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Journal Article

A Study of the Breeding Biology of the European Starling (Sturnus vulgaris L.) in North America

Brina Kessel
The American Midland Naturalist
Vol. 58, No. 2 (Oct., 1957), pp. 257-331
DOI: 10.2307/2422615
Stable URL: http://www.jstor.org/stable/2422615
Page Count: 75
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A Study of the Breeding Biology of the European Starling (Sturnus vulgaris L.) in North America
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Abstract

The preceding study on the breeding biology of the European starling (Sturnus vulgaris L.) was made at Ithaca, New York, from 1945 through 1951. The major portion of the study was carried out on an area of starling nesting boxes which bounded the periphery of a rectangular, agricultural district, 1.4 miles long by 0.8 mile wide. Observations of the annual cycle show that many starlings of the resident population display an interest in nesting sites throughout the autumn and winter months, both males and females visiting the holes in the mornings and evenings, especially in warm, sunny weather. By December some individuals begin roosting in the boxes at night, instead of returning to communal roosts; most of these early roosters appear to be ones that have previously nested on the area. Both males and females roost in the boxes. The number of birds using the boxes at night increases after mid-February, but decreases again when nest-building begins. Approximately 50 per cent of the birds nesting on the study area in one season return to breed the following year. These birds tend to choose nesting sites close to the ones they used the previous year. The starling's nesting territory includes a 10- to 20-inch radius about the nesting hole. Starlings will nest in close proximity to other starlings and other species, although observations indicate that there is a limit to how close they will tolerate neighbors. They often have communal singing perches and feeding areas. Courtship is not well defined in the starling, usually consisting of intensified song and display about the nesting box. Monogamy for a given nesting is the rule, though polygyny has been observed. The status of unpaired males is not well understood. Both members of a pair participate in nest-building, which begins about the third week of March at Ithaca. The male frequently brings materials to the nest before he obtains a mate, but when serious nest-building begins, the female is usually the more diligent worker. Copulation occurs upon the invitation of the female. The action of the female in pecking the male in the neck or shoulder region appears to be a "releaser" to the male in the mounting act. The date of the laying of the first egg for the first brood varies from mid-March on the Gulf Coast to mid-June on the Labrador Peninsula. Day-length appears to be the primary factor influencing the date of egg-laying, but annual variations in egg-laying dates in a given locality appear to be due largely to temperature differences. The minimum threshold for incitement of rapid gonad development appears to be about 40 to 43⚬F, and the birds must be exposed to this mean environmental temperature for a minimum of seventeen days before they will lay eggs. The mean clutch size in 301 layings at Ithaca was 4.9 eggs. In first broods the mean was 5.5 (199 clutches); in intermediate broods, 5.0 eggs (42 clutches); and in second broods, 4.1 eggs (110 clutches). Annual variations in mean clutch sizes proved significant. The mean weights of eighty-five eggs in sixteen first brood clutches was 7.0 grams (5.5-8.5 grams). After the laying of the first egg, the female lays one egg a day until the clutch is completed. Females tend to lay clutches of similar size each year after the first year. Incubation usually begins with the laying of the last egg, though it may begin earlier. Incubation lasts for twelve days; it is usually shared by both parents, although only the female incubates at night. If a clutch is destroyed, at least one renesting will be attempted; most renestings begin within two weeks of the time the first nest is destroyed. If the female is lost during the incubation period, the male will toss the eggs out of the nest within thirty-six hours and renew his courtship activities. The parents remove the shells of the hatched eggs by carrying them from the nest in their bills. Both parents brood and feed the young and participate in nest sanitation. The female only, however, broods at night, staying with the nestlings in the box until they are eight days old. If the male disappears during the brooding period, the female usually continues to raise the family; however, if the female is lost the young usually die. The mean brood size in 304 broods was 3.9 young. In first broods the mean was 4.5 young (230 broods); in intermediate broods, 3.5 young (41 broods); and in second broods, 2.9 young (78 broods). Comparisons with Dutch data on clutch and brood sizes and on egg-to-fledging success indicate that the starlings at Ithaca have a higher nest mortality than those on the Continent. At hatching, the nestlings are helpless and naked, except for sparse tracts of down feathers, and weigh about 6.5 grams. Energy during the first ten to twelve days is utilized primarily for size increases. Weight gains, yielding a sigmoid growth curve, are rapid during this period, and at twelve days the weight of the young approaches that of the parent birds. Feathers do not begin to break their sheaths until the sixth and seventh days. Eyes open on the sixth or seventh day. At twelve days the nestlings begin to show signs of fear and attempt to escape when handled. After ten days, feather growth becomes rapid; and before the young leave the nest at twenty-one days, they are almost as fully feathered as their parents and can fly well. The young, especially in the first broods, usually lose some weight before they leave the nest. In second broods the young are somewhat slower and more irregular in their development, usually averaging lighter than those of the first brood even at the time of fledging. There is some indication that the broods of first-year females may be poorer than those of adult birds. There is also some indication that mean nestling weights decrease as the size of the brood increases. After leaving the nest, the birds gather in small flocks which rove about the countryside during the day and join communal roosts at night. Many juveniles apparently stay in the vicinity of their place of hatching for a while after leaving the nest. The young are largely independent when they leave the nest, and the adults stay with them for only a short time, about four to eight days. Nesting success studies at Ithaca showed that 78.6 per cent of the nests were successful; 86.6 per cent hatched; 85.2 per cent of the young fledged; and 76.1 per cent of the eggs that were laid produced young that fledged. In first broods, these per cents were 89.1, 90.5, 86.0, and 81.4, respectively; in second broods they were 63.3, 80.3, 81.6, and 68.3 per cent, respectively. Some starlings, especially females, breed when only one year old. In 1950, five, or 14 per cent of the banded female nestlings of the preceding year returned to their place of hatching and bred in their first year. The reason that more first-year males do not breed can be attributed, at least in part, to immaturity and to the lack of suitable nesting sites. The mean clutch size for the first brood of the first-year females was significantly smaller than that of the adults (5.1 and 5.6, respectively). Most first-year females begin laying for their first brood at the same time as the adult females, but some do not begin until the period of intermediate brood layings. Within four to six weeks after the young leave the nest, many have begun their post-juvenal molt. This molt is complete and follows the same pattern as that of the adult post-nuptial molt. The first-winter plumage is worn for a full year, until the first post-nuptial molt when the adult plumage is gained. After this second molt, the starling normally molts once each year, after the breeding season. The molt of the starling is essentially the same as that described by Dwight (1900) for passerine birds, but there are two major differences: 1) The capital tract does not begin to molt until the wing and body molts are well advanced. 2) The caudal tract, instead of molting consecutively from the central pair of feathers (pair I), outwards, usually molts pair I first, then pair II, pair VI, pair IV, and finally pair V and pair III. Generally, as a group, the adult males are the most advanced in their molt at any given time; the adult females and juvenal males follow; and the juvenal females are the least advanced. There is a predominance of females in the sex ratios of nestling starlings, but nest mortality is slightly higher in the females, causing the per cent of males to increase during the period in the nest. During the winter, six to eight months later, collections show an even sex ratio in first-year birds. In adult populations there are significantly more males than females. Thus, data from Ithaca indicate that the females have a higher mortality rate than males. On the basis of 205 banding returns of starlings that were banded as young and survived to their first January 1 of life, the mean annual adult mortality rate in the starlings of northeastern North America is about 50 per cent. The average length of life, after their first January 1, is 15.6 months. The months of heaviest mortality are January, February, and March. One-third of the annual mortality occurs in February-March; three-fourths from January through May. The mortality rate for the first year of life, including the first summer and autumn, is approximately 60 per cent. About 20 per cent of the young fledged survive to breed.

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