This is a report of a study of nesting activites of a Ruby-throated Hummingbird, Archilochus colubris (Linnaeus), made at the University of Michigan Biological Station, Cheboygan County, Michigan, during the summer of 1947. An attempt is here made to add to the limited knowledge of parental care in this species. Other observations such as nest location, nest construction, nest sanitation, defense, fledging and nest leaving are also included. The period of observation began July 17, at which time there were two eggs in the nest, and ended August 17, when the young left the nest. The total time of observation was 62 hours. Nearly all of this was spent in a blind mounted on a 25-foot tower placed three feet from the nest. Some observations were made from a nearby position on the ground with the aid of 8-power binoculars. SUMMARY A nest of the Ruby-throated Hummingbird was under observation from July 17 to August 17, 1947, a period of 32 days. The nesting site was a clump of white birches in an AspenBirch Association. The nest was attached to a down-sloping small branch, 26 feet from the ground. Only the female carried on incubation, brooding and feeding of the young. No male was observed at any time during this study. The length of the incubation period was not determined. The two eggs hatched with an interval ot 24 hours between. The female was seen removing a portion of egg shell with her bill. The female followed a brooding schedule which was regulated more by the time of day than by the weather conditions. Brooding was more continuous during a thunderstorm. Brooding ended nine days after the first egg hatched, at which time the nestlings had a growth of pin feathers and had their eyes open. The first young was observed to be fed 11 hours after it hatched. The feeding interval was 45 to 60 minutes, being more frequent in the morning and late afternoon. The female regurgitated about three times for each bird during a feeding. The total time required to feed both young was one and one-half minutes. Nest sanitation was first accomplished by the female’s removing bits of material with her bill. After that the young defecated over the side of the nest. The female was seen defending the nest from a Least Flycatcher. By the 14th day white pin feathers on a slate-gray skin gave the young a striped appearance. They feathered out on the 15th day. Wing exercise began on the tenth day. One of the young left the nest on the 29th day after hatching, the other on the 31st day. The long nestling stage may have been due to the late start and to food shortage. The young flew directly from the nest.