Source

File: wp-includes/link-template.php

function get_author_feed_link( $author_id, $feed = '' ) {
	$author_id           = (int) $author_id;
	$permalink_structure = get_option( 'permalink_structure' );

	if ( empty( $feed ) ) {
		$feed = get_default_feed();
	}

	if ( ! $permalink_structure ) {
		$link = home_url( "?feed=$feed&author=" . $author_id );
	} else {
		$link = get_author_posts_url( $author_id );
		if ( get_default_feed() == $feed ) {
			$feed_link = 'feed';
		} else {
			$feed_link = "feed/$feed";
		}

		$link = trailingslashit( $link ) . user_trailingslashit( $feed_link, 'feed' );
	}

	/**
	 * Filters the feed link for a given author.
	 *
	 * @since 1.5.1
	 *
	 * @param string $link The author feed link.
	 * @param string $feed Feed type. Possible values include 'rss2', 'atom'.
	 */
	$link = apply_filters( 'author_feed_link', $link, $feed );

	return $link;
}