1. **State the problem:** The bird population is decreasing by 2% per year. We need to find the decay factor. 2. **Formula and explanation:** The decay factor for a percentage decrease of $r\%$ per year is given by: $$\text{Decay factor} = 1 - \frac{r}{100}$$ This is because a decrease means the population retains only a fraction of its previous amount each year. 3. **Apply the formula:** Here, $r = 2$, so: $$\text{Decay factor} = 1 - \frac{2}{100}$$ 4. **Simplify:** $$\text{Decay factor} = 1 - 0.02 = 0.98$$ 5. **Interpretation:** The population retains 98% of its previous year's size each year, so the decay factor is $0.98$. **Final answer:** The decay factor is $0.98$.