Why is female bird referred to as heterogametic?

Heterogametic organisms:

1. Heterogametic organisms have different sex chromosomes, hence producing different sex gametes.
2. The female bird is known as heterogametic as it produces two different types of gametes.
3. Heterogametic female birds have one Z and one W chromosome, whereas males have two Z chromosomes.
4. Another example of heterogametic organisms is humans, males are heterogametic (XY) but females are homogametic (XX).