filter-by-birthday.php

<?php 
$current_date = date( 'md' ); // first load the current date into a variable, formatted as md

$employee_birthdays = new WP_Query(
    array(
        'posts_per_page' => -1,
        'post_type'      => 'member', // employee
        'meta_query'     => array(
            array(
                'key'     => 'birthday', // assumes the date picker field is called birthday. Stored in Ymd format.
                'value'   => '\d\d\d\d'.$current_date,
                'compare' => 'REGEXP',
            ),
        ),
    )
);
while ( $employee_birthdays->have_posts() ) {
    $employee_birthdays->the_post();
    echo '<li>' . get_the_title() . ' : '.get_field('birthday', get_the_ID()).'</li>';
}