There are varying forms of aggression, but first I would like to point out the difference between displays of aggression and actual aggression. Both the displays and acts of aggression are encompassed by the term behaviour; however, displays tend to be signals that have evolved, and those displays that are maintained may actually communicate something, such as an impending act of aggression. This individual may or may not act upon receiving those signals. Further, these signals may be tools used by animals to assess the strength and "will" of potential competitors.

    For a number of years, this behaviour was hard to assess. As the image below suggests, the acting penguin (either a male or female, generally at the nest) tucks its head under one wing and raises the opposite wing in motion. The penguin may vary this behaviour by growling or by moving its head to the opposite side of the body.

    Although males and females perform the bill-to-axilla, it is given most often by males and it seems to indicate ownership of a general area. However, this behaviour does not seem to be directed towards particular individuals, rather groups, neighbors or passer-bys.

    There are two types of stares: the sideways stare and the alternate stare, the latter being the more intense and aggressive behaviour.

    Penguins have poor binocular vision, and some penguins have no binocular vision at all. For this reason, the sideways (and alternate) stare act as methods for focusing on the threatening object(s). These stares may be performed while lying down or while standing. In either case, the head of the staring penguin is placed in such a position that affords it ample time to strike with its bill. The flippers are kept close to the body but (as I have noticed in the Humboldts) may extend to either preserve balance or appear more threatening while leaning forward. These stares are given by territorial penguins.

    The sideways stare may also be given by females as they approach the nests of unpaired males who have been giving Ecstatic Displays (see Displays Between Sexes).

    The alternate stare serves the same purpose of the sideways stare but is separated from the former because it is employed during more intense situations. The focal penguin may perform an alternate stare if an intruder persists in the territory or if neighbors tend to move about frequently near the nest.

    The point and gape are closely related and often the gape is the logical progression from the point behaviour. Both these behaviours are more aggressive than the stares or bill-to-axilla.

    The point is a simple behaviour recognized by Penny (1968) which involves, as the name implies, the penguin's pointing (with bill) directly at the threatening object. Generally given in the pointer's territory, the object of the point may be intruding or moving very closely to the territory. Often the object of the Point is another species. The Point does not have to be given within a territory and may be employed to warn others when they approach too closely to the pointing individual.

    "The Gape" was originally seen as part of the Direct Stare that Penny had observed but is widely regarded now as separate, though related, behaviour. The gaping penguin often employs the same offensive (or defensive) posture as it does while pointing, opens its beak and emits a loud, growling sound. This gaping may culminate in what is known as Beaking. Gaping individuals may rock back and forth until they both lock beaks and begin a bout of tug-of-war.

    The Charge is seen as the most reliable indicator of attack, since it involves the most intention. As the name implies, the focal bird "charges" the threatening object, though it may not attack. Generally, the penguin leans forward, extending its wings and opening its bill, while it locomotes toward the other penguin. Penny (1968) noted that this movement may be silent or may involve general squawking.

    Penguins incubating eggs or guarding chicks will not, for obvious reasons, leave the nest to charge an approaching individual.

See a video of Macaroni penguins fighting (no audio).