Typescript error :TS2304 cannot find name

Question

Burning through a typescript course I came across these code pieces that fail to compile and give error TS2304. Any help is appreciated.

file ZooAnimals.ts:

namespace Zoo {
    interface Animal {
        skinType: string;
        isMammal(): boolean;
    }
}

File ZooBirds.ts:

/// <reference path="ZooAnimals.ts" />
namespace Zoo {
    export class Bird implements Animal {
        skinType = "feather";
        isMammal() {
            return false;
        }
    }
}

The command to compile the files:

tsc --outFile Zoo.js ZooAnimals.ts ZooBirds.ts

Throws error:

ZooBirds.ts:3:34 - error TS2304: Cannot find name 'Animal'.

3     export class Bird implements Animal {