What makes the Great Crested Grebe Podiceps cristatus so successful ? Breeding and feeding strategies of a widely distributed, opportunistic bird

Abstract

This paper is a survey of the recent knowledge on the behavioural ecology, habitat choice, feeding ecology and breeding-strategies of the Great Crested Grebe Podiceps cristatus. It is based on the work of many authors, e.g. Leys & De Wilde 1968, Leys et al. 1969, Melde 1973, Berndt 1974, Simmons 1974, Spletzer 1974, Cramp et al. 1977, Zang 1977, Asbirk & Dybbro 1978, Vlug 1979a, 1979b, Blinov et al. 1981, Fuchs 1982, Fjeldså 1983a, Vlug 1983, Kraak 1984a, 1984b, Van der Poel 1984, Büttiker 1985, Koshelev & Chernichko 1985, Van der Poel 1985, Goc 1986, Mayr 1986a, 1986b, Moskal & Marszałek 1986, Fjeldså 1988, Piersma et al. 1988, Renevey 1988, Ulfvens 1988, Renevey 1989a, 1989b, 1989c, Ulfvens 1989, Ulenaers & Dhondt 1991, Henriksen 1992, Adriaensen et al. 1993, Henriksen 1993, Van Eerden et al. 1993, Wiersma et al. 1995, Piersma et al. 1997, Simmons 1997, Abramowitz & Johansen 2001, Fjeldså 2004, Konter & Konter 2004, Vlug 2005, Kjeldsen 2008, Konter 2008a, Neubauer 2008, Abt & Konter 2009, Berndt 2010, 2011, Konter 2011, Vlug 2012.

The breeding and feeding habitat of the Great Crested Grebe is characterized by ecological instability, e.g. by wave action, changing clarity of the water, changing water levels, fluctuating availability of nesting cover and suitable nest-sites, varying quantities of prey fish, and competition for food with predatory fish such as Zander Stizostedion lucioperca and European Perch Perca fluviatilis. The species is a generalist and shows a large flexibility in its adaptations to these unstable and varying conditions, more than the other species of grebes Podicipedidae, which are, as a rule, to larger extent specialists. There is perhaps one exception: it is possible that the flexible adaptations of Little Grebes Tachybaptus ruficollis (and T. novaehollandiae) are comparable in their intensity to those of the Great Crested Grebe.

Among the 22 species of grebes the Great Crested Grebe has the largest breeding distribution in the world (158,000,000 km², BirdLife International 2017), nesting in four of the five world climatic zones (main climates of Köppen-Geiger, cited in Kottek et al. 2006): the species lives in equatorial (a scattered occurrence), arid, warm temperate and boreal climates, and is only absent in the polar climates. It nests preferably in large, eutrophic, open lakes with limited floating growth, sparse or no dense submerged vegetation, reed-fringed banks and bays and many fish, e.g. Lake Schwerin, Mecklenburg-West Pomerania (maximum 2,020 breeding pairs), Lough Neagh, Northern Ireland (c. 2,000 bp), Lake Mälaren near Stockholm, Sweden (1,630 bp), Lake Neuchâtel, Switzerland (c. 1,600 bp) and Lake IJssel, The Netherlands (c. 1,500 bp). However, the Great Crested Grebe colonizes many habitat types and inhabits the broadest spectrum of breeding waters among the Palaearctic grebes: water bodies in lowland plains and mountains (e.g. up to 4,590 m above sea level in Tibet), large and small lakes (from 0.5 ha to 31,500 km²), oligotrophic and hypertrophic water bodies, freshwater, brackish and saltwater lakes, lagoons, bays of the sea and exposed archipelagos (e.g. c. 3,500 breeding pairs in the skerries near Stockholm, Sweden), rivers, city parks, canals in towns, fish ponds, flooded gravel pits, reservoirs, etc., and of the larger species it has the greatest numbers of individuals in the world (920,000–1,400,000, BirdLife International 2013).

More than its relatives, the Great Crested Grebe in the (western) Palaearctic increased in numbers in the 20th century, e.g. in The Netherlands (1932 c. 300, 1998–2000 13,000–16,000 breeding pairs), North Rhine-Westphalia (mid-1960s 170–240, 2005–2009 2,000–3,000 bp) and Finland (1950s c. 5,000, 2006–2010 25,000–35,000 bp). This increase was especially an opportunistic reaction to the eutrophication and hypertrophication of water bodies and the following growth of its main food (fish, especially of the carp family Cyprinidae). Is it only by luck (abundant food) that the Great Crested Grebes prosper? Most other species of grebes are much less piscivorous and suffer from an increasing eutrophication since it reduces their food, especially aquatic macroinvertebrates. Is the Great Crested Grebe merely a specialist profiting from favourable circumstances? The answer must be no: many Great Crested Grebes also live in oligotrophic, mesotrophic and dystrophic water bodies. It appears, that Western- Aechmophorus occidentalis and Clark’s Grebes A. clarkii, two North-American fish-eating specialists, do not profit from the growing eutrophication in their homeland. In any case, these two species do not show the increase in numbers, which is so characteristic of Great Crested Grebes.

The Great Crested Grebe reacts less specialistically than (most) other species of grebes. It is justified to call its behaviour generalistic, i.e. the species has a broad pattern of resource usage. It is, for example, less a fisheating specialist than the Aechmophorus-species and locally its food can mainly consist of invertebrates. It is also flexible in its feeding techniques: usually, the birds are daytime feeders, but in certain circumstances they forage during the twilight periods (at dusk and dawn) or at night ; normally, they first spot the prey by peering from a surface position or they swim about under the water surface looking for prey beneath or before them, but they may also approach their prey from below, spotting the fish against the light ; typically, they catch the fish high in the water column, but they sometimes dive to c. 40 m depth ; as a rule, the birds hunt alone, but they forage occasionally in flocks ; usually, they feed on the breeding water, but in certain circumstances they undertake food-flights from the breeding lake to a larger water body.

In contrast to other grebes the species sometimes nest in large numbers in or near human settlements, for example in city parks and in canals in towns (e.g. 1983 c. 175 breeding pairs in Amsterdam) and in yachtharbours (e.g. up to 125 breeding pairs in a colony next to a landing stage in a harbour in Enkhuizen, The Netherlands). This confirms that the Great Crested Grebe is a generalist: generalist species with a large geographical distribution usually thrive in anthropogenic environments better than specialists.

Great Crested Grebes usually build their floating nests in the reed marsh (especially Common Reed Phragmites australis) bordering their breeding waters, or in offshore reed beds. They use also many other species of plants such as Lesser Bulrush Typha angustifolia and Common Club-rush Schoenoplectus lacustris. However, a number of water bodies have little or no vegetation cover, so that the birds must be flexible in their choice of nest sites. Many Great Crested Grebes place their nests on quite open sites on floating waterweeds, in narrow ribbons of shoreline vegetation, among branches of trees overhanging the water, on dead tree stumps, and sometimes ashore, i.e. on dry land. At the sea coast, especially in the naked skerries in archipelagos in the Baltic Sea where aquatic vegetation is lacking, they may build their nests on or among emerging rocks, stones or boulders.

The species often breeds near or in colonies of small larids, especially small gulls such as Black-headed Gulls Larus ridibundus, and marsh terns such as Black Terns Chlidonias niger. Larids not only provide an early warning system, but they also actively pursue predators and drive them away. Great Crested Grebes in mixed colonies synchronize their breeding activities with those of gulls and terns.

The breeding season in the Great Crested Grebe is very protracted, more than the other Palaearctic Podicipedidae, reflecting early breeding in locally wintering birds, late breeding in migrants, and high incidence of re-nesting and (locally) multiple brooding. In some countries in Western Europe, such as The Netherlands and England, the breeding season can cover all months of the year.

The nesting dispersion behaviour of Great Crested Grebes is very flexible, more than in other Palaearctic grebes. Normally, the birds are highly territorial and defend large territories against conspecifics as well as other species of grebes and waterbirds. Surprisingly, this fiercely aggressive territorial species is able to breed in high densities or semi—colonies, i.e. nesting aggregations in which individuals are independently attracted by a particular environmental feature. It should be stressed, that the Great Crested Grebe never forms a true colony, this in contrast to true colonial species such as Black-necked Grebes, in which the nest site selection is highly social. In a number of large lakes, semi-colony formation of Great Crested Grebes may be explained by a shortage of nesting sites in relation to the extensive feeding areas. Here the birds are frequently forced to nest in semi-colonies. In such lakes the grebes often have to breed in inferior habitat and frequently lose their clutch or young. However, some semi-colonies of Great Crested Grebes are located in breeding waters with alternative nest-sites, and there the birds occupy relatively rare, high-quality habitat. Many of the semicolonies are small, but sometimes large ones are found, e.g. near Villeneuve-Les Grangettes in Lake Geneva, Switzerland (300–500 breeding pairs), and near the Mirnser Klif, Lake IJssel, The Netherlands (c. 500 bp). The nest density can be high: the average nearest neighbour distance in a semi-colony in Enkhuizen, The Netherlands, was 2.19 m, with a minimum distance of c. 0.2 m, i.e. the rims of the nests almost touched one another (n = 112 nests). 

Great Crested Grebes have a surplus of stable survival habitats (moulting and wintering areas) at their disposal, but their breeding habitat is fairly limited and unpredictable. So it is likely that they have long life expectancies and rather confined prospects for successful breeding. Selection generally leads in such situations to allocation of energy to survival adaptations (K-selection). The species belongs, among the grebes Podicipedidae, to the K-strategists. However, Great Crested Grebes show high flexibility in their breeding-strategies. The birds breeding in large lakes and those which undertake food-flights to these lakes or the sea, are adapted to forage in wetlands with extensive stretches of open water and live in relatively stable and predictable survival habitats during the entire year. The prospects for breeding are not predictable, especially because huge numbers of nests and clutches are, time after time, destroyed by high waves. The Great Crested Grebes in large lakes demonstrate a more pronounced K-breeding strategy than their conspecifics in smaller water-bodies, i.e. they show a rather small annual reproductive potential and contribute to the future gene pool by a long life with many breeding seasons. Great Crested Grebes breeding in small water bodies, i.e. a relatively unstable habitat, belong to a lesser extent to the K-strategists, and show some characteristics of r-strategists (higher reproductive potential, frequently multiple broods). It seems that the distance between breeding and wintering grounds also influences the breeding-strategy: locally wintering birds in The Netherlands tend to invest significantly more in breeding (early breeding, multiple broods, higher annual reproductive potential and lesser adult survival rate) than Dutch grebes wintering in Switzerland (arriving later on the breeding grounds, lower annual reproductive potential and higher adult survival rate).

Other species of grebes show many flexible and opportunistic behaviours which we also find in the Great Crested Grebe, such as a rapid occupation of newly formed wetlands, variable incubation times, changing clutch-sizes, asynchronous hatching and its effects (desertion of the nest before all the eggs have hatched, sibling competition), brood-division, fluctuations of the period in which the young become independent, nesting in semi-colonies and undertaking food-flights. However, many of those adaptations are found more frequently and often more pronounced and noticeable in the Great Crested Grebe. This is possibly caused by the fact that the species is the largest of the Podicipedidae in its distribution area and possesses superiority in strength so that it can outcompete the other species and has a wide choice of adaptive strategies, habitats and other resources. The other species of grebes in its distribution area are:

1. The Red-necked Grebe Podiceps grisegena is more limited in its habitat and food selection than the Great Crested Grebe. However, in parts of its distribution area, where Great Crested Grebes are lacking (northern Finland, northern Russia, eastern Siberia and North America), it shows adaptations which match the Great Crested Grebe in some anatomical details. These adaptations allow the Red-necked Grebe to occupy more or less the niche of the Great Crested Grebe. The geographical variation of the Red-necked Grebe might be interpreted as a character release in response to allopatry of Great Crested Grebes, or as character displacement in response to sympatry of Great Crested Grebes (see Fjeldså 1983a).

2. The Horned (Slavonian) Grebe Podiceps auritus has, compared with the Great Crested Grebe, a relatively narrow spectrum of habitat utilization. Interspecific aggression between Horned and Great Crested Grebes is rather rarely observed. It is possible that the Horned Grebe is sometimes driven away by the Great Crested Grebe. However, there are many records of Red-necked Grebes entering the territory of Horned Grebes, and attacking them with the result that the Horned Grebe in many districts is restricted to rather poor habitats.

3. The Black-necked Grebe Podiceps nigricollis is the most numerous of all grebes in the world. By far the largest populations are found in North America. These numbers are perhaps not comparable to those of Great Crested Grebes: Black-necked Grebes are much smaller birds than Great Crested Grebes and, in general, smaller animals have higher population numbers. Although the Black-necked Grebe is a successful species and shows flexible behaviour, it is not as opportunistic as the Great Crested Grebe, since it has a very specific habitat choice. Breeding Blacknecked Grebes rely to a great extent on isolated or newly formed wetlands without fish, or with small fish populations, and with large quantities of invertebrates. Outside the breeding season the species depends strongly on stable salt-lakes without fish and with high densities of brine shrimps Artemia, brine (alkali) flies Ephydra or other macro zooplankton. The large numbers of the Black-necked Grebe are due to their specializing in a life on those lakes. Because of the strong dependence on this habitat, the species may be vulnerable. In contrast to the Great Crested Grebe, the Black-necked Grebe is not a real generalist, but much more a specialist profiting from favourable circumstances.

4. The Little Grebe Tachybaptus ruficollis has an overlapping breeding distribution with the Great Crested Grebe in the Palaearctic and Africa. The Australasian Little Grebe T. novaehollandiae, a close relative of T. ruficollis, occupies areas in Australia and New Zealand, where Podiceps cristatus australis also lives. To understand behaviour, feeding and habitat selection in the Little Grebe, we have to consider its size and shape. The birds are not only much smaller in size than Great Crested Grebes but have also another shape: not long, slender and streamlined as the Great Crested Grebe, but rather broad, short and plump. Their body is adapted to the active pursuit of rather agile prey. The Little Grebe is not able to dive as deep as the larger grebes, so that large areas along the sea coasts and open areas in large lakes are unsuitable for its feeding. High waves in marine and other large water-bodies impede the capture of prey. As a result, outside the breeding season Little Grebes have much less habitat at their disposal than Great Crested Grebes. In the Palaearctic, the species may suffer high mortality by the ice covering of the freshwater wintering areas during severe winter weather since migrating to the sea is not a good solution. The small sized birds easily fall victim to predators and suffer from competition with the larger grebes. They tend to stay within one dive’s distance from cover of some kind (at the edges of submerged vegetation or reed marsh, among floating vegetation carpets, or near dead trunks in the water), where they can hide if there is imminent danger. They are shy and skulking and are often able to disappear completely in the reed-marsh of their breeding ponds, especially in the presence of other species of grebes. As the Little Grebe nests in unstable habitats, it has developed a large number of opportunistic behaviours. By its flexible behaviour, the species is able to nest in all types of water bodies with emergent vegetation and food, and to occupy a large breeding distribution area in the world (134,000,000 km², Bird-Life International 2017). It is difficult to assess if the Little Grebe is as opportunistic in its behaviour as the Great Crested Grebe as the former is a typical r-strategist and has, for example, larger clutches than the Great Crested Grebe and produces multiple broods more often. More than the Great Crested Grebe, the Little Grebe leads a nomadic life and rapidly colonizes newly formed wetlands ; it seems that even in the breeding season larger number of birds may move from one wetland to another.

5. The Hoary-headed Grebe Poliocephalus poliocephalus in Australia lives in areas which are also inhabited by Great Crested Grebes, but the two species nest in very different habitats. Podiceps cristatus australis prefers large, deep and open freshwater lakes, but the typical breeding habitat of the Hoary-headed Grebe is semi-permanent inland swamps, and the species is well adapted to the rather unpredictable conditions of Australian wetlands. Because of its nomadic behaviour it can take advantage of large concentrations of tiny invertebrates that sometimes abound in temporary inland waters, and in sewage ponds. In contrast to the Great Crested Grebe, the Hoary-headed Grebe only thrives in particular feeding and habitat circumstances.

Not really fitting with the view of the authors is the fact that outside its Palaearctic distribution range, the Great Crested Grebe is less successful. The birds in Africa, Australia and New Zealand have a rather scattered occurrence, and they are absent from many apparently suitable habitats. As the most related species of Great Crested Grebes, notably the Red-necked and Horned Grebes, have a circumpolar distribution, it is likely that the Great Crested Grebe was Holarctic in origin and spread to Africa, Australia and New Zealand. It might be postulated that the species has not yet had the opportunity to develop adaptations to the new environment. 

Dramatic declines by anthropogenic factors are reported from Kenya, Uganda (probably by the introductions of Tilapia and the increased use of monofilament fishing nets) and in New Zealand since the arrival of Europeans (probably by loss of habitat, increased disturbance from water-based recreation, introduction of trout Salmo, weasels Mustelidae, cats and rats). Such problems are also reported from the Palaearctic, but only have local significance. They have a large negative impact on populations in Africa, Australia and New Zealand, perhaps as the birds have not yet found adequate mechanisms to cope with these dangers.