Here's why:

* Complexity: Birds are complex creatures with specialized organs, tissues, and systems like a digestive system, nervous system, respiratory system, and more. These systems are made up of many different types of cells working together.

* Size: Birds are macroscopic, meaning they are visible to the naked eye and are significantly larger than a single cell.

* Cellular Organization: Like all animals, birds are composed of many different types of cells organized into tissues, organs, and organ systems.

Unicellular organisms, like bacteria and amoebas, are single-celled organisms that carry out all life functions within that single cell.