It’s instructive to compare the feeding behavior of birds at our sunflower feeders. Some like House Finches, Purple Finches, Northern Cardinals and Evening Grosbeaks will sit on the feeder and crack seed after seed with their powerful, crushing bills. Others like Black-capped Chickadees, nuthatches and Tufted Titmice swoop in to grab a seed and then fly away with it.
Birds in the latter group have relatively small bills and can’t easily crush a sunflower seed to get to the nutritious kernel inside. Instead, one of these birds will take the seed to a more protected area, hold the seed firmly beneath its feet as it perches and chisels the sunflower seed apart with well-placed strikes at the suture line of the sunflower seed.
This process takes some time and focused vision so a chickadee’s guard is down. Chiseling a seed open in full view at a feeder is risky. Finches can crush a seed easily without having to use their eyes to accomplish the task. These birds can keep a watch for predators without having to retreat to a protected area to extract the kernels.
If you watch a chickadee after it flies away with a seed, you can often see the chickadee perch and hammer the seed open. Sometimes the chickadee will eat the kernel but at other times the chickadee will take the kernel and cache it behind a scale of conifer bark, in a crevice or among needle clusters of evergreens.
Many species of birds from diverse families hoard food. Usually the hoarded food is seeds although insects and other animal matter may be cached. One can consider voles or birds impaled on a barb by Northern Shrikes to be a type of hoarding behavior.
Hoarding makes good sense for resident birds. Food supplies always vacillate. Birds that are non-migratory have two choices: wander widely in search of patches of food or store food when it is abundant to use in times of food scarcity.
Hoarding has been well studied in Black-capped Chickadees. We’ll use this species as a case study in food hoarding.
Chickadees mostly cache food in the fall. October and November are the peak months and hoarding is only rare observed after December. During peak hoarding, a Black-capped Chickadee can store hundreds and even thousands of food items in a day. A study of a related species in Norway indicated 50,000-80,000 seeds were stored each autumn.
Chickadees store each seed in a separate location. This dispersed storage is called scatter hoarding as distinguished from larder formation where all the food is stored in a single place.
One might surmise that hoarding would be more important at higher latitudes where the winter is more severe. The available information supports this conjecture. Hoarding by Black-capped Chickadees is common in Ontario and New York but documented only once in southern Illinois.
Scatter hoarding is advantageous because a competitor cannot possibly find all the cached food items. However, hiding food singly demands a prodigious spatial memory. Chickadees are up to the task.
Work by David Sherry at the University of Toronto showed Black-capped Chickadees can accurately relocate caches 24 hours later. Subsequent work shows these spatial maps persist for as long as 28 days.
Chickadees spent more time in the vicinity of their caches than in areas where they had not hoarded seeds. Furthermore, chickadees allocate most of their time in the vicinity of caches where their most nutritious food has been hoarded.
Most food items are recovered between one and four days after hoarding. How can hoarding then be of value in the dead of winter? The answer seems to be that chickadees remove cached items and hide them again in a new place for a few days, moving them again and again until they are finally eaten.
[Originally published on December 21, 2014]