public static int migratoryBirds(List<Integer> arr) {
    return arr.stream()
        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
        .entrySet().stream()
    .sorted((a, b) -> (int) (b.getValue() - a.getValue()))
    .mapToInt(es -> es.getKey())
    .findFirst()
    .getAsInt();
}