During Chapter 1, you will need to discover who attacked Aipom in Tahnti Park in order to Find the Necklace that Aipom originally stole from the Little Girl. By examining the assaulted Aipom, you will find a Black Feather on it. By speaking with the one Park Caretaker, you will then learn that a number of different Flying Type Pokemon can be found in the park, including Pidove, Taillow, Starly and Murkrow.
In order to Find out which of these birds was the one who attacked Aipom, you will need to go through the various areas of the Park and obtain Eyewitness Testimony, including from all of the birds themselves. In the Cafe Area, you will eventually find a Taillow Feather after speaking with everyone there, quickly eliminating Taillow as the culprit. After obtaining bird food in the Cafe Area and throwing it out for the Pidove in the Fountain Area, you will earn yourself a Pidove Feather and will then be able to eliminate it as a suspect as well.
From there, you will have to head to the Nature/Lake Area of the Park in order to Find both Starly and Murkrow. Starly will gladly give you one of its feather, which will then eliminate it of suspicion as well, leaving you with Murkrow as the culprit. After discovering where Murkrow can be found, you will need to interrogate it. Even though Murkrow will admit to attacking Aipom, you will learn that it actually doesn't have the Necklace you are searching for.