I’m not sure it’s a good idea, but that’s what the recipe says.
18 Fieldfares (reckolter fogel)
Take fieldfares (turdus pilaris) that have been prepared cleanly, and when you take out the innards, thrust the stomach back in. Boil them in a good meat broth, then fry them in fat. Take the liver of a calf or a sheep, pound it in a mortar with an equal quantity of bread and pour in a little wine and vinegar. Pass this through a cloth, spice it, and colour it well. Boil it up in a pan and serve the fieldfares in it.
The fieldfare, known in German as the Wacholderdrossel (literally ‘juniper thrush’) or Krammetsvogel (‘juniper bird’), has a long culinary tradition. The migrating birds were caught in large quantities for the table and augmented the income of many rural households in season. This recipe provides a rich, meaty sauce that will probably overpower any flavour the birds have, but seems to have been very popular at the time. Interestingly, this recipe is paralleled almost verbatim in the Rheinfränkisches Kochbuch (#53):
53 (A sauce) For birds or for chickens. Boil fieldfares well in a pan with fresh meat broth, and they must boil up in it. Then fry them well in fat. Now take a liver of a sheep or calf and boil it and pound it very small with the same quantity of bread and add (lit. pour) wine or vinegar or both with it. Pass it through (a cloth) and season it and colour it. Let it boil up and serve the birds in it.
54 But if you would eat it sweet, add a good part of honey to it according to your will. You may serve partridges in this, domestic chickens, brawn, roasted deer liver, or other things.
It was also clearly good for more than fieldfares, which as far as we can tell were more typically fried and served as crunchy tidbits.
Bound together with medicinal, veterinary, and magical texts, the culinary recipes of Munich Cgm 384 were partly published in 1865 as “Ein alemannisches Büchlein von guter Speise“. The manuscript dates to the second half of the fifteenth century. My translation follows the edition by Trude Ehlert in Münchner Kochbuchhandschriften aus dem 15. Jahrhundert, Tupperware Deutschland, Frankfurt 1999, which includes the first section of recipes not published earlier.