REPRODUCTIVE EFFORT OF COLONIAL AND SOLITARY BREEDING TREE SPARROWS PASSER-MONTANUS L

A total of 250 nestboxes were arranged in five plots in a suburban are a of Budapest, Hungary (19-degrees 04'E, 47-degrees 41'N). In each plo t, 25 were placed at 50 m intervals to simulate solitary breeding and 25 3-5 m apart to simulate colonial breeding. Length of nest building period, feeding frequency, nestling mortality, nestlings' diet, produc tivity and parental condition were compared for colonial and solitary breeding tree sparrows Passer montanus. Parents with long nest-buildin g periods, including the majority of first-year females, produced fewe r young than parents which built over short periods. Parents fed nestl ings more frequently and nestlings had lower mortality in second than first broods; whether or not a third brood was reared was determined b y the costs invested in first and second broods. Females that laid a t hird clutch had reared fewer young in first and second broods and were heavier than females that reared many young in two broods. Colonial b irds had higher feeding frequencies, more similar diets and suffered l ower nestling mortality than solitary parents for first broods, but th ey fed less frequently, diets were less similar, and nestling mortalit y was higher in second and third broods. It is suggested that colonial breeders benefited from the social stimulation of simultaneous feedin g in first broods, but the advantage of synchronicity in feeding decli ned in second broods and the sparser breeding spacing of solitary pare nts was more advantageous for feeding in second and third broods. Bird s that changed nest spacing between broods had fed nestlings less freq uently and had higher nestling mortality before changing than birds wh ich retained their spacing. Parents which changed from colonies to sol itary nests fed more frequently with lower nestling mortality in the n ext brood than parents which retained colonial nests for their second (and third) brood. Solitary parents did not show such a relationship. The rearing of three caused higher weight loss in colonial than solita ry parents.