In the photo - a black and white fan-tail (Rhipidura leucophrys) attacks the Australian crow (Corvus coronoides). Most likely, a fan-tailed nest is nearby, and crows are not averse to feasting on eggs or chicks.
Fantail - very territorial and brave creatures and can drive away not only a crow, but even a cat or a dog. But the fan-tails will attack the hawk in a flock. Because in birds, the protection of offspring is a dynamic process, and not established once and for all. The birds choose the strategy of protection and its intensity for each specific case, and this choice depends on the ratio of risks for the offspring and for the parents.
If the threats to the parents are too serious, birds will not risk themselves protecting the brood, regardless of the level of danger to the clutch or chicks. In other cases, the type of protection depends on the capabilities of all participants involved in the conflict. Birds take many factors into account to determine the optimal protection strategy or to decide not to protect the offspring.
The cuckoo intended to lay her eggs in the robin's nest (Erithacus rubecula). She herself does not pose a mortal danger to the robin, but, after hatching, she will throw her half-brothers and sisters out of the nest. Having weighed the risks for itself and future offspring, the robin decides to drive away the parasite (why birds cannot always distinguish other people's eggs from cuckoo eggs - see the Kukushka eggs problem). Photo from the site dailymail.co.uk
To protect offspring, birds can use direct attacks and / or vocalizations (for example, alarms), as well as various distractions and masking techniques. Feathered parents also attack wisely. For example, a fan-tail flies up to the enemy from behind, avoiding meeting with a powerful beak, toothy mouth or sharp claws.
The screaming plover pretends to be wounded in order to divert the predator from the nest. This is one of the many strategies for protecting offspring in birds.
If direct attacks are too dangerous, then birds resort to vocalization. With the help of alarms, they warn chicks, a partner and neighbors of danger, revealing the location of the enemy and his identity. This can cause the predator to leave (or fly away) to hunt elsewhere. Some birds are larger than fan-tails, such as the wandering thrush (Turdus migratorius), can voice warn a predator of their intention to attack.
Whitethroat fledgling (Sylvia communis). At the command of one of the parents, he stopped squeaking, hid in dense vegetation and waited for the "enemy" (that is, me) to leave for a safe distance. Photo © Alena Shurpitskaya, Ryazan region, June 14, 2019
Vocalization, like fly-overs, can be a distraction (to take the predator away from the nest) or a trick. When an enemy appears, small birds silently fly away from the nest so as not to reveal its location. In wandering thrushes, on the other hand, one can observe the "Geiger counter effect": when the crow approaches, the signals of the parents become more frequent. More crows flock to bird trills, which, in turn, attract the attention of the hawk. And now the crows have to choose the strategy and the intensity of the defense.
It is not uncommon for others to join the alarms of one bird, and not necessarily birds of the same species. Polyphony can be accompanied by flights to the predator and even direct attacks. This behavior is called mobbing (from English to mob - "to attack in a crowd", see Mobbing) and is found not only in birds, but also in other animals. A vivid example: monkeys, jumping on trees, scream and throw branches at the lion. As for birds, you can often see a flock of birds chasing a cat or a crow, and the crows themselves can chase an owl or a hawk. In this case, predators can get serious injuries and even die. With the help of mobbing, birds not only confront enemies, but also train young birds. And males can once again demonstrate their masculinity and win or retain the lady's favor.
A flock of starlings chases Cooper's hawk (Accipiter cooperii). At the same time, the birds follow a certain construction pattern. Photo from lookoutnow.com
A vivid example of how birds choose a protection strategy depending on the ratio of risks for themselves and for their offspring is the behavior of blue titmice (Cyanistes caeruleus) and field thrush (Turdus pilaris).
Fieldbirds more often settle in colonies and when corvids appear, they throw their feces on them. The droppings can stick together the plumage so that birds lose their ability to fly. However, it makes no sense to "fire" the ermine and weasel from the air. Therefore, in those years when there are few rodents in the forest and martens in large numbers destroy birds' nests, blackbirds nest farther from each other so as not to jeopardize all the eggs of the colony at once.
Raven sling (Corvus corax), attacked by a "fecal line" of field thrush. You can see how the feathers from the droppings have glued together. Photo © Julia Mikhnevich, Bitsevsky forest, Moscow, May 10, 2017
How desperately parents will attack a predator and use alarms (that is, how intense the defense will be) depends on the value of the offspring, the condition of the parents and the danger of the predator.
Chicks are protected more actively than eggs: chicks are more likely to survive to reproductive age. The same applies to the first offspring of the season: for feathered parents, it is more valuable than the subsequent ones. Obviously, this is due to the fact that early chicks have more food and time to grow under optimal conditions. In some cases, the number of offspring can also play a role, especially if the species is characterized by a small number of clutches. Also, the visibility and accessibility of the nest affect the intensity of protection. The more vulnerable the offspring, the more the parents usually protect them.
The intensity of protection is often influenced by the gender and physical condition of the parents. Females may be weakened after laying and are unable to defend the nest as actively as males. The same applies to those species where only the female incubates eggs, since her death will lead to the death of the offspring. If sexual dimorphism is expressed, then this may affect the choice of strategy. For example, smaller and more agile male snowy owls (Bubo scandiacus) are more likely to attack Arctic foxes that steal eggs, while less agile females are more likely to use distracting tactics.
Indian ringed parrots, or Kramer's necklace parrots (Psittacula krameri), banish the Bengal monitor lizard (Varanus bengalensis), who wanted to feast on eggs laid in the hollow of a tree. Photo © Rathika Ramasamy from conservationindia.org, Keoladeo National Park, Bharatpur, India
When choosing a defense strategy and its intensity, the experience of interacting with various predators is also very important. Birds often do not have innate recognition, so they learn to recognize and respond to enemies from older relatives or from their own experience. However, common predator traits, such as the combination of beak shape and eye color, or body color and size, help them identify a threat in an unfamiliar object. Despite these clues, young birds sometimes cannot distinguish between a harmless partridge and a sparrowhawk.