Absolutely need to school that pup. Our dogs are taught what is off limits and what is not. Right now I have an Irish Setter laying next to a batch of hatched chicks - they are 7 days old. The chicks are off limits. The dog is not remotely interested in them (keeping in mind an Irish Setter is a bird hunting dog). When she was 4 months old she would have killed them.
Put that pup on a leash so you have control - walk towards the chickens and have a "NO" word. It doesn't not have to be NO - my word of schooling to tell the dogs something is off limits is "aught O". I have the leash - show the pup the chickens and say my word strongly and pop them with the leash. Do not let them get close enough to frighten the chickens - you do not want the chickens to run. If the pup advances to the chickens, a pop on the lead followed by my word. When the pup looks away from the chicken - praise the pup - make a bid deal positively. If you only correct negatively you are going to frighten the pup.
Scold for approaching the chickens, praise for looking away. NEVER have the pup off leash around the chickens. When you know the pup is not lunging or interested in the chickens, start getting closer to the chickens with the pup. When the pup is trust worthy, walk through the chickens. Try not to have the chickens run - that will make it tougher on the pup. When the pup is ignoring the chickens, throw some food to the chickens so the come closer to the pup. The pup can not react. If it does, you are not done working on a short leash - keep working. If the pup doesn't respond to a pop on the lead, increase the pop to a stronger pop. Eventually graduate to a long lead - small rope or lunge line of 20 feet. Do the same thing. You want the pup to think the sky is falling when it looks at or even approach the chickens.
When the pup chooses not to look at the chickens - wow - go crazy with praise. Show the pup the difference - chickens mean the sky is falling. Staying away from chickens means the world is AWESOME. NEVER let the pup loose with the chickens again until they know the difference. If you don't stop it now, you might not be able to have the dog around the chickens - ever.
IMPORTANT - be sure to ALWAYS follow with praise - even if the pup looks away from the chicken and at you for a second. Too much negativity will get you nowhere. Always follow up with praise for the smallest positive act by the pup. The pup will figure it out. Baby steps. At first just a look away from the chicken gets praise. When the pup decides to turn away from the chicken praise. When the pup walks through the chickens on a leash and doesn't look at them - praise. Praise for everything positive not matter how big or small.