Using streams -

static int migratoryBirds(List<Integer> arr) {

    return arr.stream().collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
                                    .entrySet()
                                    .stream()
                                    .sorted(Map.Entry.<Integer, Long>comparingByValue().reversed())
                                    .findFirst()
                                    .get()
                                    .getKey();
}