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Eggpouchers
No bird has ever evolved ovoviviparity or viviparity, probably not because of evolutionary constraints, but simply because of no significant selective pressure. Evolution is not intelligent: millions of years can pass before a potential new trait develop, even the most trivial. Mammaliamorphs for example appeared in the Triassic but didn't evolve a placenta until the middle Jurassic, 60 million years after.
No holocenic bird is capable of retaining eggs, but there are exceptions: egg-binding is a relatively diffuse condition in all bird groups and at least two genera of cuckoos are naturally able to retain eggs for 25 hours. One of these two lineages, the Clamator genus, has given rise during the Lentocene to a now cosmopolitan group of fully ovoviviparous birds: the eggpouchers (sometimes called endopulcids) (clade: Endopulcidae).
Eggpouchers' eggs are retained inside a special "pouch" of the oviduct and they are laid shortly before the hatch; special hormones warn the mother's body when chicks are fully-formed and must be expulsed from the oviduct pouch. Chicks use their large egg tooth to break the shell, sometimes with the help of their parents.
We have no information on how this group has evolved: one hypothesis is that their capacity of retaining eggs was selected to reduce food competition with other birds during the breeding season. Eggpouchers in fact can raise prematurely their chicks in cold or monsoonal environments, when the good season is not yet started and food supplies are low, since they don't need to spend energy on creating a nest and protecting eggs from cold, heat and predators.
Egg deposition (and subsequent hatch) happen in protected shelters (like hollow trees or cliffs) in sync with the start of the good season; chicks are usually precocial and can already take advantage of the abundant food, while other bird species have just started to prepare their nests.
Eggpouchers are generalist omnivorous and can feed on anything, from invertebrates and plant material to eggs and small vertebrates, which is a further reason for their global success.
Eggpouchers arrived in Antarctica about 8 million years ago using islands as stepping stone corridors. Some species are even migratory, reaching Antarctica during early spring/late winter.
The overall cold climate of Antarctica favored the expansion of this clade, which is physiologically adapted to frost environments.
They're all medium-large birds weighing between 500 to 1500 grams (1.1 to 3.3 lbs) and mainly search for food on the ground like a gamefowl.
Lots of species tend to become terrestrial, developing long legs to run. An example is the mountain eggpoucher (Endobiovum inexpectatus) a large endopulcids which prefers to run from predators instead of taking flight.
The species can be found in the Central Massif mountain ecoregion: during summer they can be found in the upper tundra, while in the winter period they move at lower altitudes.
They are vicious predators of large snails in the warm season, which are hit with large rocks to break their shells.
Mountain eggpouchers live in reproductive pairs that annually congregate during winter and form large social flocks to better detect food.
Thanks to their relatively large size and swiftness, these eggpouchers possess few predators: only incubos and hawktules can pose a threat to these birds. If abundant, eggpouchers can occasionally fall prey to some wotters and small sheardogs.
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