Rumbling on the run
The alphabetabrumble

While terrariders are undoubtedly the rostrids most adapted to running, we can't overlook other efficient runners such as brumbles. These birds typically have a graviportal structure and are far from being built for speed, but they possess proportionally long legs that enable them to take long strides. Brumbles also have tremendous stamina, making them the most adapted birds for endurance pursuit. Besides the almost trample steppe-exclusive webrumble and the coldvanna-exclusive stiltbrumble, the brumble group actually includes a third and final species found in both biomes, although it is more common in coldvannas: the alphabetabrumble (Glacirostris unicum).

This moderately large brumble is the most abundant species of rostrid during interglacials and inhabits a wide range of woody ecosystems, from the dense thickets of the northern tip of Antarctica to the sparse shrublands in the hills of the northernmost trample steppe.
The great adaptability of this bird is due to its dietary plasticity: unlike its ancestors, this brumble is not exclusively a browser but can significantly feed on low-abrasive herbaceous plants, encompassing a large portion of forbs (yet not graminoids). This makes this bird well capable of dwelling in shrubby trample steppe, but it largely disappears when erect shrubs become infrequent due to the strong grazing impact of other megaherbivores.

This bird exhibits marked sexual dimorphism, a unique characteristic compared to contemporary rostrids. While females mostly have a brown and dotted coat with yellow eyes, males are grayer on their back and have a white supercilium. Both sexes display a dark line running across their entire cheek, giving the impression that the animal has a larger mouth than it actually does, like a clown. This line, present regardless of sex, could hold communicative significance, perhaps for recognizing other individuals of its species or social group.

These birds do not live in breeding pairs like most rostrids but live in well-separated groups depending on age and sex. Females and young usually stay together and form herds of up to 40 individuals, while adult males form small herds of five or fewer individuals that separate during the breeding season, when dominant males become territorial. The rearing of the young is solely in the hands of the female, who often shares her eggs with other non-reproductive females. This behavior compensates for the absence of the male in caring for the young and enhances the parental bond among individuals of the same herd.

Alphabetabrumbles derive their name from their large size variation depending on food quality. Coldvanna populations (Alpha ecotype) are usually larger due to the abundant foliage availability, but high-density populations and those living in the trample steppe (Beta ecotype) can rapidly reduce their size to just 60 kg (less than half compared to the largest individuals found in the coldvanna).
The small size of Beta ecotypes is also caused by a small yet constant genetic outbreeding with another species of rostrid, the webrumble. Despite several adaptations to avoid hybridization, the two species still infrequently breed in a large habitat overlap, in the western coast of the Antarctic Peninsula. The most common hybridization is the F. palta ♂ x G. unicum ♀, which gives birth to the webeta-B. The male hybrid is usually fertile and can breed with both species, while females are completely infertile, in accordance with Haldane's rule*.

*female birds, unlike mammals, are heterogametics (ZW), while males are omogametics (ZZ)