Shortcode to output get_header

I’m trying to create a shortcode that will output the contents of one of my template header files.
I thought I just needed to create it in my functions.php file

function zoo_header(){
	return 'test'.get_header('navbar');
}
add_shortcode('zoo-header', 'zoo_header');

And use it in my templates: [zoo-header] or do_shortcode(‘[zoo-header’])

However, the output of the shortcode is simply ‘test’
Why is the content of my header not showing up?

header-navbar.php

<?php wp_nav_menu( array(
	'theme_location' => 'right-menu',
	'container' => false,
	'menu_id' => 'navbar-right',
	'menu_class' => 'nav navbar-nav navbar-right'
)); ?>
<p>testing 123</p>

It’s as if get_header is not returning anything.
I have 2 headers that I need to show, and I wanted to give the template authors the option to choose between one or the other.