Vindication!

Long ago I read a paper on the migratory habits of Sedge Warblers.

They cross the Sahara to overwinter in Africa every Autumn. To fuel the flight they double their bodyweight, which they achieve by switching to high carb insects - plum reed aphids - which are basically little protein bags full of sugar from the plant sap. Then they burn the fat on the migratory flight.

I predicted at the time that they would switch on insulin resistance to convert the sugar and store it as fat, then switch it off again to access it. Likely the same for other birds who put on fat for the winter by eating berries.

Thanks to Tim Noakes

https://www.researchgate.net/publication/223683158_The_body_mass_cycle_of_the_migratory_garden_warbler_Sylvia_borin_is_associated_with_changes_of_basal_plasma_metabolite_levels

well Garden Warblers are similar but perhaps less extreme.

This is from nearly twenty years ago.

Ah yes, they switch to berries to gain weight. Their closely related cousins Blackcaps have taken to overwintering here and may be found in gardens in autumn and winter stuffing their faces with honeysuckle berries. Well actually it turns out that "our" Blackcaps still fly south for the winter while the ones from Germany and elsewhere in Europe have taken to migrating west and ending up here.

I've seen and heard more Garden Warblers than usual this year, they may be replacing the Willow Warblers in the south and east, too soon to tell. They have rather similar songs, especially when a Blackcap imitates a Garden Warbler, one way to tell them apart is that Blackcaps tend to be much more visible while GWs pour out their hearts while concealed. Used to be that Blackcaps preferred treetops while GWs stayed lower down but this year I've been hearing them (and occasionally seeing them) in the treetops also.