Weapon Power - Going WP is all about critting and shredding down the enemy with your Glimmershot as the ability can apply on-hit effects that include Crit Chance, Bonesaw, and Breaking Point. Most people generally take WP Kestrel into tankier team compositions.

Crystal Power - The CP route is based on nuking enemies with Glimmershot and One Shot One Kill because of how absurd the CP ratio scalings are. CP Kestrel is most effective against squishy team compositions.

- Patch 3.3 Notes to Kestrel : Rejoice! Kestrel has slightly increased base WP and now has a stronger WP Ratio Scaling on her Glimmershot. 100% → 100/100/100/100/110%

- Patch 3.4 Notes to Kestrel : Glimmershot CP ratio from 135% -> 120%

Patch 3.5 Notes to Kestrel : Active Camo Trap only triggers upon direct damage

Adrenaline - Landing basic-attacks and Glimmershot will reduce the energy costs of her abilities by 10%, stacking up to 50% and wears off after 8 seconds. For CP Kestrel this isn't as useful as going WP since as CP you will be buying Clockwork to help with your energy pool and cooldowns.
-WP : Basic-attack and Glimmershot to reduce Energy Costs of your abilities which will be very helpful as you won't be building Energy items with the exception of Spellsword.
-CP : Not as important with Clockwork in your build, but it always helps to conserve Energy.

Glimmershot - Kestrel's basic poke and damaging ability. Since it scales with both WP and CP AND procs on-hit effects on the primary impact this is what allows Kestrel to build both routes. Weave in basic-attacks while using this for maximum efficiency. This ability has passive armor pierce as well.
-WP : Repeat between using Glimmershot and basic-attacking, both will Crit and deal massive amounts of damage.
-CP : Chunk down the enemy with the splash damage and stay a safe distance away while keeping in range.

Active Camo - The stealth from this helps Kestrel with escaping and repositioning since she has no little to no mobility. It can be used to either Stealth and move or to blow someone up with the massive CP ratio and stun.
-WP : Reduces the time it takes for you to be able to Stealth again after being attacked by an enemy. Building enough WP also allows you instantly Stealth even after being hit by an enemy.
-CP : The time you stay stealthed and the damage from the Mist Trap are increased by building CP.

One Shot One Kill - The only reason you play Kestrel for those sweet, sweet montage snipes across the map and Kraken / Dragon steals. Practice until you're familiar with the delay time so you can easily land this on enemies more consistently. Use this to help someone fighting an enemy if you can't reach them in time by chunking down their HP. Also, look for low health enemies that you can pick up a kill on.
-WP : Less WP scaling (120%) but will still deal enough damage to really hurt.
-CP : MASSIVE CP scaling (260%) that will obliterate anyone that doesn't have Defense items or isn't naturally tanky. (ie. Catherine)

Crystal Route

Shatterglass - More CP to scale into Kestrel's completely stupid scaling.

Clockwork - Helps with keeping the Energy Pool full and lowers the CD of One Shot One Kill for Kestrel to keep sniping fools.

Broken Myth - A must-have in every build since it adds Shield Piercing onto the passive Armor Piercing that Kestrel already has.

Dragon's Eye - If you're staying in long fights often, picking this up will ramp up Kestrel's CP to deal even more damage.

Spellfire - If your enemies are barely alive after using One Shot One Kill buy this to burn them for even more damage and finish them off. Also good for making your Glimmershot even more powerful by adding in Mortal Wounds and burn damage.

Frostburn - Very situational if you need help kiting enemies with fast mobility.

Aftershock - A personal preference for dealing percent max health damage
since you can proc this while using Glimmershot.

Weapon Route

Sorrowblade - More WP for Kestrel to deal damage with.

Tyrant's Monocle - As Glimmershot applies on-hit effects on the primary impact, this means that Kestrel can Crit with it.

Spellsword - If you find yourself running low on Energy and need Journey Boots instead of Halcyon Chargers then this will help with that. Also gives a nice amount of WP and CD.

Bonesaw - Situational buy if you need the Armor Shred on top of your passive Armor Pierce on Glimmershot.

Breaking Point - Similar to Dragon's Eye, this is most useful in long fights where Kestrel will stack up more WP.

Poisoned Shiv - Mortal Wounds and Lifesteal against sustain heroes like Krul.

Tornado Trigger - More Crit and more attack speed for more damage.

Defensive Items

Aegis - Most often the only thing you'll need since Kestrel keeps her distance while raining down Glimmershots. Reflex Block will save you from CC and damage.

Metal Jacket - If the enemy team is mostly WP or they have a hyper-carry such as Ringo who is the biggest threat dealing WP damage, then this will help you.

Slumbering Husk - Very rarely bought as Kestrel won't be in range for massive burst damage, but can be useful at times.

Boots

Journey Boots - CP Kestrel will usually buy this to help with mobility since Clockwork will do what Halcyon Chargers does with helping the Energy Pool. Great for chasing and escaping.

Halcyon Chargers - Stats help with keeping the Energy Pool filled and lowers CD even more, especially helpful on WP Kestrel.