Final answer:
The scientific investigation that does NOT include dependent and independent variables is observing and counting birds.
Step-by-step explanation:
The scientific investigation that does NOT include dependent and independent variables is option C: Observing and counting birds. This is an example of an observational study, where the researcher simply observes and counts the birds without manipulating any variables. In option A, the independent variable is mass and the dependent variable is the speed at which an object falls. In option B, the independent variable is the model car's design in the wind tunnel, and the dependent variables are air flow resistance and fuel economy. In option D, the independent variable is temperature and the dependent variable is bacterial reproduction rates.