A nest cup can be plastic, cardboard or molded wood fiber shaped at 4”, a.k.a, “jiffy pot”, found in garden or nursery stores.
We all monitor our bluebird boxes regularly, but opening the door and peering into the darkness to count accurately the number and color of the eggs and the number of chicks can sometimes be challenging. Wouldn’t it be nice if we could pull out the whole nest and its contents ? Then examine it in the sunshine.
The nest cup acts as a template for the mother bluebird to weave her soft grass or pine needle nest. She completes the nest in record time as less material is needed. The cup acts as a moisture barrier when and if wind blows rain into the box. The cup does not allow an egg to get pushed up onto the brim of the nest away from incubation and possibly roll out when the door is opened for monitoring. These nest cups are reusable and bluebirds may prefer boxes with the cups as opposed to plain, square, wood walls.
Sometimes even bluebirds make a skimpy nest or have to lay the eggs before the nest is really finished. The result could be that the eggs or the nestlings fall out when the door is opened or that the chicks get their legs/wings pinched when the door is closed. The nest cup has high sides so that even if the nest is shallow, the chicks have a wall around them keeping them safe and free from drafts.
For the monitor, removing the nest for a weekly check is quite simple. With a nest, eggs or chicks in the nest cup, the cup may be lifted out, checked, photographed and returned to the box in just a few seconds without moving one blade of grass or pine needle. By removing the entire nest, a good check on ants and earwigs can be made. If necessary, diatomaceous earth or cinnamon can be sprinkled under the cup. When the nestlings have fledged, the nest cup can be easily cleaned and replaced.
Sometimes we look in and see a nest & chicks that are soaking wet from wind-driven rain! Wouldn’t it be nice to replace that wet nest? You can then transfer the eggs or chicks into a new, dry liner containing a “hand-made nest” of dry grass or pine needles that you can construct in advance for this immediate nesting need. It is so satisfying and easy to provide a dry nest.
I think the North Carolina Bluebird Society website, http://ncbluebird.org , says it most succinctly. In the Monitoring Tips section: 7. “Use a molded wood fiber nest cup in nest boxes to facilitate nest building as well as monitoring activities.”