You are probably correct about the white substance smeared on the trees. Lard or suet is a common, high-calorie food usually offered to birds only in the winter when other food is scarce and their fuel requirements are high.
Suet is "the hard white fat on the kidneys and loins of cattle, sheep, and other animals, used to make foods including puddings, pastry, and mincemeat." and lard is just the fat from pigs.
There are lots of recipes combining these fats with seed, cornmeal, peanuts or peanut butter to make tempting treats for our frigid feathered friends. It is usually hung in wire cages or net bags but smearing it on trees could work, too.
Paul