You can use the Hearts of Ice in Treasure Hunter to freeze certain award categories from being in the chest. This will increase the chances of you receiving a certain item in one of the unfrozen categories. You can freeze up to 10 categories at a time, but you need to have a lot of Hearts of Ice to do so. Each time you open a chest, you will lose one Heart of Ice for each category you have frozen (a maximum of 10 per key use). If you receive a reward that you've elected to freeze with the Hearts, it will be rejected, and another reward will be granted. Note that you lose Hearts on every key use, regardless if your reward is a frozen category item or not.
Silverhawk Feathers are considered an Agility item according to the rewards page for Treasure Hunter, so you will want to keep that category unfrozen. Knowing this, you can freeze whichever categories you'd like to help increase your odds of getting Silverhawk Feathers.
You can earn more Hearts of Ice by obtaining more keys:
Whenever a key is obtained, a Heart of Ice will also be obtained as a bonus. Additionally, whenever 5 keys are used in a single day, 5 extra hearts of ice will be given.
An easy way to get keys is by completing the Daily Challenges. You will be granted one key for each challenge completed. They are often easy tasks that scale to your skill levels, with some taking a little bit longer than others. You can have up to 5 Daily challenges at a time, and a new challenge is given each time the game resets at 0:00 UTC. The new challenge will replace the oldest one. Another way to get keys is simply play the game. You can be awarded them randomly through skilling and as drops from monsters. This way is obviously much slower, and also random since there is no guarantee you will get a key. Completing quests also awards you keys.