Can I put tinned mackerel in spring water out for the birds

Hello Helen,  unless you have gulls in your area (and it is often not recommended to feed them as large numbers can become a nuisance)  I doubt the regular garden birds would be interested in fish as their natural food requirements will be seeds, nuts, fat balls/suet, at this time of year and berries during the season etc.,      If you would like to feed the regular garden birds something different then you can give them homemade raw pastry:   plain (supermarket brand flour) lard,   as you would for home pastry but leave the salt content out;     add suet pellets or a little seed and a small handful of mild cheddar cheese (just a little as it contains salt),   mix together with just enough water to bind the pastry together so it is not sticky;     add more flour if sticky.     Place the pastry in a fat feeder or pinch off little bits and place on shrubs/twigs in your garden and you will see a lot of birds love it except for the finches who will prefer seeds.