Others can guide you better but from my knowledge
The bird was sparrow carrying stones in their beak.
Well prehistoric records show there were but that thousands of years ago since the events you are asking preislamic Arabia..
They were there to invade mecca as a part of conquests by abreha
Abreha was their owner who came with an army to take control of mecca.. Basically spreading and consolidating his rule to the only area that hadn't been conquered by Persia and Byzantines.. He was a general or king of the kingdom of aksumite a kingdom formed around the horn of Africa which spanned from there to Yemen and parts of Qatar and parts of modern day Saudi Arabia... It is considered the fourth most strongest empire preislamic with others being China, byzantine, Sassanid...
Its really not weird to imagine an elephant army in Arabia.. Sassanid had elephant corp and could being their armies to modern day Iraq and syria with ease due to the proximity of their supply lines...
The mud clay stone could be brimstones but I heard stones when I was young...
Anyhow the others can definitely expand our knowledge..