According to https://www.rspb.org.uk/birds-and-wildlife/wildlife-guides/bird-a-z/bar-tailed-godwit/ https://www.rspb.org.uk/birds-and-wildlife/wildlife-guides/b

According to https://www.rspb.org.uk/birds-and-wildlife/wildlife-guides/bird-a-z/bar-tailed-godwit/ https://www.rspb.org.uk/birds-and-wildlife/wildlife-guides/b..., the bar-tailed godwit weighs around 230-450g. So the tracker would add somewhere around 1-2% of its weight.