Red-throated Diver E-Book

Jörg Hemmer

• Herausgeber: Books on Demand
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 63

Veröffentlichungsjahr: 2020

Beschreibung

The biologist Jörg Hemmer presents a comprehensive portrait of the Red-throated Diver, also known as Red-throated Loon (Gavia stellata), with sections on evolution, habitats, reproduction, diet, migration, population numbers, and conservation. Captivating photographs document the family life and the spectacular territorial behaviour of this charismatic traveller between fresh and saltwater.

Leseprobe

Contents

Preface

Divers

Phylogeny

Appearence and Moult

Distribution

Population and Threats

Locomotion

Breeding Biology

Diet

Behaviour and Vocalisation

Bibliography

PREFACE

Red-throated Divers, also known as Red-throated Loons in America, spend most of their life in the coastal waters of the northern Atlantic and Pacific Ocean. Who would take notice of these unpretentious seabirds if their appearance and behaviour would not undergo such a miraculous transformation when spring arrives? That is the time when they change their modest winter outfit for an elegant suit of black, white and grey, complemented by a rust-red waistcoat. Splendidly dressed, they leave the open sea to spend some summer weeks on one of the numerous ponds of the Nordic tundra and taiga where they nest and raise their young. What may seabirds motivate changing the freedom of the ocean with the constraint of a little lake somewhere at the edge of the Arctic? This surprising decision is the outcome of an evolutionary dilemma. Though Red-throated Divers may spend their whole lives on the sea, they still need a piece of land to breed their eggs. While seashores are bare and exposed to tides and waves, divers prefer nesting under the cover of shoreline vegetation of tiny inland freshwater bodies, which offer their offspring protection from predators and, at the same time, reduce the contact with the unloved land to the inevitable. But Red-throated Loons remain persistent seabirds, which continue to fish in the open sea to feed their young. The short period of breeding and rearing gives discreet observers the opportunity for participating in the life of these mysterious wanderers between salt and fresh water. Hours and days pass quickly when studying their territorial behaviour and courtship ceremonies. The rich behavioural repertoire of these magnificent birds, however, would remain a secret of the diver family if not their strident calls would point the way to their remote breeding pond. The German zoologist Alfred Brehm once described the vocal utterances of the Red-throated Diver as "an ocean chant which birds develop when listening to the storms and the roaring of waves". Could it be put in better words?

Professor Dr. Jörg Hemmer

Nersingen, June 2020

DIVERS

The diver or loon family (Gaviidae) includes a single genus only, Gavia, which represents five species: the Red-throated Diver (Gavia stellata), the Yellow-billed Diver (Gavia adamsii), the Common Loon or Great Northern Diver (Gavia immer), the Black-throated Diver (Gavia arctica), and the Pacific Diver (Gavia pacifica). Whether appearance, habitat, diet, breeding biology, behaviour or way of living, divers have a lot in common. Only the smallest of them, the Red-throated Diver, deviates a bit from the line. All divers breed on freshwater lakes in the far north of the American and Eurasian continent and spend the cold season along the coasts of the northern Atlantic or Pacific Ocean, sometimes even far away from land. However, while the four larger species use fish-rich inland lakes for raising their young, the Red-throated Divers prefer small and mostly fishless ponds, from where they continue to obtain the food for themselves and their young from the sea or, in some regions, also from larger neighbouring freshwater bodies [Johnsgard 2008, Solovyeva et al. 2017]. Despite a certain otherness of the Red-throated Diver, which also signifies a special phylogenetic position, the members of this species-poor bird family are nevertheless equipped with many matching characteristics.

During the breeding season, their distinctive black, white and silver-grey breeding plumage gives them an unmistakable appearance. The sexes are equally coloured in all five species. Apart from the similar Black-throated and Pacific Divers, the other species are well recognisable in breeding plumage (pages → and →). Distinguishing between the members of the Gaviidae family in winter and juvenile plumage is less easy.

Seen from a distance, the deep, cormorant-like swimming position is diagnostic for divers if compared with other seabirds. Divers fly fast and straight, with the neck and head held slightly below the body axis. With three to four wing beats per second, they reach a maximum velocity of up to one hundred kilometres per hour. Divers are unable to fly slowly and their manoeuvrability in the air is thus limited. Therefore, all species require a quite long water surface and a free airspace for take-off and landing.

All Gavia species are excellent divers, which are perfectly adapted for an aquatic lifestyle. A dense plumage insulates against the coldness of their northern environment. The backwards positioned legs and the webbed toes give their torpedo-like bodies the perfect propulsion an underwater hunter needs that mainly feed on fish.

As elegant as they dive and swim, divers appear extremely clumsy on land. Their element is the water, leaving it only for mating and breeding. As their anatomy is not made for walking, they build their nests, easily accessible from water, on vegetation islets or directly on the lake shore. The nesting sites must provide protection from terrestrial predators. Despite the apparently conspicuous colours and patterns of the feathering, the plumage of the incubating parents provides a reliable camouflage also against airborne enemies. In case of danger, however, they often prefer slipping carefully into the water and leave their inconspicuously dark eggs behind. Usually the clutch consists of two eggs, rarely there are one or three, which they place in a nest of hastily collected vegetation or directly onto the soil. Both parents share in the incubation duty, which takes about thirty days [McIntyre 1994]. As incubation starts with the first egg, the second chick hatches one or two days later. They leave the nest already after a few hours, whereby one partner already takes care of the firstborn while the other one continues breeding. The young must grow quickly and become fully fledged before the onset winter.

Divers live solitary during most of the winter and form pairs when on their breeding grounds. Observations suggest that both partners frequently return to the same nesting site. Since also the females contribute to the defence of their territory against intruders, there is a high probability that the young are genetically descendants of both partners [Piper et al. 1997]. All divers have a complex courtship and territorial behaviour in common, whereby it is hardly recognisable on which occasion and to what extent the striking performances and plaintive utterances are displayed to strengthen the couple's bond or to mark territorial claims [Johnsgard 2008].

The breeding areas of the divers extend over the entire north of Eurasia and America, with overlapping (sympatric) occurrences of two to four species in most regions. Reproduction in the tundra and northern forest zone means that their breeding waters usually freeze in winter. That implies that the entire reproduction process must be completed in a narrow timeframe and that the birds need to migrate to open waters before lakes are frozen. They find their wintering grounds along the northern Atlantic and Pacific coasts, in estuaries, lagoons or, rarely, on large inland lakes of the temperate climate zone, where they feed on fish and crustaceans. Moulting takes place at sea where a water bird is less reliant on its flight capability. Adult divers spend most of the year on the ocean. Juveniles even stay in their marine habitat for the first three to four years of their life, until sexual maturity.

Unlike other seabirds, the Gaviidae