Defence mechanisms
WITH the breeding season in full swing, a great deal of a bird’s time is spent protecting its nest and chicks from intruders. While most nests are well hidden and protected, any approaching predator is usually greeted by a response aimed at deterring or confusing it. Individual birds which are under threat may also use particular behaviour aimed at repulsing whatever threatens them. Some even have built-in means of keeping threats away.
Distraction display is very effectively used by a wide range of birds. Plovers are best known for luring away a predator by walking away with one wing outstretched and dragging as if it is broken. When the intruder is far enough away from the nest or young, the parent will simply fly off. Other birds which do this include some owls, such as Snowy Owl, and Eurasian Skylarks, which will flutter away with tail spread, one wing raised and the other lowered. Common Chiffchaffs will lure an intruder away by flying and hopping just in front of it, calling, and even Alpine Accentor will pretend to be injured. Another version is used by several waders, such as Purple Sandpiper, and also many North American warblers, in which the bird will crouch and run with its wings dangling down to appear like some kind of small mammal, and so entice the predator away.
Mobbing is another way to warn off a predator. Small birds will make a lot of noise around a roosting owl when discovered, which may result in the individual moving on. Some breeding waders may react to an intruder by giving loud calls and swooping down at it. Common Redshank and Northern Lapwing are good examples, and they may creep quietly away from a nest before flying up in this manner. Any birder who has approached breeding terns or skuas knows that they will dive-bomb anything which threatens them and will actually hit them with a beak or foot. Threats to adult birds are treated differently.
Many owls will react by puffing out their feathers, stretching out and inverting their wings to appear much bigger than they are. This may be accompanied by a loud hissing. Small owls, such as Eurasian Scops Owl, when threatened by a large intruder may do the opposite, compressing their feathers and making themselves thin to disguise themselves. They will even narrow their eyes, which are usually so prominent. Many seabirds have an interesting defence which involves ‘projectile vomiting’. Northern Fulmars are well known for this practice, coughing up a stream of oily, sticky and very smelly liquid when threatened at the nest, coating a predator up to 2 m away. An intruding gull may get this on its feathers, leading to a loss of waterproofing and subsequent chilling, which could prove fatal. Other tubenoses, such as albatrosses, also do this.
Chemical defences are unusual in birds but several species use them. Young European Rollers will vomit when threatened, covering themselves in acids from their grasshopper prey and making themselves distasteful to a predator. Hoopoes secrete an oil from their preen gland while they have eggs and young in the nest. This contains dimethyl sulphide, which smells awful and keeps intruders away. The adult ceases producing it once the young have fledged.
Some chemicals are so toxic that the bird itself can become poisonous. Common Quail is hunted in the Mediterranean, but if you catch and eat one in the autumn you might be poisoned as they contain the chemical coniine which may be present from eating hemlock seeds. Red Warbler of Mexico contains alkaloids which render it inedible, and Spur-winged Goose of Africa has a diet of toxic blister beetles which give its flesh a high level of the toxin cantharidin, making it extremely toxic to humans. The pitohuis of New Guinea contain a type of batrachotoxin from their food which even makes their feathers harmful if touched.