A woodland food chain is shown below: oak tree → caterpillar → blue tit → sparrowhawk. Explain why a sparrowhawk must consume many blue tits in order to obtain enough energy and biomass to survive.

Only about 10 % of the biomass of the oak tree is transferred to the caterpillar, and only 10 % of that is transferred to the blue tit. Thus the biomass available to the sparrowhawk is very small.
The rest of the energy is lost as heat during respiration and as waste (faeces, urine, excretion). These losses mean that each trophic step receives only a fraction of the previous level’s biomass.
Because the sparrowhawk is at the fourth trophic level, the amount of biomass it can obtain from a single blue tit is tiny. Therefore it must eat many blue  tits to accumulate enough energy and biomass to survive.