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The presence of amniotic eggs, a four-chambered heart, and lightweight, honeycombed bones strongly suggests that the species in question belongs to the group of birds, or specifically, the clade that includes birds (Archosauria).

Amniotic eggs indicate that the species has adaptations for reproduction on land, reducing the reliance on water for the development of offspring. A four-chambered heart is characteristic of birds (and mammals), allowing for efficient circulation and oxygen delivery, which is vital for flight. Lightweight, honeycombed bones are a hallmark of avian anatomy, providing the necessary strength for structural support while minimizing weight, which is crucial for flight.

Feathers for flying (the additional characteristic in question) align perfectly with the characteristics described. Birds possess feathers, which are essential for flight, thermoregulation, and sometimes display. This is consistent with the evolutionary adaptations that allow birds to thrive in various environments.

The other options are less relevant; mammary glands are specific to mammals, external fertilization is more characteristic of fish and amphibians, and ectothermy generally pertains to reptiles and fish, making them unsuitable with the provided traits. Thus, feathers for flying is logically the most fitting additional characteristic based on