Influence of Age and Habitat on Reproduction by the American Goldfinch

E-ISSN： 1939-9170|60|2|418-432

ISSN： 0012-9658

Source： Ecology, Vol.60, Iss.2, 1979-04, pp. : 418-432

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Abstract

During the summers of 1968—1975 the breeding biology of a color—marked population of goldfinches was studied in natural, nursery, and city habitats at Guelph, Ontario, Canada. Nesting began in early July, the mean date for the laying of the first egg laid each year varying by only 7 days and the mean date for the 1st clutches by 2.6 days. Old ♀ ♀ (birds in their 2nd breeding seasons or older) were mainly responsible for early nesting, and the percentage of such birds involved with breeding became less as the season progressed. The height of nesting occurred in the latter half of July, and no new nests were begun after August. The largest clutches (mean 5.3 eggs) were laid by old ♀ ♀, which in turn laid a higher percentage of 6—and—7—egg clutches than did presumed 1st—year birds. The age of the @M may also influence clutch—size since the largest recorded clutches (mean 5.6 eggs) belonged to old ♂ ♂ paired with old ♀ ♀. Habitat also affected clutch—size, the nursery having the highest (5.6 eggs) which was followed by the city (5.2 eggs) and finally by the natural habitat (5.0 eggs). The differences in clutch—size may have been influenced by the age structure of the breeding populations in each habitat, by differences in time of nesting and breeding population density, and by effects of cowbird parasitism. Clutch—size diminished with season, July clutches being significantly larger than August clutches. Nesting records suggest that 6.0 eggs forms the most productive clutch—size for the American Goldfinch. Old ♀ ♀ had a higher success, both in terms of numbers of young fledged/nest and ratio of successful nests, than presumed 1st—time nesters. Age of ♂ ♂ may also affect nesting success, as in matings between old ♂ ♂ and old ♀ ♀ all chicks that hatched fledged, and a greater number of fertile eggs was produced in this mating combination than in any other. Fledging and nesting success were highest in the city, followed by the natural and nursery areas. City nests suffered a lower predation rate than those in the other 2 habitats, but a higher percentage of city nests was abandoned. Overall, the city was the most successful nesting habitat. In the natural habitat 74.1 of nests were built in shrubs, trees being used exclusively in the nursery and city. Maple was the most commonly used tree in all 3 habitats; whereas 67.9% of all nests built in maple failed in the natural area, only 22.2% failed the city. Heights above ground at which nests were built also increased with reliance on trees. City nests were significantly higher than those in the other 2 habitats. Six 2nd broods were raised from the 9 nests in which second broods were attempted. Old ♂ ♂ were known to have been responsible for 6 of these 9 nests. The latest date for the laying of a second clutch following the rearing of a first brood was 16 August. In all but 2 nests the 2nd clutch was laid before the first brood left the nest. Replacement clutches were laid by ♀ ♀ that had lost the 1st nest. The mean time between the loss of the 1st nest and the laying of the 1st egg of the new clutch was 6.9 days. The latest date on which the 1st egg of a replacement clutch was laid was 21 August. All replacement clutches were smaller than 1st clutches. Renesting proved successful in 60% of the cases. Whereas double—brooded birds produced a man of 7.3 fledglings/pair, renesting birds produced 2.8 fledglings/pair. Banding data showed that goldfinches commonly changed mates from year to year. Whereas ♂ ♂ were likely to change their nesting area, ♀ ♀ remained faithful to their original nesting area in subsequent years.