela (BAH).

On marine coasts, foraging activity is largely dictated by tidal conditions. Rarely wades in water >2–3 cm deep (BAH). Generally forages on eroded peat banks during southward migration in New Jersey and Massachusetts during 2 h on either side of low tide.

Food Capture And Consumption

Figure 3 . Pecks at surface prey or probes for prey buried below surface substrate. Uses pecking mostly when hunting epifaunal prey such as horseshoe crab eggs, small snails and mussels, and may have higher catch frequencies (perhaps of smaller prey) with this strategy than with probing (Moreira 1994). In Massachusetts, knots pulled mussel spat from peat banks at higher rates than from adjacent rocks (BAH); during at least one year when mussel spat were abundant, however, knots spent more of their foraging time hunting for infauna than pecking the readily accessible mussel spat (B. Bailey unpubl.). According to Bailey, forages by 3 methods: pecking, plowing, and probing. Uses pecking, typically involving a twisting-tugging motion, when foraging on mussel spat. Uses pecking without tugging for other epifauna. Plowing knots insert bill 1–3 cm into sandy, wet (usually with a film of surface water) tidal flat, with tip of bill pointed slightly forward. With rapid up-and-down motions of the head, knot moves forward slowly, seeming to plow a small furrow in the sand. Direction of travel ap-pears aimless. Probing involves deep penetration of substrate, often to full length of bill, while knot remains in one spot. Probing often follows bout of plowing on sand flats having high water content. Knots evidently can search habitats where bivalve stocks are too low to be directly detected at profitable rates by probing. Experiments by Piersma et al. (1995b) suggest that individuals can detect buried bivalve prey through a self-induced pressure mechanism, with detection rates exceeding those predicted through touch senses alone.

Swallows captured prey, including mollusks, whole, so there are upper limits on size of prey that can be swallowed; lower limits also exist, with respect to profitability (e.g., ratio of shell volume to digestible mass; Zwarts and Blomert 1992, González et al. 1996). After being swallowed whole, mollusks are crushed by the muscular gizzard, which can atrophy through disuse (Piersma et al. 1993b).

Major Food Items

Away from breeding grounds, marine invertebrate animals, especially small mollusks, which are swallowed whole (Dekinga and Piersma 1993). Common prey include bivalves, especially of the genera Mytilus (Prater 1972, T. Bailey unpubl.), Mulinea (BAH), Donax (Harrington et al. 1986), Macoma (Beukema et al. 1993), Tellina (Prater 1972, Piersma 1991), Myadora and Nucula (Piersma 1991), and Gemma at one location in California (Recher 1966), but rarely taken although abundant at knot migration staging sites in New Jersey (Botton 1984) and Massachusetts (Schneider 1978, BAH). Gastropods also are common prey—e.g., Hydrobia (Prater 1972), Littorina (Alerstam et al. 1992), and Heleobia (Harrington et al. 1986).

The difference in use of Gemma, a small, thick-shelled bivalve, at different locations (see above) could be related to the probability that individuals in migration have reduced stomach sizes and reduced capabilities to process thick, hard-shelled bivalves (Piersma et al. 1993b). On the other hand, knots generally prefer small (5–15 mm), thinner-shelled bivalves of species providing favorable meat-mass ratios (Gonzalez et al. 1996).

Other intertidal invertebrates commonly eaten by Red Knots include amphipods (Corophium [Prater 1972], Emerita [Harrington et al. 1986], and Acanthohaustorius [BAH]) and polychaete worms (Prater 1972, Piersma et al. 1993b). One prominent departure from the species’ penchant for mollusks at marine locations occurs at a key migration staging site in Delaware Bay (mid-Atlantic Coast, U.S.) which individuals visit during northward migration. The key food here is the eggs of horseshoe crabs, tens of thousands of which come ashore to nest during second half of May (Botton et al. 1994).

During southward migration in Massachusetts (B. Bailey unpubl.), mussel (Mytilus edulis) spat are the most common prey in knot stomachs during Jul and Aug, but amphipods and naticid snails also were common in 2 collected specimens. A few gem clams (Gemma gemma) also were eaten, but given their extraordinary local abundance it was clear that they were not a preferred food. In the Dutch Waddenzee, tellin clams (Macoma balthica) were the preferred food (Zwarts and Blomert 1992, Piersma et al. 1993a), but when these were less accessible, other mollusk prey commonly eaten were the snail Hydrobia ulvae, the mussel Mytilus edulis, and the cockle Cerastoderma edule . Other prey identified were the shrimp Crangon crangon, the crab Carcinus maenas, and the polychaete worm Nereis diversicolor . In Río Negro, Argentina, knots fed mostly on mussels (Brachidontes rodriguezi; González et al. 1996).

Knots often arrive in arctic breeding areas before snow cover is gone, and before insects and other macroinvertebrates are active and accessible to predators. During this stage and sometimes in unusual situations during migration, individuals will eat vegetable foods, which contrasts with their general use of animal foods during the rest of the year. On the Forsheim Peninsula, Ellesmere I., Parmelee and MacDonald (1960) reported that early-season knots (1 Jun) were probing and jabbing for grass shoots, seeds, and other vegetable matter, and that 0 of 13 specimens collected had consumed animal food. By 19 Jun and thereafter, foraging was mostly by pecking, and all specimen stomachs contained insects, and occasional seeds and/or marine invertebrates. At Hazen Camp, Ellesmere, Nettleship (1974) also found mostly vegetable matter in stomachs early in the season, with increasing volumes of insects as the season advanced, but never exclusively insect contents. He also found that chicks ate mostly insects, especially adult midges (Diptera: Chironomidae). Morrison (1975) suggested that Red Knots arrive on breeding grounds at Alert (n. Ellesmere I.) with sufficient fat to survive if foraging conditions are severe, but nevertheless starvation and death sometimes occur (Nettleship 1974).

Quantitative Analysis

Spring migrants at Delaware Bay consume mostly eggs of spawning horseshoe crabs and visit beaches having greatest availability of eggs (Botton et al. 1994, Tsipoura and Burger 1999). In Massachusetts (BAH) during Jul and Aug, knots removed mussel spat ranging in size from 0.9 to 2.0 cm, averaging 1.4 cm in length (n = 51). In Argentina, length of Brachidontes mussels eaten by knots was 5–20 mm (average 10.5, n = 192; Gonzalez et al. 1996); larger and smaller individuals, although present, were avoided. In New Zealand, knots select bivalves <10 mm long, rarely taking larger size classes although present (Piersma 1991); Zwarts and Blomert (1992) found knots commonly taking Macoma bivalves >10 and <18 mm long.

Prey taken may vary with changes of accessibility. Thus, where Macoma may be principal prey during fall or spring in Waddenzee, it is less important food during winter, when Macoma descend to greater depths in the substrate (Reading and McGorty 1978). An exception exists in the case of small Gemma gemma clams, which knots avoided consuming where abundant at a Massachusetts (B. Bailey unpubl.) and a New Jersey (Botton et al. 1994) site. González et al. (1996) note that some bivalve prey are not taken by knots because the food value is minimal because of a low meat-shell ratio.

Selection of horseshoe crab eggs during spring migration at Delaware Bay represents a major divergence from other foods preferred by Red Knots. It is unclear why this is the case, but Piersma et al. (1993b) suggest that the species’ ability to digest hard-shelled prey may atrophy following sustained fasting, which presumably occurs during flight from South America to North America.

This species is evidently specialized for consuming bivalves, but clearly not all bivalves have the same value to the birds (see Food selection and storage, above). As noted by González et al. (1996), digestive efficiency is strongly affected by the shell–meat mass ratio, which in turn is a function of the size (as well as species) of bivalve eaten. On the basis of an observed defecation (dropping) rate, and calculation of numbers and meat-shell ratio of mussels consumed to achieve the rate, Dekinga and Piersma (1993) and González et al. (1996) estimated biomass and energy content of food consumed by knots as roughly 25–35 mg ash-free dry mass/dropping, or roughly 0.3 g/min of foraging. At a migration stopover site in Argentina, individuals feeding on mussels of average size were estimated to be able to increase their body mass by 5 g/d (maximum rate); individuals with average dropping rates would have gained only 0.4 g/d. Daily weight gain rates at some other staging sites have averaged 3–4 g/d (Davidson and Wilson 1992, Piersma et al. 1992, Morrison and Wilson 1992).

There is almost certainly a complex relationship among foraging strategies of Red Knots, habitat availability, size structure of prey populations, digestibility, and prey behavior; taking these factors together, modeling (Zwarts and Wanink 1993) suggests that knots wintering in nw. Europe have meager and unpredictable food resources.

Migration And Energetics

Red Knots visiting one of the first known stopover sites during northward migration—Valdés Peninsula in Chubut Province, Argentina—did not show an increase of mass between 11 and 20 Apr (Table 2). In Río Negro Province, Argentina, Gonzalez et al. (1996) estimated that individuals during Mar consumed mussels (Brachidontes spp.) at a rate enabling an average mass increase of <1 g/d. During early Apr, knots with partial or complete Alternate plumage at Point Raza in Buenos Aires Province, Argentina, had mean mass of 138.9 g ± 16.9 SD (n = 30), significantly heavier than Basic-plumaged individuals (mean 110.0 g ± 7.9 SD, n = 7). During early May 1984, mean mass of Alternate-plumaged knots in s. Brazil was 202.2 g ± 19.4 SD (n = 280), the heaviest ever recorded for a sample of C. c. rufa; Basic-plumaged birds weighed significantly less (mean 149.9 g ± 15.5 SD, n = 11; Harrington et al. 1986).

At Delaware Bay, U.S., little change of mass before third week of May (e.g., mean 153.1 g ± 20.8 SD, n = 129 on 19 May; BAH), but rapid increase during last 10 d of the month (e.g., mean 175.9 g ± 17.5 SD, n = 265 on 24 May; BAH). Working with a number of assumptions, Castro and Myers (1993) estimated that 95,530 knots stopping at Delaware Bay during spring would consume 226.1 metric tons of horseshoe crab eggs and gain 5.2 metric tons of fat; during the stopover, average individual would consume 1,052 g of Limulus eggs and gain 54 g. But see Tsipoura and Burger 1999 .

Individuals have been found to have different basal metabolic rates (BMRs) at different wintering latitudes, with levels tending to be lower at tropical than at temperate latitudes (Kersten et al. 1998). Moreover, average BMR values correlate positively to lean body mass. Kersten et al. (1998) proposed that Red Knots (and other shorebirds) have BMRs capable of adjusting to prevailing conditions. For example, increased physiological demands, as encountered by shorebirds during migration, or wintering in colder climates, require increased processing of food for fattening (e.g., in preparation for migration or cold weather). This requirement, in turn, increases demands on functional components (e.g., stomach, liver, gizzard), which in turn require an increase of BMR for their “support.” A conclusion from this work is that knots wintering at tropical latitudes require less food not only for thermal maintenance, but also for BMR maintenance.

Sunbathing, Thermoregulation, Temperature Metabolism

Metabolism studied in context of understanding the energetics and evolution of spending winter in temperate versus tropical latitudes and for relationships between basal metabolism and metabolism required for high energy demands (Piersma et al. 1994, 1995a, 1996). Basal metabolic rates of Red Knots wintering in tropical climates are lower than those of individuals wintering in colder climates, evidently because nutritional organs require less work and become smaller, and because basal metabolic rates adjust commensurately with the mass of the nutritional organs (see Metabolism and temperature regulation, above). Red Knots wintering at tropical latitudes appear unstressed by extreme heat (Klaassen 1990).

Individuals can and do drink water (BAH), including salt water; indeed, major wintering zone of C. c. canutus in Mauritania has relatively high salinities, and fresh water is nowhere available (Wolff and Smit 1990). Nevertheless, laboratory studies (Verboven and Piersma 1995) suggest that individuals may be able to meet their water intake requirements through their diet, even in hot climates.

Not known to cast pellets; instead, excretes hard parts of consumed prey with feces, a fact that has enabled food consumption rates to be estimated in different parts of the range (see Nutrition and energetics, above).

Individuals foraging on tidal flats have variable defecation rates depending on location (González 1996) and probably other factors such as stage of migration and types of foods consumed. Defecation rates in 3 studies were 0.3 defecations/min (Alerstam et al. 1992), 0.8/min (Zwarts and Blomert 1992), and 0.4/min (González 1996).