Ardea Linnaeus, 1758. Syst. Nat. ed. 10, 1, 141. Type by subsequent designation Ardea cinerea Linnaeus (Gray, 1840, List Gen. Birds, p.66).
Ardea cinerea Linnaeus, 1758. Syst. Nat. ed. 10, 1, p. 143, Sweden.
Subspecies: Ardea cinerea jouyi Clark, 1907, Proc. U.S. Natl. Mus. 32, 468, Korea; Ardea cinerea firasa Hartert, 1917, Bull. Brit. Ornith. Club. 38, 6, Antinosy country, Madagascar; Ardea cinerea monicae Jouanin and Roux, 1963, Oiseau, 33, 104, Ile Arel, Banc d'Arguin, Mauritania.
The Grey Heron is a large grey heron having white and black accents, a white crown with black plumes, black belly and white thighs.
ADULT: The adult has a white head (including crown, sides, throat) with a broad black eye stripe extending from above the eye to the back of the crown and continuing as a crest including several elongated, black plumes. The long and heavy bill is yellow with a dull brown suffusion along the lower bill and top of the upper bill. The irises are yellow and lores are yellow turning darker green around the eye. The chin, throat, and neck are light grey to white, with the neck being tinged buff at its base. The fore neck is grey white with two distinct broken black streaks running parallel down the median. The upper back and hind neck are pale grey, the lower back and upper wing blue grey. Pale grey lanceolate feathers occur along the back. The flight feathers are dark grey to black contrasting with the paler upper wings and uniform grey to white under the wing. At rest, a black “shoulder” patch with a few white feathers is formed at the forward bend of the wing. The flanks are grey, the sides of the belly black, but the rest of the under parts are light grey to white, including the feathered thighs. The breast feathers are loose and elongated. The tail is grey. The legs and feet are green grey to yellow brown, varying in shade with age and season; the upper leg is paler (more yellow) than the lower leg.
During breeding, the black crest plumes attain full development and long white lanceolate plumes develop along the back, lower fore neck and breast. The iris, bill and legs flush deep orange to red. The bill may retain some of this color until after the young have hatched.
VARIATION: The sexes are identical in plumage, but the male averages larger (Boev 1987a,b, c). Considerable individual variation can exist, including birds that are various shades of grey, white, black, brown, or buff (e.g., Krotoski 1983, Sanders and Ebels 1998). Geographic patterns of variation are recognized taxonomically. The European-African race, H. c. cinerea, is as described above. The buff color on the neck decreases geographically from east to west, and the eastern race jouyi typically lacks the buff color and is markedly paler on the neck, upper wing feathers, and back plumes (Vaurie 1965). The island race firasa averages larger, especially in bill and legs than cinerea. The West African monicae is paler, looking sufficiently pale grey to white to be distinguishable from migrant H. c. cinerea (Erard et al. 1986). In the western Indian Ocean, Grey Herons breeding on various islands vary from firasa-type to cinerea–type (Penny 1974, Langrand 1990).
JUVENILE: Juvenile Grey Herons are more uniformly grey than adults and lack black plumage highlights and ornamental plumes. They have a grey to dark grey crown. The chin is white and fore neck appears brown grey. The upper parts are grey brown and under parts grey with brown grey streaking on the fore neck. The soft parts are duller than in the adult’s. Legs are dark grey, the upper legs being paler and tinged yellow green. In the first autumn and winter the upper parts become more blue grey. The feathers of the back and breast are moderately elongated. From the second autumn sub-adult plumage is assumed, differing from the full adult in having the forehead and crown grey instead of white and the black of the sides of the belly less developed. Juvenile plumages are retained into second year and vary among individuals from relatively more juvenile to more adult.
CHICK: The chick is brown grey above, with notable head crest, and white below. Irises are yellow, bill grey and legs green grey.
VOICE: The Grey Heron is a rather vocal heron. Its distinctive vocalization is the “Frarnk” call, a loud far carrying flight call. The “Go” call, rendered “go, go, go,” is an alarm call. The “Oooo” call is an aggressive call, also given in the Forward display. The Oooo call is also used in the Stretch display, becoming a more gurgling “oooo” when given by both sexes in the crouched part of the Stretch. At the breeding colony, the species is very vocal, uttering a variety of yelps, squawks and other, softer notes. The “Rwo” call is the male's advertising call. The “Arre” call is a landing call, rendered “arre, arre,” tending to a clucking on alighting. The Snap Display ends in a “Clop.” A Greeting Display includes bill-snaps. Bill Clappering occurs among pairs during formation and when in contact.
WEIGHTS AND MEASUREMENTS: Length: 90-98 cm; weight: 1020-2073g.
The Grey Heron is identified by its large size, grey coloration, distinctive eye stripes, black “shoulder patch” (wing edge), black belly, and white thighs. At rest, the black shoulder patch is a useful field character; and in flight the leading edge of the wing and especially the carpal joint are noticeably white against the general blue grey of the body and under wing. The flight action is slow, with wings bowed. When descending to a feeding area or to the nest, the Grey Heron can be very agile in parachuting downwards. These herons usually are seen alone or in small numbers either standing erect, or hunched over next to the water.
It is distinguished from the Purple Heron by its white head and lack of chestnut color. Juveniles and darker than normal adults are distinguished from Purple Herons with more difficulty (and wild hybrids are reported (Passarella et al. 1999), but Grey Herons are larger and lack rufous brown coloration. It is distinguished from the Black Headed Heron by its white head, larger size, grey (not black and white) under wing color. Immature Grey Herons have lighter legs (skin and feathers) than immature Black Headed Herons. The Grey Heron is distinguished from the Goliath Heron by its smaller size and has a white or grey in the juvenile neck (not chestnut). The Malagasy race of the Grey Heron is distinguished from the Malagasy Heron by being smaller and lighter and having a black shoulder patch. It is distinguished from the Imperial and Sumatran herons by being smaller and more brightly black and grey.
Distinguishing Grey from Great Blue and Cocoi herons where each is vagrant poses challenges (Gantlett 1998). Fundamentally, the Grey Heron is a smaller and slighter bird (about 60% of the Great Blue Heron and 40% of Cocoi by weight), with a smaller bill, shorter and light colored neck, and lacking chestnut thighs and chestnut edging to the “shoulder patch” of the Great Blue. If a field comparison is possible, the Grey Heron is more similar in size and shape to the American Great White Egret, but heavier (the Great Egret being about 60% of the Grey Herons weight). The general appearance of the Great Blue is of a larger, longer legged, longer necked, thicker billed bird with “warmer” color tones (derived from the venous to chestnut wash to the neck and chestnut thighs) contrasted with the flat grey of the Grey Heron. The Cocoi Heron is a starkly black and white bird, with the general appearance, size and shape of the Great Blue, with white neck and thighs like the Grey Heron, but with an entirely black cap that may be paler on the fore crown.
Out of range identification of the three species is even more difficult among juveniles. All three have a brownish wash to their plumage, which is both variable and changeable, dark crowns, dark upper bill, and shorter plumes. Juvenile Grey Herons are quite pale, have grey thighs and strong black and white throat and breast streaking, light legs, and a white “shoulder patch.” They tend to have a lighter crown than the other species, although this is variable among subspecies, jouyi being lighter than cinerea. Juvenile Great Blues are rather darker (except for some in southern Florida), have a pale to intense rufous tinge to the thighs and “shoulder patch,” and grey and rufous throat and breast streaking. Thigh color, so important in identifying adult Great Blue Herons, is grey as in the other species. Juvenile Cocoi Herons are duskier than the others, with dusky streaked thighs and stripes on the throat; most importantly have a distinct cap that is dark grey to black, paler on the fore crown, even in young juveniles. In all the species, adult plumage is assumed gradually over the first or second year with the grey of the crown diminishing very gradually, accounting for much of the variation to be seen, and much of the confusion in identification. The most important marks are the very dark and distinctively edged crown of the Cocoi Heron, the larger size and bill of the Great Blue Heron, and the and thigh and “shoulder patch” coloration.
The Grey Heron is a typical heron related most closely the other Ardea and the Egretta herons. Indeed in many ways it typifies the heron family - in nomenclature, dispersion, habitat, and behavior. This is the Eurasian form of the large, solitary heron type, which is replaced by similar species in other biogeographic regions – the Great Blue Heron in the Nearctic, Cocoi Heron in the Neotropics, and the White-necked Heron in Australasia.
Of the four subspecies recognised here, two distinguish extremes of clinal variation across Eurasia. The third subspecies represents an isolated island form, a taxonomic distinction with analogies in other herons. The pale Banc d’Arguin has a highly restricted distribution. Erard et al. (1986) proposed the interesting hypothesis that it is a remnant form, more closely related to populations of Asia and Madagascar, and worthy of specific rank. More typical Grey Herons nest in nearby Senegal, and an individual in Senegal was reported as showing intermediate features between cinerea and monicae (Baillon 1989, Dowsett and Dowsett-Lemaire 1993).
Other described subspecies are problematic. Grey Herons on islands in the Asia-Oceania region include the heavy billed birds described as A. c. altirostris, occurring in Sumatra and Java (Voous and van Marle 1988). Those on Comores have been attributed to A. c. cinerea, whereas those on Aldabra have been considered intermediate between cinerea and firasa (Penny 1974).
RANGE AND STATUS
BREEDING RANGE: The breeding range of the Grey Heron covers most of the Old World south of the Arctic Circle, including Europe, Africa, Asia, East Indies islands to Wallace’s Line.
H. cinerea cinerea occurs across northern Eurasia from Outer Hebrides, Great Britain (Bourne 1999), and Scandinavia, generally south of the Arctic Circle, through Russia (to Sakhalin). More to the south, it ranges from France, Spain, the Mediterranean, east Europe, Turkey, Iraq, possibly Israel (Ilani and Shalmon 1984), north Iran, north Afghanistan, Turkmenistan, Pakistan, India to Sri Lanka. Its status in Africa is intriguing. In north Africa, it nests rarely in isolated locations - Canary Islands, Morocco, Algeria (Van Dijk and Ledant 1983), and Tunisia. South of the Sahara and north of the Equator, small and sedentary breeding populations are known from Senegal, Mali, Niger, Côte d’Ivoire, Ghana, Nigeria, Sudan, Ethiopia (Turner 2000). Further south small breeding populations occur in Kenya, Tanzania, and Rwanda. They are resident throughout southern Africa, breeding patchily in suitable wet areas.
In the east Palearctic, the race cinerea integrates with jouyi in east Russia. This subspecies ranges from India, Mongolia (Stephan 1988), China to Taiwan and Hainan, Korea, Japan (Matsunaga et al. 2000), Myanmar, Indochina, Malaysia (peninsular), Indonesia to Java and Sumatra (Danielson and Skov 1985, Silvius 1986).
The race firasa occurs in Madagascar. The race monicae breeds on islands of the Banc d'Arguin, Mauritania. It has been observed breeding elsewhere rarely, a few nests in the Aftout es Saheli located 100-200 km south of Banc d’Arguin (B. Lamarche fide H. Hafner). Outside of Mauritania it has been reported only as far as Senegal.
NONBREEDING RANGE: European nesting birds occur in winter in Great Britain, west and central Europe (e.g., Draulans et al. 1986a), the Mediterranean, Africa, and the Middle East including Saudi Arabia, and Iran. Asian nesting birds (Biondi et al.1994) occur in winter in Pakistan (Balochistan, Sindh), India, south China including Hong Kong and Taiwan, south east Asia, Japan, Thailand, Malaysia, Philippines, and Indonesia (Borneo, Sumatra). Birds occur only in winter in Hong Kong, Taiwan, Philippines and Borneo (Sabah – Sheldon et al. 2001). SubSaharan Africa north of the equator is the important African wintering area for many populations (Turner 2000). This winter range generally is from Cape Verde Islands, Senegal and west Africa, south to 4o in Zaire, east to Somalia. Western European birds winter from Senegal, Guinea, Sierra Leone, Mali, Upper Volta, Togo, Nigeria, and Zaire. Eastern European birds winter from Mali, Upper Volta, Togo, Nigeria, and Ethiopia. Birds from Russia winter from Senegal, to south Egypt, to Kenya. There is no direct evidence of northern birds wintering south of the Equator but seasonal increases in birds in parts of Tanzania are suggestive (Baker and Baker in prep.).
Individuals sometimes wander widely. The Grey Heron is regularly seen in Iceland, Faeroes, and Ascension islands. In the New World, it has been recorded in Greenland, Barbados (Smith and Smith 1990), Tobago (D. Finch pers. com.), French Guiana, Azores, Trinidad (F. Hayes pers. com.), Montserrat, Martinique, off Bermuda, Brazil (Para), and Venezuela (Marion 1988). The last six records are of birds that originated in France. It has been reported off shore in South Africa (Kirsch 1998). In Asia, it is been noted fairly often in Taiwan and in the Philippines. There are also occasional records from New Guinea, Kalimatan (Van Balen 1999), Australia, and New Zealand (Parkes 1974). Early sight records from New Zealand have been discounted (Dawson 1974).
MIGRATION: Grey Herons disperse widely after the breeding season, this movement beginning generally soon after the young can fly. Dispersal may be in almost any direction although mainly to the southwest in Europe. In southern Europe, postbreeding migration is in September and October, and prebreeding migration in February and March (Lekuona and Campos 1996c). In northern Europe, fall migration is from early September to late November and return is in February and April (Grishchenko and Serebryakov (1993).
Birds breeding in Scotland and England are generally nonmigratory (Partridge 1986), although British birds have also been recovered in Spain and Gambia (Christmas 1994). In southern Europe, some birds remain in winter but others migrate (Lekuona and Campos 1996a,c, Marion 2000a). Migration is generally to the southwest in west and east Europe, southwest in west Asia and to the south and southwest in east Asia (Hancock and Kushlan 1984, Li et al. 1989, Round 1995). In the Mediterranean, migration is broad but corridors are along both the eastern and western shores. Birds nesting in Africa, India and south east Asia appear to be sedentary.
STATUS: The Grey Heron is the most abundant and widespread heron in Eurasia, with a breeding population in Europe between 150,000 and 180,000 pairs (Marion 2000). In Britain, where the longest running census occurs (Bourne 1999), the population was nearly 6,000 pairs in 1991 (Carter 1992). The core of the European population is in France, Russia, Ukraine, Germany, the United Kingdom and Netherlands. With few exceptions, Grey Heron populations have increased markedly in numbers and range in Europe in this century. Northern expansion may be enhanced by post Pleistocene climate amelioration, but population increases are due principally to decreased hunting and depredation control (Moller and Olesen 1983, Marion and Marion 1987a, Bakka 1989, Bara 1988). In Britain, for example, the nesting population increased from 4,000 to 6,000 pairs between 1928 and 1991, rising at an annual rate of 8% per year during the 1980’s (Tomlinson 1992). In France, the nesting population increased from 4,500 to 27,000 pairs between 1974 and 1994, becoming the larger European population due to protection (Marion 1997).
In West Africa, the nesting population outside Mauritania is small, perhaps less than 500 pairs (Turner 2000). In east Africa, it is widespread but scattered. The resident population in Tanzania may be about 8,000 birds (Baker and Baker pers. com.). In southern Africa, the Grey Heron is increasing in Zimbabwe and South Africa, due principally to irrigation and reservoirs. Aquaculture has provided substantial additional feeding opportunities to which populations have respond (Kurz and Schmidt 1994).
The coastal Mauritania population is also highly isolated, although the known population there has increased from 1100-1600 pairs in the 1960’s to over 4188 in 1997 (Hafner et al. 1998). The Malagasy subspecies is largely restricted to the western portion of Madagascar and is vulnerable due to restricted range, exceedingly high levels of habitat alteration, hunting, and predation at the nesting colonies (Turner 2000). The Grey Heron is increasing on the Seychelles (J. Gerlach, pers. comm.).
The Grey Heron is generally abundant summer residents in China, Mongolia, Japan, Korea, and parts of Indonesia (Matsunaga et al. 2000, Lansdown et al. 2000). Breeding populations are small in Indochina and peninsular Malaysia.
Grey Herons are generalists in habitat use. They are typically found in and around shallow water, generally along watercourses and shorelines, and usually in locations having roost trees nearby. They may occur in inland fresh waters, along estuaries or in marine habitats. Shallow water, relatively large prey, and four or five months of an ice-free breeding season seem to be essential characteristics of suitable habitat. Among typical habitats are lakeshores, rivers and floodplains, reeds (Jeanmonod and Roulin 1989), swamps, ponds, mudflats, beaches, mangroves and salt marshes. Dry or damp grasslands away from water are also used. Generally most abundant in the lowlands, Grey Herons can be found at surprising altitude, breeding to 2000m in Armenia and occurring to 4000m in northwest India.
Use of manmade habitats is the rule over many parts of its range. Rice fields and fishponds provide important feeding sites nearly range wide. Individual birds and even local populations may become dependent on these habitats and so are affected by changes in agricultural management, as well as other anthropogenic factors (Fasola 1986, Biondi et al. 1993).
They nest and roost in trees, with exceptions such as the Mauritanian population. So trees or other high vantage points are generally desirable. Some degree of isolation and protection is typical of places chosen for roosting and nesting.
The Grey Heron usually hunts solitarily, but in situations where food is more concentrated, birds may feed in loose aggregations or even mixed species flocks. If conditions are especially favourable, quite large feeding aggregations may form, particularly after the breeding season. Herons feeding alone defend their feeding territories. Defence may be vigorous, and killing of intruders is known (Richner 1985). Aggression varies seasonally being most intense when young are being fed (Lekuona 1999). Sites may be near to the colony site or as many as 38 km away. When available feeding areas are poor and/or more distant from the colony, adults use two or three feeding areas and are no longer territorial (15% of breeders, Marion 1984, 1989).
Depending on prey availability and distance, herons use three types of feeding sites (L. Marion pers. com.): 1) the individual defended feeding area composed of a single patch of about 20 h, usually in wetlands or rice fields; 2) individual nonterritorial feeding areas composed of 2 or 3 patches in poor habitats or in typical habitats more distant from the colony, and 3) neutral feeding sites used by many herons, where individual site appropriation is not possible. The latter are either ephemeral sites where high densities of food occur for short periods or sites too close to the colony and therefore attracting too many birds to be defended successfully. Herons appear to make trade-offs between nearer and distant sites, between using energy in territorial defense or no between site fidelity and exploring new feeding opportunities. Non-breeders and birds with little experience in an area spend more time exploring than do birds with local experience (van Vessem and Draulans 1987a).
Although considered in the literature to be typically a diurnal feeder, in fact it feeds at any time of the day. Generally, Grey Herons feed most actively at dawn and at dusk and roost – usually in trees - during the middle of the day and at night. But Grey Herons also feed at night, particularly during breeding when adults have to feed up to 23 hours a day to care for the young (Marion 1988). In fish farms, herons generally feed at early twilight or night, sometimes using artificial light but often feeding in the dark, where probably not coincidently they are out of sight of fish farmers which disturb their daylight feeding (Amies 1990, Carss 1993, Draulans and van Vessem 1985a, Duquet 1987, Marion and Marion 1987b, b).
Time budgets actually observed depend on the interaction of food availability and food demand (van Vessem and Draulans 1987a, b). As noted, during the peak of nesting energy demand, birds feed nearly all day. In tidal situations, birds forage according to how tides make food available, selecting high or low tides (Matsunaga 2000). In France, they fed predominantly during four hours of low tide (Lekuona 1999).
By far, the most common feeding technique (used half or more of the time) is Standing, often thigh-deep in water, but also in shallower water, on land next to water, or on dry land away from the water. Walking Slowly is used to move between Standing bouts or for slow exploration. Walking is done at a very slow pace. When fishing from a bank, herons frequently use Peering Over. They also may use more active behaviors such as Running with wings half-open, Swimming Feeding (for many minutes at a time), Diving and Plunging, but these behaviors are rather exceptional (e.g., Brown 1985, Craney et al. 1989, Guntert 1986, Husband and Siering 1991, Laurenti 1986, Taylor and Carss 1990). Despite its size, it feeds while in flight by performing aerial manoeuvres of surprising dexterity (Keighley and Hall 1995, Bowey 1997).
Scavenging along the shore or in pastureland is not uncommon (Hiraldo et al. 1991), and it follows ploughs feeding on what is scared off or unearthed (Fox 1989a). In winter, it generally feeds in pastures on small mammals. The usual method of capturing prey is by a rapid Bill Thrust, preceded by retracting its head and neck. Pecking is used on dry ground, such as for earthworms. Food is nearly always dipped in the water, if available, before being swallowed. It is handled by biting, stabbing, and pounding to soften, to remove spines, to position for swallowing (fish are swallowed head first) or for breaking into smaller pieces.
Being a relatively large bird, it is not often harassed on its feeding ground. In the air, it is usually able to avoid aerial attacks from other species by sideslipping, but is not always successfully as there are instances of its being harried to its death by crows (Walters 1983). Being among the larger birds in mixed feeding situations, it can steal prey from other species such as gulls, cormorants, bitterns, and grebes (Cooper 1984, Kingston 1991, Hume 1992, Sellin 1986). In turn it is subject to piracy and other aggressions from smaller, more agile birds such as crows and gulls (Skeen 1988, Todhunter 1987, Walters 1983, Warner 1986). It also feeds symbiotically with species such as mergansers and osprey (Laurenti 1990, Jacobs 1994).
The Grey Heron’s flexibility in diet and feeding behavior suggests it can make use of a complex set of foraging tactics in face of different or varying prey abundance (Draulans 1987, Draulans and Hannon 1988, Draulans et al 1986b, Draulans and van Vessem 1985a,b, Marion 1989). Evidence suggests that the species is highly opportunistic in prey selection. Prey do differ in profitability, depending on prey size, morphology, behavior (Moser 1986), and Grey Herons rapidly respond to changes in prey availability (Adams and Mitchell 1995). Fishing success of Grey Herons increases with increasing prey density, to a point, due to higher encounter rates and use of more intensive hunting methods. Prey intake is also highest when birds first arrive at a feeding site, declining with time. Nonetheless, the ability of herons to adjust their foraging intensity, behavior, and expectations to conditions means that success rates turn out to be similar over different habitats.
during non breeding season, when herons can easily change feeding to more productive sites in response to environmental conditions (wind, rain, clouds) that alter the sites’ relative value. However, during breeding season the territorial feeding strategy induces large differences in feeding success between territories and thus adults, and directly influences the breeding success (Marion and Marion 1987a, 1988).
Individuals differ in their behavioral choices, juveniles especially being less effective in both feeding and selection of the site and time for feeding. It is not clear how much of this ineffectiveness is due to juvenile choice or to dominance of adults at prime sites (Draulans 1987, Draulans and Hannon 1988). Juveniles, being less experienced, improve their foraging efficiency at fish farms more than did adults. Overall, juveniles appear to be less flexible than adults in use of time, but more flexible in use of space (Draulans and van Vessem 1985b).
That prey capture success, energy intake and frequency of feeding strikes of Grey Herons increases at higher prey densities indicates the importance of herons finding and feeding in locations that have high food abundance. At very high prey density, success stops increasing but the energy spent foraging decreases. Thus an important aspect of Grey Heron ecology is the ability to access sites supporting acceptable to high foraging success. Diet variability follows, as herons change diet to take advantage of more catchable prey (Adams and Mitchell 1995, Boldreghini et al. 1995, Peris et al. 1995, Draulans and van Vessem 1987a). For example at a fish farm, herons focused on caged fish that were blind or in poor condition (Carss 1993, Carss and Marquiss 1991). Diet differs among colonies and among years (Draulans et al. 1987). Differential feeding success becomes exceptionally important during nesting. When feeding opportunities are distant and/or poor, for example if only frogs or voles are available, no more than two young can be produced whereas if the feeding sites are rich, such as with eels, even if as far as 35 km away from the colony, herons can produce four young (L. Marion pers. com.). Herons have more options for feeding site selection outside the nesting season, when energy demands are lessened.
Most studies have shown that Grey Herons do not seem to severely impact their prey supplies, taking only 6-8% of fish standing stock available in ponds (Draulans 1988a, Feunteum and Marion 1994), although effects may be greater in trout streams (Kramer 1984). These are crucial findings in that the well being of local populations are often tied to their interactions with fish farms.
The Grey Heron takes a very wide variety of prey, the diet varying according to habitat and season (Draulans et al. 1987, Marquissa and Leitch 1990, Sawara et al. 1994). It is something of a specialist on larger prey, up to 19-25 cm long. Some may be much larger or longer. The size of prey eaten or attempted is often remarkable and at times this leads to swallowing problems (Bentzien 1991, Cuenoud 1994, Davis 1982, Geroudet 1993a). The typical strategy is to catch a few large prey during the course of the day.
Depending on location, fish or crustaceans tend to dominate a diet; amphibians, small mammals, reptiles, crustaceans, worms, insects and small birds are also eaten (Fox 1989b, Bentzien 1991, Mienis 1991, Greaves 1991, Camici and Zimmerli 1993, Besson 1994, Cuenoud 1994, Carruette 1995, Hewson 1995, Lekuona and Campos 1996c, Kkreuziger and Achenbach 1998). It is remarkably versatile and is reported taking such prey as muskrats, rats, grebes, wood hoopoe, frogs in winter, scavenged fish, and mussels. It also swallows a good deal of vegetable matter and even other foreign material (Hill 1988), but it is unclear if this is functional. It can scavenge carrion, dead fish, and scraps from fishing boats (van der Kelen 1993, Dies 1999a,b). Prey and size of prey varies seasonally, especially with respect to the food needs and food handling capacity of young in the nest (Lekuona 1999).
Time of nesting differs across the extensive range of this species. In temperate areas, breeding season is restricted to spring and summer. In northern China, herons arrive in late March and in some years not until May. In coastal Russia (Maritime Province), herons arrive mid March and lay eggs in the first half of April (Litvinenko 1983). In Britain, nesting begins in February and March. In Spain nesting can be as early as January, corresponding to when minimum daily temperatures reach 8 C (Campos and Fernandez Cruz 1991). Nesting in most situations is early relative to other herons, in Spain three months before other herons (Prosper and Hafner 1996).
In the tropics, nesting season is more flexible, often occurring in the rains, the timing of which may differ from place to place. For example, in north India nesting is in July-October, but in south India in November-March. Birds breed at the height of the rainy season in Africa, from Senegal through west Africa, Sudan, Ethiopia, Kenya, Tanzania, and Rwanda (de Naurois 1976, Turner 2000). The coastal monicae nest from April to December peeking in April –May (H. Hafner pers. comm.) Even in monsoon areas, nesting may start relatively early, well before the rains actually begin. In some tropical areas, nesting can occur nearly year round, such as in coastal Kenya, where birds feed in the littoral zone.
Most frequently these herons nest in patches of trees located where they offer some degree of isolation and privacy. They generally choose higher of the tress available, up to 50 m, and in mixed colonies are the highest of the nesting herons (Fasola and Alieri 1992a). However, the total range of nest substrates and colony sites used is very large, including wood lots, rocky islands and city parks, herbaceous marsh plants, rock cliff ledges, buildings, walls and even the bare ground (e.g., Litvinenko 1983, Helm 1996, Polasek 1991). Nesting sites need to be within convenient flying distance of feeding areas. They fly 2 – 38 km to feed (Litvinenko 1983, Marion 1989).
The Grey Heron is primarily a colonial nester, although often in small colonies of 2-10 nest (e.g., Korvin 1987). The largest colony in Europe is 800-1300 pairs (Feunteun and Marion 1994). Being the first nesters, their colonies attract other species such as ibis, spoonbills, cormorants, other herons (e.g., Carpega et al. 1990, Commecy 1989a) and especially corvids (Tkachenko and Shakula 1983). Heron nests are attractive to and are taken over by other species such as hawks (Boonman and de Vrieslaan 1992).
The nest is generally a stick platform with grass liner about 50 cm in diameter, constructed from the most easily available material at the site. As noted above, nest sites vary widely, but they tend to be built toward the top of the canopy. While new nests may be so flimsy that the eggs are visible through them, pre-existing nests are more often reused, being repaired and enlarged annually becoming bulky masses. Windstorms can destroy nests, so they tend to be situated on secure tree branches. On bare ground, herons use bush twigs, herbaceous plants and seagrass for nesting material (e.g., Litvineenko 1983). Males do more stick gathering than females and females more of the nest building (van Vessem and Draulans 1986c).
The courtship of the Grey Heron have been intensively studied (Witherby et al. 1939, Lowe 1954, Bauer and Glutz 1966, Milstein et al. 1970 and others). The male on arrival at the courtship site or the colony site claims an advertising site, an old nest if available. In small populations, males may gather and courtshipoccurs at display sites prior to the birds moving as a group to the colony site (L. Marion pers. com.). In larger colonies, there are generally several waves of birds arrivals in the nesting area during two or three months, and each group of new birds tend to nest together forming subcolonies (Marion 1988).
While displaying, through the day, the male repeatedly gives the Rwo call, its yelping advertising call. It also performs Upright displays with plumes erected, a bill-down version of the Stretch display, and the Twig-shake display. The male defends his territory against other males (and approaching females) mainly with Forward displays. The main pairing displays are the less aggressive version of the Stretch with up-pointed bill, combined with a milder version of the lunge movement or Bow-Snap display. Males Fly Around, when landing at the display site give a landing display, alighting with powerful wing beats and arched neck with raised neck and head plumes.
Females approach the male, who is at first aggressive toward the intrusion giving Forward, Upright, and Stretch displays. As aggression wanes, Snap displays increase. After pairing occurs, Snap displays decrease, Stretch displays continue, as do much mutual preening, billing, and Bill Clappering. Pairs may Fly Around, and when one lands with a Landing Display, the other responds with Upright and Stretch displays. The individual permutations of the display repertoire can be complex, varying a great deal with age, temperament and the stage of the pair bond. As the pair bond strengthens, the intensity and elaboration of display diminish, particularly for males. Copulation occurs with the female standing and this can continue will into egg laying. At larger colonies extrapair copulations can occur, usually with neighbouring females (Ramo 1993, Lekuona and Campos 1998). The pair bond appears to last a single season only.
The eggs are pale green blue, relatively large but variable, 57-61 x 41-43 mm, laid at intervals of two days or longer. The clutch size differs with latitude. In temperate zones (Europe) clutch is usually 3-5 eggs (3.6 in Belgium – van Vessem 1991). In the tropics, egg clutches are reported to be 3-4 in India, 3-5 in Africa, and 3 in Madagascar. Complete clutches of 1 egg to 10 eggs are on record, but latter seem unlikely to be attributable to a single female. Replacement clutches are reported, but this is less common that has been assumed because birds more typically mate with another bird or both leave the colony rather than renest (Marion 1988). Nests from failed attempts may be reused by other birds. In Spain early season losses led to a bimodal laying peaks, in March and again in April-May (e.g., Campos and Fernandez Cruz 1991).
Incubation takes 23-28 days, averaging about 26 but in tropical conditions may be only 21 days. Both parents incubate. They attend the nest at comparable rates through the day (van Vessem and Draulans 1986b,c). Young hatch asynchronously, depending on laying date. Young are fed upon hatching; so earliest chicks are much larger than later chicks. This leads to differential survival, influenced by sibling competition, aggression, and even cannibalism (Lekuona and Campos 1993, Litvinenko 1983). In years of food shortage or other difficulty the smaller chicks do not survive, and in larger broods young chicks rarely survive irrespective of food supply.
Chick’s eyes open at hatching, and begging begins soon after. Parents brood young for up to 18 days. One remains on guard through about day 30. Thereafter, both parents forage at the same time. Both parents feed the young, which take food directly from adult's bill except during the first days. When young cannot eat the sizes of prey most profitably caught by adults; the larger prey are removed from the nest by the adult, consumed and predigest so that young are able to ingest again (Marion 1988).
Males tend to guard very young nestlings and spent less effort feeding chicks than females. Parents feed chicks first at the nest and then on nearby branches. Chicks that fall to the ground are not usually tended, although there are contrary reports (Lowe 1984). After three to four weeks the young start clambering from the nest, but take a total of about 50 days to fledge and do not leave the nest area until two weeks later. Generally juveniles are not fed or attended away from the nesting site.
Feathers begin to grow about 7 days, and chicks are fully feathered at 28 days. Chicks remain in the nest for up to 10 to 20 days, where they spend most of the time standing, sleeping, begging, and tussling among sibs. They perform Upright, Forward and Snap displays, defending the nest. They can step away from the nest in 20-30 days, fly about 50 days of age, and are independent about 60-70 days. After fledging, generally juveniles are nor fed or attended away from the nesting site.
Many factors affect the success of breeding. Egg failure can be high in some populations; in
Belgium, egg failure was 17% (van Vessem 1991a, Campos and Fernandez Cruz 1991). Also in Belgium, nestling mortality was 33.3% (van Vessem 1991a). In Spain, brood size averaged 2.2 nestlings from a clutch size averaging 3.5 (Fernandez Cruz and Campos 1993).
Intracolony differences occur in success. For example, brood size increased with increasing age of
parents, suggesting higher production by older, experienced birds. Breeding success was not affected by the number of birds present or by synchronisation or by location in colony. The most critical factor in nesting success is availability of food through the nesting season. Young can be killed by predators, especially large aerial predators such as eagles (Berthelot and Navizet 1987, Yiraberg 1992, Naoroji 1990). Breeding success can increase for birds using fish farms (Carss and Marquiss 1996). Nesting high in trees puts nests and nesting at risk to windstorms (Lekuona and Campos 1995).
The population dynamics of the Grey Heron are among the best understood of all the herons (Marion 1980, 1981, Marion and Marion 1987a, Campos and Fernandez Cruz 1989, van Vessem 1991b, North 1979, Marquiss et al. 1983, Tomlinson 1992). Grey Herons ggenerally do not breed until after the second year, although some frequent colonies as young yearlings.
Mortality is estimated to be 66.7% for yearlings (Fernandez Cruz and Campos 1993), which nonetheless supported a population increase of 34.6% for 8 years prior to stabilisation. The demography of the species is driven by its longevity (Cordonnier 1985), suggesting that adult survival is the most critical factor in population growth and stability. In Spain, the breeding population is skewed toward older adult birds suggesting saturation of the breeding population (Campos and Fernandez Cruz 1989, 1990, 1991).
Killing, hunting and accidental deaths of Grey Herons associated with humans can be a significant risk to stability (Kriedemann 1991). Century-scale population trends in Europe are attributable in large part to changes in its protective status (Marion 2000a). Hunting is less of a threat for most populations, although it is a serious problem in some areas, such as Bavaria (Utschick 1983)
Fish farming has a double influence. Populations can increase as birds, especially inexperienced young birds, feed at aquacultural facilities and perhaps increase their survivorship (Sawara et al. 1992, Samusenko 1993). However, mortality can also increase due to antidepredation activities at farms. In England water pollution has caused deaths of breeding adults and eggshell thinning, but has had no obvious effects on total populations.
Winter survival is a critical factor in temperate populations. In Britain survival of both adult and first-year herons depends on the severity of the winter (North 1978, Marquiss et al. 1983). The largest annual decreases in the British population occurred after the winters of 1947 and 1963. Although in Brittany, France, where the species is widely dispersive, no difference in mortality rates was noted between hard and mild winters (Marion 1981). Young appear to be more susceptible than adults to winter kill (Tomlinson 1992).
The interactive effects of humans killing birds and hard winters have been demonstrated by Utschick (1983) showing that the potential exists for hunting to decrease populations to levels that inhibit recovery following a severe winter.
Range wide, as populations have increased in this century under protection, hunting has decreased as a factor range wide and density dependent factors, such as territoriality, and winter mortality may have become the more important factors in regulating population size (Marion 1984, Marion and Marion 1987a). It is likely that in much of Europe population size is becoming governed by environmental quality, which determines the location and size of heronries (Utschick 1983, Fasola and Alieri 1992b).
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