Personality (consistent differences between individuals in behavior) and plasticity (changes within individuals in behavior) are often viewed as separate and opposing phenomena. We tested this idea by analyzing parental care reaction norms in a bird that exhibits biparental care. Personality in provisioning behavior existed (r(ic) = 0.11) and persisted despite being reduced after accounting for individual differences in environment. Plasticity was also evident and differed between the sexes. Male visit rate was associated with changes in brood size and time of day, but female visit rate was associated with changes in nestling age and date. In both sexes changes in visit rate were positively correlated with changes in their partner's visit rate. Both sexes also exhibited multidimensional reaction norms; interaction terms revealed that within-individual visit rates increased more steeply with brood size when nestlings were older, and the effect of the partner's visit rate was sensitive to variation in date, precipitation, and the focal bird's age. Individuals also varied in how they responded (reaction norm slope) to changes in nestling age and partner visits. Moreover, parental personality was interdependent with individual plasticity in several ways. Individuals of both sexes with a high visit rate also responded more positively to changes in nestling age, and males also showed this pattern with changes in partner visit rate. Explicit use of the behavioral reaction norm integrated personality and plasticity, revealed that these are not opposing concepts, and stimulated new hypotheses about sexual conflict over care and provisioning as a life-history trait.

Brood parasitism by brown-headed cowbirds (Molothrus ater) reduces reproductive success in many passerines that nest in fragmented habitats and ecological edges, where nest predation is also common. We tested the hypothesis that parasitism and predation are often linked because cowbirds depredate nests discovered late in the host's nesting cycle to enhance future opportunities for parasitism. Over a 20-year study period, brood parasitism by cowbirds was a prerequisite to observing marked inter-and intraannual variation in the rate of nest failure in an insular song sparrow (Melospiza melodia) population. Nest failure increased with the arrival and laying rate of cowbirds and declined when cowbirds ceased laying. The absence or removal of cowbirds yielded the lowest nest failure rates recorded in the study. The absence of cowbirds also coincided with the absence of an otherwise strong positive correlation between host numbers and the annual rate of nest failure. Host numbers, cowbird parasitism, and nest failure may be correlated because cowbirds facilitate nest failure rather than cause it directly. However, an experiment mimicking egg ejection by cowbirds did not affect nest failure, and, contrary to the main prediction of the predation facilitation hypothesis, naturally parasitized nests failed less often than unparasitized nests. Higher survival of parasitized nests is expected under the cowbird predation hypothesis when female cowbirds defend access to hosts because cowbirds should often depredate unparasitized nests but should not depredate nests they have laid in. Where female cowbirds have overlapping laying areas, we expect parasitized nests to fail more often than others if different cowbirds often discover the same nests. We suggest that nest predation by cowbirds represents an adaptation for successful parasitism and that cowbirds influence host demography via nest predation.Brown-headed cowbirds (Molothrus ater) are brood parasites of over 220 species of North American passerines and are thought to reduce host productivity, mainly because adults remove host eggs and nestlings usurp host parental care (1, 2). Nest predation also reduces productivity in cowbird hosts, and parasitism and predation have been jointly linked to population declines in several species, including threatened and endangered neotropical migrants (3-9). This link is currently believed to be a coincidental result of the preferences of nest predators and cowbirds for fragmented habitats and ecological edges (10-15).In contrast, we have argued that cowbirds regularly depredate nests that are discovered too late in the host's nesting cycle to be suitable for parasitism, because this enhances future laying opportunities (16,17). Nest predation may also improve a cowbird's ability to synchronize its own laying with that of its host. This is because most common hosts are territorial and, barring renesting dispersal, will renest within 5-7 days of nest failure (1,8,(18)(19)(20)(21)(22)(23). The potential benefits of nest predatio...

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Phenotypic plasticity is a widespread phenomenon and may have important influences on evolutionary processes. Multidimensional plasticity, in which multiple environmental variables affect a phenotype, is especially interesting if there are interactions among these variables. We used a long-term data set from House Sparrows (Passer domesticus), a multi-brooded passerine bird, to test several predictions from life-history theory regarding the shape of optimal reaction norms for clutch size. The best-fit model for variation in clutch size included three temporal variables (the order of attempt within a season, the date of those attempts, and the age of the female). Clutch size was also sensitive to the quadratics of date and female age, both of which had negative coefficients. Finally, we found that the relationship between date and clutch size became more negative as attempt order increased. These results suggest that female sparrows have a multidimensional reaction norm for clutch size that matches predictions of life-history theory but also implicates more complexity than can be captured by any single model. Analysis of the sources of variation in reaction norm height and slope was complicated by the additional environmental dimensions. We found significant individual variation in mean clutch size in all analyses, indicating that individuals differed in the height of their clutch size reaction norm. By contrast, we found no evidence of significant individual heterogeneity in the slopes of several dimensions. We assess the possible mechanisms producing this reaction norm and discuss their implications for understanding complex plasticity.

A common life history pattern in many organisms is that reproductive success increases with age. We report a similar pattern in house sparrows Passer domesticus , older individuals performed better than yearlings for most measures of reproductive success. Older males and females began breeding earlier in a given season and fledged more young than their yearling counterparts. Individual males also fledged more young in their second breeding season than they did in their first, but individual females did not show consistent improvement in reproductive success from year one to two. A path analysis indicated that age in both sexes acted primarily through the timing of breeding; earlier nesters laid more eggs and hence fledged more young but did not have more nesting attempts. We tested whether the increased reproductive success with age arose from high quality individuals surviving to be older (selection hypothesis). In contrast to the main prediction of this hypothesis that reproductive success and survival should be positively related, we found that survival from one year of age to two years of age was negatively related to reproductive success in the first year for males and females combined. Additionally, individuals that survived to breed as two-year-olds did not differ in total young fledged in their first year from those that did not survive to their second season of breeding. Our results indicate that fledgling production increases with age due to improvements in timing of breeding, particularly in females, and not because of the loss of poor breeders or increased output. Mechanisms producing age-related differences in timing of breeding warrant further study.

I studied nest defense in 53 female Song Sparrows (Melospiza melodia) on Mandarte Island, British Columbia, between 1994 and 1995. A total of 75 trials was conducted by presenting a Northwestern Crow (Corvus caurinus) and a Dark-eyed Junco (Bunco hyemalis) mount near nests during incubation. Female Song Sparrows spent more time perched close to the crow than the junco mount, and alarm called more often in the presence of the crow. Female responses were unrelated to their age and were not correlated with the subsequent success of their nests. Individual female responses to the junco, but not the crow mount, in 1994 were significantly positively correlated with their responses in 1995. Individual responses to a predator model may not be consistent from year to year because of experiences with live predators or the inherent variability of nest defense behavior.

Early arrival at the breeding grounds for migratory birds is associated with greater reproductive success. According to the condition-dependent arrival hypothesis, only those individuals in superior physiological condition are able to bear the costs (e.g., poor environmental conditions, limited food availability) of early arrival. Condition has usually been measured in terms of energy reserves or mass but other physiological measures of condition such as hematocrit and immune function have been gaining attention. We examined several measures of condition and their association with date of first capture in Gray Catbirds (Dumetella carolinensis) arriving at breeding grounds in northeastern Pennsylvania. Earlier arrivals had higher hematocrit and H/L ratios and lower lymphocyte counts. Arrival date was also negatively associated with fat score. Fat score was positively related to hematocrit, total number of leukocytes, and number of lymphocytes, but the other hematological parameters were not associated with traditional measures of condition (keel score, fat score, or a body condition index). Our results provide some support for the condition-dependent arrival hypothesis and suggest that there may be immunological differences between early-and late-arriving birds.

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