Ratites in Animal Collections: Ostrich, Emu, Rhea, Cassowary

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Cassaworry preening his feathers, note the fabulous head of colours

Ratites are the birds most visitors recognise before they can name them. A tall neck above a fence, a heavy body on long legs, and no obvious sign that the bird has the slightest interest in flying. In animal collections, the group usually means four: the ostrich, the emu [Dromaius novaehollandiae], the rhea [Rhea americana] and the cassowary. Each has its own management quirks, its own record-keeping demands and its own way of making a keeper's day more interesting.

This article looks at what these birds share, where they differ, and what that means for zoos, wildlife parks and open farms that hold them.

What Counts as a Ratite

Ratites are large flightless birds. The defining structural feature is a flat breast bone with no keel. The group is generally described as including ostriches [Struthio camelus], emus [Dromaius novaehollandiae], rheas [Rhea americana] and cassowaries [Casuarius casuarius], along with kiwis [Apteryx spp.] and several extinct species. The fossil record stretches from the Paleocene through to the Holocene, roughly 56 million years ago to the present day.

Ratites are also described as a polyphyletic group, and that single word explains a great deal of the confusion surrounding them. A polyphyletic group is one held together by shared appearance and lifestyle rather than by one common ancestor that gave rise to all of them and nothing else. DNA evidence has been rearranging the branches of the avian family tree, and ratites sit right in the middle of that reshuffling.

The Four Ratites Most Often Kept in Collections

Ostrich, emu [Dromaius novaehollandiae], rhea [Rhea americana] and cassowary are the four that come up most often in collection planning. They are not interchangeable, and the table below sets out where each one sits within the group.

Bird	Position in the group	Notes from the literature
Ostrich	Largest living ratite; assigned to its own order	One of the two African ostriches [Struthio camelus]
Emu [Dromaius novaehollandiae]	Second-largest living ratite; shares an order with the cassowary	Australia's emu [Dromaius novaehollandiae]; paired ceca vestigial
Rhea [Rhea americana]	Third-largest living ratite; assigned to its own order	The greater rhea [Rhea americana] is South American
Cassowary	Shares an order with the emu [Dromaius novaehollandiae]	Paired ceca vestigial and of minor functional importance

Ostrich

The ostrich is the largest living ratite. The two African ostriches [Struthio camelus] are only distantly related to the rhea [Rhea americana] and to Australia's emu [Dromaius novaehollandiae], despite the obvious family resemblance. In current classifications the ostrich is assigned to an order of its own, which tells you how far apart it sits from the others in taxonomic terms. In a collection, the ostrich's scale drives everything: boundary specification, viewing distance and the amount of space a keep team can manage safely.

A majestic emu [Dromaius novaehollandiae] stands tall in the grass, showcasing its handsome plumage. Often called "boomers" for their distinctive calls, these impressive birds add charm to the surroundings.

Emu [Dromaius novaehollandiae]

Australia's emu [Dromaius novaeA curious rhea strolls across a grassy enclosure, showcasing its unique plumage and representing the intriguing ratite family from South America.d-largest living ratite, and it shares an order with the cassowary. That shared placement is one reason the two are so often discussed together in veterinary and husbandry literature. Emus [Dromaius novaehollandiae] are polyandrous, and the males incubate the eggs and rear the young. That reversal of the usual pattern matters when you plan breeding groups, interpret behaviour for visitors, or write signage that genuinely explains what people are looking at.

Rhea [Rhea americana]

The greater rhea [Rhea americana] is South American and is the third-largest living ratite. Rheas [Rhea americana] are assigned to their own order in current classifications and are distantly related to the two African ostriches [Struthio camelus] and to the emu [Dromaius novaehollandiae]. In collection terms, rheas [Rhea americana] are often the ratite that suits a mixed layout, but the same record-keeping, veterinary and welfare considerations apply as for their larger relatives.

Cassaworry: A Handsome Bird from AustralasiaThe Cassaworry, known for its striking appearance, is a remarkable bird native to Australasia. One of its most distinctive features is its beautiful sky blue head, which adds to its overall charm and elegance.CharacteristicsColoration: The sky blue head is a standout feature, complemented by other vibrant colors in its plumage.Size: Cassaworries are typically medium to large-sized birds, making them quite noticeable in their natural habitat.Habitat: They are commonly found in forests and woodlands across Australasia.BehaviorDiet: Cassaworries are omnivorous, feeding on a variety of fruits, seeds, and insects.Social Structure: They often live in small groups, showcasing social behaviors that are fascinating to observe.Conservation StatusEfforts are being made to protect the habitats of Cassaworries, as they play an important role in their ecosystems. Conservation initiatives aim to ensure that future generations can enjoy the beauty of these handsome birds.

Cassowary

The cassowary shares an order with the emu [Dromaius novaehollandiae] and, like the emu [Dromaius novaehollandiae], it is polyandrous, with males incubating eggs and rearing young. One detail that stands out in the veterinary literature concerns the paired ceca, which are vestigial in emus [Dromaius novaehollandiae] and cassowaries [Casuarius casuarius] and of minor functional importance, particularly in the cassowary. That is a meaningful point for anyone designing a feeding plan or interpreting digestive health in these birds.

Size, Order and Why the Ranking Matters

Within the group the size order is consistent in the sources: ostrich first, emu [Dromaius novaehollandiae] second, rhea [Rhea americana] third. Cassowaries [Casuarius casuarius] are not placed within that particular ranking. Size drives practical decisions in a collection, including how much space a pair requires, how tall a boundary needs to be, how far back a visitor path should sit, and how many staff are needed for routine tasks. It also shapes transport planning and the equipment a team keeps on hand.

Size is not the only variable. Temperament, social structure and breeding behaviour differ between species, and the ranking above says nothing about which bird is easiest to manage. It is a starting point for space and risk planning, not a verdict.

Taxonomy That Refuses to Sit Still

Current classifications assign the ostrich to its own order. The cassowary and the emu [Dromaius novaehollandiae] sit in the same order. The kiwi [Apteryx spp.] and the rhea [Rhea americana] each have their own order. The ratites as a whole do not form a tidy single branch, and DNA evidence has been rearranging the avian family tree for years.

Why does that matter in a collection office? Because scientific names, order-level groupings and common names all feed into records, signage and licence paperwork. If your records system uses a naming convention that no longer matches current thinking, you end up with inconsistencies between what keepers call the birds, what the vet notes say and what the public reads on the sign. Agreeing one authoritative source for nomenclature and sticking to it saves a lot of tidying up later.

Breeding, Incubation and Rearing

Cassowaries [Casuarius casuarius] and emu [Dromaius novaehollandiae] are polyandrous, with males incubating eggs and rearing the young. That is unusual enough among birds to be worth explaining properly to visitors, and it has real consequences for how a collection manages breeding groups.

If the male sits on the eggs and raises the chicks, then his condition, diet and stress levels are the limiting factors on a successful season. Keepers need to identify individuals reliably, watch male behaviour closely, and record what they observe in enough detail that patterns emerge over time. Public interpretation benefits too: a sign explaining male incubation turns an apparently idle bird into something visitors will actually stop and watch.

Digestion and Feeding Considerations

The paired ceca in emus [Dromaius novaehollandiae] and cassowaries [Casuarius casuarius] are vestigial and of minor functional importance, particularly in the cassowary. Veterinary reviews of the group cover gastrointestinal passage times alongside other aspects of ratite anatomy and physiology. The practical takeaway is that these birds do not follow the digestive model many keepers are used to from other large herbivorous birds.

That makes professional veterinary involvement in diet design essential rather than optional. Any change to a feeding regime should be agreed with the attending vet, introduced gradually and monitored with written records. If you are unsure how a proposed diet fits with the anatomy described in the literature, ask before you change anything.

Practical Considerations for Collections

• Space and boundaries: size drives footprint, viewing distance and barrier specification.

• Social structure: polyandrous breeding in emus [Dromaius novaehollandiae] and cassowaries [Casuarius casuarius] affects group composition and timing.

• Records: keep species-level data consistent with one agreed nomenclature source.

• Veterinary input: diet, digestion and breeding condition all warrant professional oversight.

• Signage: behaviour such as male incubation is worth explaining, and accurate signs reduce visitor misunderstanding.

• Compliance: licensing, movement and welfare requirements change, so confirm current requirements with the relevant authority before acting.

Records, Licensing and Compliance

Collections holding ratites sit inside a regulatory framework covering licensing, welfare, movement and record-keeping. The detail differs depending on where you are, what kind of collection you run and which species you hold. Because those requirements are reviewed and updated periodically, the safest approach is to verify the current position with the relevant authority and with your veterinary adviser rather than working from an old template.

Good records pay for themselves. They support inspections, they make veterinary decisions easier, they underpin breeding planning, and they give you the evidence you need when something unexpected happens. Whether your system is software or paper, the aim is the same: one place where the history of each bird can be found quickly.

Frequently Asked Questions

What is the largest living ratite?

The ostrich is the largest living ratite, with the emu [Dromaius novaehollandiae] second and the greater rhea [Rhea americana] third. Those three also happen to be the three largest living ratites according to the sources used here. If you need exact figures for records or public signage, confirm them with a current taxonomic or zoo reference rather than relying on a general summary.

Are cassowaries [Casuarius casuarius] and emus [Dromaius novaehollandiae] related?

Yes, closely enough to be placed in the same order in current classifications, whereas the ostrich, the rhea [Rhea americana] and the kiwi [Apteryx spp.] are each assigned to their own orders. Ratites as a whole are described as a polyphyletic group, which means the shared flightless body plan does not map neatly onto a single branch of the family tree.

Why are ratites flightless?

Ratites are large flightless birds with a flat breast bone and no keel. The group includes ostriches [Struthio camelus], emus [Dromaius novaehollandiae], rheas [Rhea americana], cassowaries [Casuarius casuarius] and kiwis [Apteryx spp.], along with several extinct species, and it spans a very long fossil record running from the Paleocene through to the Holocene. The absence of a keel is the structural feature most often cited when describing them.

Can a cassowary breed with an emu [Dromaius novaehollandiae]?

This is a question that comes up regularly, and the sources consulted for this article do not resolve it. Hybridisation questions of this kind are best directed to a specialist avian veterinarian, a zoo breeding programme coordinator or a current taxonomic reference. Do not rely on forum answers or general summaries when planning breeding groups or making records entries.