Martin L. Morton

Extra-pair fertilizations in mountain white-crowned sparrows

SummaryA population of white-crowned sparrows (Zonotrichia leucophrys oriantha) located near Tioga Pass, in the central Sierra Nevada of California, was studied during the 1985 and 1986 breeding seasons to determine how often chicks were not the offspring of both adults who reared them. Blood samples were obtained from 35 nesting pairs and feather pulp specimens were collected from all 110 young they fledged. Electrophoretic analyses of 4 polymorphic loci revealed that a minimum of 9 nests (26%) contained a chick that could not have been conceived in a mating between the attending pair, and that at least 15 chicks (14%) were not offspring of both attending adults. Using two recently developed probability indices, it was estimated that 34%–38% of chicks hatched at the study site were not offspring of one of their putative parents. Because there was no evidence that intraspecific brood parasitism occurred, all the mismatched young apparently resulted from extrapair copulations. Our data, along with similar results from other species, raise new questions about sexual selection and patterns of parental care in monogamous birds.

Annual Gonadal Cycles and Pituitary Gonadotropins in Zonotrichia leucophrys gambelii

The essential role of the anterior pituitary gland in the development and the function of the avian gonad has been recognized for more than three decades. However, little is known concerning annual variation of gonadotropic function in species with marked gonadal cycles. The only extensive information available appears to be that of Greeley and Meyer (1953) who investigated the gonadotropic activity of the anterior pituitary in relationship to the annual testicular cycle in Ring-necked Pheasants (Phasianus colchicus) held in captivity under semi-natural conditions. In order to extend our understanding of the regulation of the hypothalamo-hypophysio-gonadal axis in birds under natural conditions, we have investigated the annual cycle of pituitary gonadotropic potency in relation to gonadal cycles in free populations of the White-crowned Sparrow, Zonotrichia leucophrys gambelii Nuttall. Such data are also indispensable in the interpretation of the results of experiments conducted under controlled conditions. The annual testicular cycle of Whitecrowned Sparrows has been investigated by Blanchard and Erickson (1949), by Oakeson (1954), and by Oakeson and Lilley (1960). Our data extend these investigations, and describe also the annual ovarian cycle.

Effects of temperature on photoperiodically induced reproductive development, circulating plasma luteinizing hormone and thyroid hormones, body mass, fat deposition and molt in mountain white-crowned sparrows, Zonotrichia leucophrys oriantha.

The mountain white-crowned sparrow, Zonotrichia leucophrys oriantha, breeds in subalpine meadows throughout many mountainous regions of western North America. Mathematical analysis of 20 years of egg-laying dates at Tioga Pass, California (3030m elevation) indicated a highly predictable breeding season suggesting that precise environmental cues such as the annual change in day length were important for regulating reproductive function. Additionally, it appeared that there was sufficient yearly variation in the timing of breeding to suggest that other environmental cues may also be important for regulating adjustments in reproductive development and regression. Captive populations of Z. l. oriantha showed strong responses in gonadal development following transfer to longs days (15L 9D) and low temperature (5 degrees C) slowed down photoperiodically induced gonadal growth and subsequent regression, in both males and females. High temperature of 30 degrees C tended to accelerate gonadal development and regression whereas gonadal development was intermediate in a group exposed to 20 degrees C. Prior exposure to these temperature regimes while on short days (9L 15D) had no effect on body mass, fat, or plasma levels of luteinizing hormone (LH) and thyroid hormones. Curiously there was no effect of temperature on photoperiodically induced rises in LH in either sex despite marked effects on gonadal growth. Brood patch development was also enhanced in females exposed to 30 degrees C. Corticosterone levels measured in a subset of plasma samples from this experiment indicated no effect of temperature suggesting that the retarded gonadal development at 5 degrees C was not a result of thermal stress. Although there was a robust effect of photostimulation on thyroid hormone levels in blood of both sexes, temperature treatment had no effect on tri-iodothyronine (T3) concentrations. However, plasma levels of thyroxine (T4) were lower initially at 5 degrees C versus 20 and 30 degrees C treatments. This may be related to the protracted gonadal cycle at 5 degrees C versus the truncated gonadal cycle at 30 degrees C. Molt score, an indication of post-reproductive state and onset of photorefractoriness, was delayed in birds exposed to 5 degrees C. Body mass, and to a lesser extent fat score, tended to be lowest in birds exposed to 5 degrees C compared with those at 20 and 30 degrees C. These results demonstrate that ambient temperature significantly affected photoperiodically induced gonadal development and regression in these birds. The endocrine mechanisms underlying these effects require further study.

Aspects of circulatory physiology of montane and lowland birds.

Abstract o 1. Weights of body, heart, and lungs, tarsometatarsal lengths, and several hematological parameters (red blood cell count, hemoglobin concentration, and hematocrit) were measured in closely related birds at four montane and lowland locations. 2. Lung and heart weights of highland birds average 37 and 11% higher, respectively, than those of lowland individuals of similar size. Average hematological parameters are 23% (red blood cell count), 17% (hemoglobin concentration), and 7% (hematocrit) higher in the former animals than in the latter. 3. Pine siskins (Spinus pinus) captured in midwinter in Michigan are the only birds that have significantly elevated blood properties over those of their montane relatives. A comparison of blood values of American goldfinches (S. tristis) during summer and winter in Michigan reveals an increase in blood characteristics of winter individuals similar in extent to that of winter pine siskins. This raises the possibility that the blood parameters of high altitude birds reflect adjustments in oxygen carrying capacity to meet the thermogenic demands as well as hypoxia characterizing cold montane habitats. 4. Birds that migrate seasonally between high and low altitude do not differ in any of the morphological or vascular characteristics analyzed from those of individuals residing continuously in the mountains. 5. Since the physiological characteristics of montane birds probably reflect responses to multiple demands of the physical environment, identification of the responses specific to hypoxia associated with the altitudes at which these birds live will require further study.

Stimulatory effects on the reproductive axis in female songbirds by conspecific and heterospecific male song.

Courtship vocalizations of male songbirds can profoundly enhance the reproductive physiology and behavior of conspecific females. However, no study has fully investigated the selectivity of conspecific song effects on reproductive development in birds. We studied the effects of conspecific and heterospecific song on reproductive development in domesticated (canaries) and wild songbirds (song sparrows). As expected, conspecific song enhanced follicular development. Unexpectedly, however, birds exposed to heterospecific song also underwent enhanced follicular development (compared to birds exposed to no song); conspecific and heterospecific songs were equally effective in enhancing ovarian development. In canaries exposed to 18L:6D, conspecific song induced oviposition earlier and at a greater frequency than in heterospecific and no song groups, with the fewest eggs being laid in the no song group. These results indicate that conspecific and heterospecific male song can enhance reproductive activity in female songbirds. Whether or not activation of the reproductive axis in female songbirds by heterospecific song occurs in the wild remains unclear. It is also unclear as to whether the ability of the reproductive axis to respond to heterospecific song performs a specific function, or whether it is simply a consequence of greater selection pressure acting upon behavioral responses to song.

Photoperiodically induced ovarian growth in the white-crowned sparrow (Zonotrichia leucophrys gambelii) and its augmentation by song

1. 1. Ovarian growth rates were measured in White-crowned Sparrows exposed to various photoperiods and to tape-recorded male song. 2. 2. The minimum (threshold) photoperiod necessary to induce ovarian growth was slightly longer than 11L:13D. Growth rate increased linearly thereafter until the photoperiod was 16L:8D. Growth was greatest under conditions of continuous light (LL). 3. 3. The addition of song to photoperiod treatments did not affect the growth induction threshold but song did augment the rate of ovarian development when regimes were 12.5L:11.5D and 14L:10D. 4. 4. Under natural conditions song may be an important stimulus to ovarian development on both the wintering and breeding grounds.

Effects of Sex and Birth Date on Premigration Biology, Migration Schedules, Return Rates and Natal Dispersal in the Mountain White-Crowned Sparrow

A long term mark-recapture study was conducted on a montane population of migratory sparrows to determine interannual variations in numbers of young produced, the behavior and physiology of young birds once independent from parental care, and their return in subsequent years as a function of sex, calendar date, birth date, and birth location. Measures of natal dispersal distance and subsequent involvement in breeding were also obtained. Sex ratio of nestlings was 1:1 but 28% more independent juvenile males than females were trapped. Return rates were 6.8% for nestlings and 12.2% for independent juveniles and were higher in males than in females in both groups. Numbers of young trapped on the study area showed a three-fold interannual variation which was attributed to the impact of snowpack on reproductive success. Following completion of molt juveniles fattened quickly and departed on migration in late September but not before most had traveled some distance from their birth site. Date of arrival on the breeding grounds from wintering quarters was not appreciably affected by age (1-year olds vs. 2-year olds +) and although males tended to arrive ahead of females, the sexes overlapped greatly in their schedules. Return rates of yearlings, among those handled as independent juveniles, were about 8 to 15%; no matter the sex or the duration they were known to linger as juveniles on a study area meadow, except for a group of males that stayed for more than four weeks. Members of this group were highly philopatric, having a return rates of 28.5%. Time spent on the same meadow as a juvenile did not affect the ability of either sex subsequently to attain breeding status although more yearling females than males were known breeders, 62.1% vs. 43.6%. The majority of yearlings (recruits) appearing on the study area each year were captured for the first time as adults. This group, called adult recruits, comprised 68.7% of the yearling males and 82.5% of the yearling females. In females, adult recruits were as successful at attaining breeding status as were juvenile recruits (birds captured and banded on the study area previously as juveniles). In males, however, previous familiarity with the area may have enhanced success because the number of known breeders was 10% higher in juvenile recruits than in adult recruits. Return rates peaked at about 10% in both sexes born 10-20 July and decreased thereafter. This was attributed to increased mortality in late-born young due to inadequate preparation time for migration. Returns of females, but not males, also decreased in those born before 10 July. This was attributed to a propensity for postfledging dispersal distances of females to increase with available time. Natal dispersal (distance from birth nest to first nest as a yearling) was greater in females than in males but sample sizes were small and the difference was not significant. Close inbreeding was rare. It occurred once in 473 pairings. Finally, the data are incorporated into a descriptive model of natal dispersal which suggests that natal dispersal is temporally divided, one part occurring in juveniles during the postfledging period and the other in yearlings soon after arrival at potential breeding areas. Natal dispersal distances tend to be greater and apparent philopatry lower in females than in males because females are more vagile as juveniles and they establish larger home ranges which they may enlarge even further as yearlings while in the process of choosing breeding sites. Current hypotheses concerning sexual asymmetries in dispersal are considered and expanded upon, a key addition being the proposal that intrasexual competition in females may contribute importantly to dispersal patterns in migratory passerines.

Postfledging dispersal, habitat imprinting, and philopatry in a montane, migratory sparrow

Movements of White-crowned Sparrow Zonotrichia leucophrys oriantha fledglings in relation to their nest sites were measured for 37 broods on a large subalpine meadow, Tioga Pass Meadow (TPM). Broods were split between the parents upon fledging and the resulting family subunits appeared to move independently. Three weeks after fledging and about when young were becoming independent, family subunits were located a mean distance of 200 m from their nests. Additional data on movements of independent juveniles were obtained from a trapline on TPM in which we captured individuals originally banded in their nests either at TPM (locals) or at other sites (nonlocals) up to 5 km away from TPM but within the same watershed. Although some siblings traveled together at least until fall migration, considerable dissolution of families occurred at 35 to 40 days of age. At that same age, nonlocal juveniles began to appear in the TPM trapline. First captures of known age nonlocals of both sexes from within a 5 km radius of TPM peaked at age 50-55 days. The majority of nonlocals were probably more than 60 days of age upon arrival at TPM, however, and we assumed that they probably had also traveled more than 5 km from their natal sites.

Sexual dimorphism in egg size and implications regarding facultative manipulation of sex in Mountain White-crowned sparrows

The volume of male eggs in Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha) was slightly larger than that of female eggs in every year of a 5-year study. The volume difference was highly significant (P < 0.01) when data for all years were combined. Male nestlings also grew faster than female siblings. Natural history observations on adults suggest that reproductive success is more variable in male than in female Whitecrowned Sparrows. Taken together, these data are consistent with facultative manipulation of offspring sex as proposed under the Trivers and Willard hypothesis (1973). In avian species it may be necessary when testing this hypothesis to consider the relationship between sex and parental investment for individual offspring rather than focusing solely on differences between broods.

Reproductive Cycle and Nesting Success of the Mountain White-Crowned Sparrow (Zonotrichia leucophrys oriantha) in the Central Sierra Nevada

Mexico (AOU 1957). Apparently, the Rocky Mountains and the Sierra Nevada provide the greatest amount of habitat suitable for reproduction, but the Cascades and small ranges within the Great Basin also harbor breeding populations (Banks 1964). The habitat elements essential for reproductive success include grass, bare ground, dense shrubs or scrub conifers, water, and on the periphery, tall conifers (DeWolfe and DeWolfe 1962). These elements are contained in certain mountain meadows that are often