Do I have to know about muscles and their placements while learning to draw realistic birds?
Knowing the anatomy of your subject will make you a better artist. Here's why...

I can think of four reasons why an artist studies the anatomy of what they intend render into a work of art:

1) Structure
The human eye is very sensitive at identifying shapes and proportions. Get the structure wrong and the eye "will" see it. Knowing how something is put together will make it easier for you to reproduce it during the drawing process. You will also be able to critique your own work and know if the image is working or not.

2) Invention
The artist may not have access to a reference photo of the exact pose they are looking to draw. In this case the artist will need to invent the pose by some means. Look up the stick figures of Terryl Whitlatch, or the maquettes of James Gurney. The stick figure and maquette are both tools available to the artist to help them draw better and be more inventive with the pose of their subject. It takes a solid understanding of anatomy to create a useable tools such as these artists use.

3)Further Learning
When you know the anatomy of the subject it will be easier for you to communicate with others when discussing the subject. Being able to discuss the subject with others will make things clearer in your own mind. As new information comes up you will be better able to catalogue and integrate it with what you already know. Look up Richard Feynman's technique of how to learn anything. Feynman's approach to learning technique works for all subjects, even those subjects important to the artist. 

4) Spiritual Empathy
... this gets into some artsy spiritual stuff ...
Learning the anatomy of your subject brings you one step closer in developing your connection to what it is you want to draw. Artists look to put their energy into their works and combine it with the energy of their subject. To be able to detect that energy you need to develop a close relationship with your subject. It through this empathetic connection of understanding that you will be able to connect on a soulful level with the energy and spirit of your subject. Better understanding of your subject (anatomy, physiology, philosophy, behaviour, spirit) will make you a better artist of the subject.