O(n) inplace solution:

static int migratoryBirds(int[] ar) {
    for(int i=0;i<ar.length;i++)
       ar[(ar[i]%10)-1] = ar[(ar[i]%10)-1]+10;
    int max = -1,q=0;
    for(int i=0;i<ar.length;i++){
        if(max<ar[i]/10){
            max = ar[i]/10;
            q=i+1;
        }
    }
   return q;
}