Birds struggle to find worms due to dry weather
In times of low rainfall, ground feeding birds such as Blackbirds are often unable to find sufficient natural food and in particular earthworms, as the worms are well below ground level and in damper soil. In these conditions, Live mealworms are the best food to put out for garden birds, as they're high in fat and protein, plus also moisture which is very important when adult birds are feeding their young. Blackbirds, Song thrushes, Robins and Starlings will all readily feed on live mealworms.
As an alternative, sultanas or raisins can also be out but it's essential that they're first soaked in water for around 30 minutes. This will mean that the previously dried food becomes hydrated, and therefore is safe for birds to feed on and in particular when given to young birds in the nest by adults, as food is their only source of fluid until they have fledged.
Sultanas and raisins are lower cost option than live mealworms, but certainly not such a good one because the food doesn't contain anything like the same level of protein. Also, if you do feed sultanas and raisins and have a dog, then be sure to keep your pet well away from the food, as both are poisonous to dogs and can lead to renal failure.