Concept:
Think about what mixed farming is built for. Its main aims are risk spreading, recycling of nutrients and use of the land in many ways. It is not built to beat a single crop on yield.
We test each statement against these aims and look for the one that does not fit.
Option 1 against the aims:
Nutrient recycling is a core aim. Manure from the animals and crop waste feed the soil again. That keeps fertility up, so statement 1 fits and is correct.
Option 2 against the aims:
Mixed cropping keeps a plant cover on the soil for a longer time. Covered soil washes away less. So statement 2 fits and is correct.
Option 3 against the aims:
Higher output than monoculture is a yield claim. Specialised monoculture farms usually aim for the highest yield of one crop with heavy inputs.
The gain from mixed farming is steadier income and safety from total failure, not always more output. So statement 3 does not fit. It is the false one.
Option 4 against the aims:
A farm with many kinds of plants, hedges and trees is a better home for birds than a field of one crop. Habitat value is a known side benefit. So statement 4 fits and is correct.
Conclusion:
Three statements match the known benefits. One is an overclaim. The odd one out is option 3.
Final Answer:
The statement that is not true is option 3.
\[ \boxed{\text{Option 3}} \]