I have no idea whether vegetable fat is good for birds. But you are right that they would not naturally feed on cows and pigs! You could give them a supplement of protein and fat by including mealworms in a dry seed/peanut mix. Apologies that this is not a vegan suggestion, but it is closer to birds' natural diet.
Chopped apple and raisins are good for blackbirds, too.
In terms of making your garden more wildlife-friendly, you could really use more shrubs/trees, I think, not only for food for birds, but also as nesting sites, shelter, homes for invertebrates, and a wide range of other benefits. They have calculated that garden trees contribute a huge amount of oxygen and clean air. Likewise, flowering plants, particularly daisy flowers, pea flowers and things like thyme, will be good for nectar-feeding insects.
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Chopped apple and raisins are good for blackbirds, too.
In terms of making your garden more wildlife-friendly, you could really use more shrubs/trees, I think, not only for food for birds, but also as nesting sites, shelter, homes for invertebrates, and a wide range of other benefits. They have calculated that garden trees contribute a huge amount of oxygen and clean air. Likewise, flowering plants, particularly daisy flowers, pea flowers and things like thyme, will be good for nectar-feeding insects.