According to the Daily Mail:
After saying a prayer for Ukraine, where at least three people were killed during the latest clashes that have seen Kiev in flames, the Pontiff helped schoolchildren release the doves from the window of the Apostolic Palace as a peace gesture.
But they were immediately attacked. The seagull pinned one of the doves against the wall of the palace and pecked at its tail, but was left with a mouthful of feathers as its victim made an escape.
The crow caught the smaller bird unawares, as it rested on a windowsill, before pecking at it repeatedly. Again, the victim managed to get away.
Eventually both doves flew off, seemingly uninjured, to the relief of tens of thousands of onlookers.
The release of the birds concludes the Vatican’s annual Caravan of Peace event.
Apparently, there have been problems with this symbolic gesture in the past:
Last year one of the doves was set upon by a gull, while in 2012 they simply flew back into the Apostolic Apartments.