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Experimental Modification of Reproductive Performance by Density in Captive Starlings

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Experimental Modification of Reproductive Performance by Density in Captive Starlings EXPERIMENTAL MODIFICATION OF REPRODUCTIVE PERFORMANCE BY DENSITY IN CAPTIVE STARLINGS ARTHUR C. RISSER, JR. ’ Department of Zoology University of California Davis, California 95616 Regulation of animal populations by density- failed to survive more than a few days, appar- dependent factors has received much attention ently because of dietary deficiencies. from ecologists. Lack (1966) has examined This study represents the first attempt to this problem extensively in natural populations determine how male and female Starlings held of birds, and contends that reproductive rate, captive under natural light and temperature as measured by the number of eggs per clutch, conditions respond to different densities during has evolved through natural selection to cor- the regular breeding season. It is also the first respond, on the average, with the number giv- study to test experimentally an hypothesis ing rise to the greatest number of offspring. regarding population regulation with respect Since clutch size varies little with density, to birds, i.e., that high population density can population regulation must, therefore, be a modify reproductive performance in a wild function of density-dependent variations in species. mortality which in some wild birds correlates well with food shortage. MATERIALS AND METHODS Lowered fecundity with high population A preliminary experiment was conducted during the density is rarely reported for avian populations 1968 breeding season to determine whether several (Kluyver 1951,1963; Lack 1966; Perrins 1965). male and female Starlings in large aviaries would form It is known, however, that external stimuli pairs and go through the normal sequence of repro- ductive events. This experiment provided a basis on associated with breeding behavior in birds can which to establish high and low density captive result in changes in the birds ’ physiological populations for comparison the following year. state which might affect fecundity. Birds with About 701 of the European Starlings from the a history of domestication are generally used preliminary study were used in the experiment con- in behavioral studies because of their adapti- ducted in 1969. These birds had been trapped in 19167 near Fresno and Santa Rosa, California. In addi- bility to confinement. Experimental tests of tion, about 350 Starlings were trapped near Santa Rosa hypotheses concerning population regulation in late summer 1968. All Starlings were housed to- in wild birds have not been conducted perhaps gether, from early October until mid-December 1968, due to difficulties in getting large numbers to in an outdoor aviary. During October, most of the Starlings developed a disease morphologically similar breed readily in captivity. Thus mechanisms to avian pox. A few of the most heavily-infected operating at the physiological and behavioral individuals were removed from this flock. Symptoms levels in wild birds which might result in repro- abated after approximately three weeks, with a ductive changes and lead to population regula- mortality of less than 5%. tion remain unclear. Birds from this aviary were removed, sexed by laparotomvI (Risser 1971). banded with a numbered , , European Starlings (Sturnus vulgaris) have aluminum band, and color banded according to sex. long been used as experimental animals Birds from the preliminary experiment were known to (Bissonette 1933; Burger 1940; Davis 1957, be at least 1.5 years old. As late as mid-October, age 1963; Mathewson 1961; Schwab and Lott could be precisely determined by the presence of juvenal plumage, and especially by the few persistent 1969), but few experiments have been con- feathers dorsal to the auricular region. When no cerned with both sexes. Burger (1942) stated juvenal feathers were found, age was determined on that female Starlings isolated during the the basis of skull ossification ( Kessel 1951) . normal breeding season laid eggs in laboratory In order to establish captive populations comparable cages, but that three or four pairs of Starlings in age structure to a wild population, Kessels’ (1957) figures for winter flock composition at Ithaca, New in 1440 ms cages did not pair. He implied that York, were used. No such data were available for space was a factor limiting the potential of winter populations in California. The age ratio thus the female to complete the reproductive cycle derived and established in each cage was four adult in captivity. Miller (1969) reported the SUC- males, one first year male, three adult females, and one cessful breeding of a captive pair of Starlings in first year female. All birds were released in their a 2.4 x 1.8 x 1.8 m cage, but the nestlings respective cages by 1 January 1969. Any bird that died during the course of the experiment was replaced with one of the same sex and age. Five cages contained 1Present address: Bird Department, San Diego Zoo, San Diego, California 92112. 10 pairs each for a combined total of 100 birds, and [1251 The Condor 77:125-132, 1975 126 ARTHUR C. RISSER, JR. FIGURE 1. Separate nest boxes used by captive FIGURE 2. Colony-type nest box used by captive Starlings in high-density Cage 12. Starlings in high-density Cage 3. two cages contained 50 pairs each for a combined pens south of the aviaries, bringing the number of nest total of 200 birds. boxes in the field area to 109. These boxes were all Starlings were caged in two large, outdoor aviaries within 2 km of the aviaries. Ten boxes attached to the north of a wooded depression along an old creek outside of the aviary were checked at least once every bottom in the Zoology Field Area of the University of 48 hr. First dates of laying were obtained directly California, Davis campus. Aviaries consisted of eight from three of these boxes, while laying, hatching, and adjacent cages, each measuring 6 m long, 3 m wide, fledging dates for other boxes in the field area were and 2.1 m high. Burlap covered the screening between determined on the basis of the condition of eggs or each cage to provide partial visual isolation. There young in the previous check, and on comparisons with was at least one vacant cage between those that were known-aged young. Data from check sheets of workers occupied. Natural vegetation grew from the dirt floor. who monitored nest-box activity during the 1964 In each cage there were at least two l-m hanging through 1967 breeding seasons were combined with horizontal perches. the above data to provide a range of laying dates and Each cage was provided with nest sites equal to the mean clutch size for Starlings nesting naturally in the number of pairs of birds per cage. Nest boxes in the vicinity of the aviaries. five IO-pair cages (low-density) and one of the 50-pair To clarify the following presentation of results, a cages (high-density cage No. 12, fig. 1) were made of number of terms must be defined. The number of rough 2-cm redwood boards and were of the type and pairs that actually formed at each density was assessed dimensions described by Kessel ( 1957). Four 13-site by the number of boxes in which at least one clutch colony boxes were erected in the second high-density was laid. This method assumes that once a pair nests cage (No. 3). Colony boxes were constructed of in a particular box, it will use the same box for 16-mm exterior plywood and coated with redwood subsequent clutches. Although Starlings are usually stain and varnish. Outside dimensions of the colony monogamous, cases of polygamy in nature have been boxes were 235 x 42 x 20 cm. Inside dimensions of reported ( Kessel 1950, 1957 ). each compartment approximated those of a separate Familiarity with both pairs in a two-pair cage and nest box. Three hinged doors across the bottom one pair in an eight-pair cage in the preliminary provided access to all compartments (fig. 2 ). A lo-cm experiment supported the conclusion that there was horizontal perch was attached perpendicular to the only one breeding pair per nest box. Even if a poly- front of each nest site about 4 cm below the hole. The gamous male did defend more than one box and mate boxes were positioned about 2 m from the floor. Nest- with more than one female, this measure serves as an ing material in the form of straw, burr clover, dried index to the females ’ reproductive performance. alfalfa hay, and chicken feathers were available at all “Clutch size” is used here as defined by Davis times. ( 1958): the number of eggs found in a nest. Mean Commercial turkey pellets with a crude protein clutch size was calculated only for successful clutches, content of 27% were always available from hanging which are defined as any group of eggs laid in the nest self-refilling feeders. Water was also available ad in which at least one egg hatches. libitum. Fresh vegetables such as tomatoes, squash, Since even aborted clutches represent reproductive apples, parsley, and lettuce were provided occasionally. effort by females, abortions were included in calculat- Cages were entered daily for nest box inspection, ing the mean number of clutches per breeding pair and usually between 15:OO and 18:O0. Nest-building activ- total clutches laid in all cages. Generally, aborted ity, number of eggs laid, and number of young present clutches were either buried under new nesting mate- were recorded and dead nestlings were removed. rial or removed by me some time after the projected From this information mean clutch size per pair, mean hatching date. Eggs were occasionally found on the total number of clutches laid per pair, and percent ground. By checking the previous days ’ records, it hatchability of eggs could be calculated for each was discovered that the adults expelled most of the density.
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