Ludwig, J. P. 1964 Most birds this AM were quietly sitting on eggs, preening, or sitting on empty nests. When incubating, the bird often sleeps with head held erect and eyes shut, and rarely with head over back in- between wings. Unemployed birds always sleep with head over back. Sitting birds do not rework the nest or attempt to maintain it in any way unless the mate brings it materials for the nest. A bird gathering twigs brings these to a sitting bird which then takes the twig and weaves it into the nest. The arriving bird always calls, but the taking (sitting) bird may not. The placement of new materials in the nest is always done by the sitting bird, never by the arriving bird. Single birds were seen to gather sticks also, but on arriving at their nest sight they looked about and usually lost interest in continuing to build. It may be that 2 things are necessary for nest building - a pair, and in later an egg. The truth may be that each part of the behavior of these birds requires a releaser (C.F. Tinberger)- That is the presence of an egg releases incubation in one of a pair while the presence of that bird incubating stimulates the mate to gather more materials to complete the nest, these acts in turn cementing and in- suring the duration of the pair-bond when the incubator accepts the preferred stick or twig. I have the impression that a bird accepting a stick stimulates the offering bird greatly - A bird having an offered stick accepted simply "looks pleased." Copulatory behavior may be stimulated by the nest building activities. Once I watched a bird bring a stick to another sitting on the edge of an incomplete nest sans egg. The sitter accepted the stick and gurgled