Plant identification books always state "it is reported" that the fruit of V.trilobum (Highbush cranberry) is more attractive to birds than berries of V. opulus (Guelder rose). Supposedly berries of V.opulus stay on the tree all winter and are only eaten by birds the next spring in desperation. This is perhaps an old wives' tale that has been taken as gospel in many quarters. I was collecting some V. trilobum berries recently and found that the berries were very loosely connected to the stem, even some of the less ripe berries. Pick one berry and another two would be jarred loose and fall to the ground. Therefore another explanation for why berries of V. opulus stay on the tree over winter could be that they are more firmly attached to the stem.