Phenology effects on productivity and hatching-asynchrony of American kestrels (Falco sparverius) across a continent

Climate-driven advances in spring can result in phenological mismatch between brood rearing and prey availability and consequently cause decreased productivity in birds. How consequences of mismatch vary across species' ranges, and how individual behavior can mitigate mismatch effects is less studied. We quantified the relationship between phenological mismatch, productivity, and behavioral adaptations of American kestrels (Falco sparverius) across their breeding range in the United States and southern Canada. We obtained phenology and productivity data using nest observations from long term nest box monitoring, remote trail cameras, and community-scientist based programs. We collected data on parental incubation behavior and hatch asynchrony using trail cameras in nest boxes. Kestrels that laid eggs after the start of spring had higher rates of nest failure and fewer nestlings than earlier nesters, and effects of mismatch on productivity were most severe in the Northeast. In contrast, kestrels in the Southwest experienced a more gradual decline in productivity with seasonal mismatch. We attribute the effect of location to the growing season and temporal nesting windows (duration of nesting season). Specifically, resource availability in the Northeast is narrow and highly peaked during the breeding season, potentially resulting in shorter nesting windows. Conversely, resource curves may be more prolonged and dampened in the Southwest, and growing seasons are becoming longer with climate change, potentially resulting in longer nesting windows. We found that the onset of male incubation was negatively associated with lay date. Males from breeding pairs that laid eggs after the start of spring began incubation sooner than males from breeding pairs that laid before the start of spring. Early-onset male incubation was positively associated with hatching asynchrony, creating increased age variation in developing young. In sum, we demonstrate that American kestrels are vulnerable to phenological mismatch, and that this vulnerability varies across space. Northeastern populations could be more vulnerable to mismatch consequences, which may be one factor contributing to declines of kestrels in this region. Also, we demonstrate early onset of incubation as a potential adaptive behavior to advance average hatch date and spread out offspring demands, but it is unknown how impactful this will be in mitigating the fitness consequences of phenology mismatch.

Variation in incubation length and hatching asynchrony in Eastern Kingbirds: Weather eclipses female effects

Incubation length and hatching asynchrony are integral elements of the evolved reproductive strategies of birds. We examined intra- and interpopulation variation in both traits for Eastern Kingbird (Tyrannus tyrannus) populations from New York (NY), Kansas (KS), and Oregon (OR) and found that both incubation length and hatching asynchrony were not repeatable among females, after controlling for a repeatable trait, clutch size. Instead, incubation length and clutch size were influenced by ambient temperature and precipitation. Incubation length exhibited the same median (15 days) and range (13–17 days) at all sites. Model selection results indicated that incubation periods for the smallest and largest clutches were longer in NY than KS when rain was frequent throughout incubation, in replacement nests, and likely when ambient temperatures were low during egg-laying. Full hatching usually required 2 days (but up to 3), with synchronous hatching associated with small clutch sizes, short incubation periods, frequent rain during the egg-laying period, and low ambient temperatures during the first half of incubation. Nestling starvation was uncommon (5–9% of nestlings monitored) and not associated with greater hatching asynchrony. These results indicate that while clutch size, a repeatable female trait, contributed to variation in incubation length and hatching asynchrony in Eastern Kingbirds, weather was a greater source of variation, especially for incubation length.

Incubation Onset Influences Maternal Investment in Eggs and Egg and Nestling Survival in an Altricial Bird

Many birds initiate incubation before clutch completion which results in asymmetric survival of eggs and nestlings within the clutch. When parents start incubating before clutch completion, low survival is expected of the nestlings hatched from eggs laid after the onset of incubation due to hatching asynchrony. Conversely, eggs laid before the onset of incubation may have a lower survival because of extrinsic factors (e.g. ambient temperature and microbial infection). Many studies investigating the allocation of parental investment have hypothesized two different strategies wherein parents allocate investment that favors eggs/nestlings with high survival prospects or compensates for the disadvantages of eggs/nestlings with low survival prospects. Although birds could take different strategies based on incubation onset within the same breeding attempt, this idea has never been tested. We conducted an observational study to investigate the effects of incubation onset on the survival of eggs laid before and after incubation onset and parental egg allocation in the altricial wryneck Jynx torquilla. We found that survival decreased in the eggs laid earlier or later than the day of incubation onset within the clutch. Because egg volume increased with laying sequence, egg volume and the survival of eggs laid before incubation onset were positively associated, whereas egg volume and the survival of eggs laid after incubation onset were negatively associated. Furthermore, late-hatching nestlings grew to similar weights to early-hatching nestlings. These suggest that females proportionately invested in egg size before incubation onset, but that investment in egg size after incubation onset was compensatory. Our observational study proposes a possibility that female wrynecks adopt two different investment strategies before and after incubation onset during a breeding attempt.

Parental feeding preferences rather than sibling competition determine the death of smaller nestlings in asynchronous broods

Hatching asynchrony is a reproductive tactic that, through the creation of competitive hierarchies among offspring, allows parents for a quick adjustment of brood size via the death of smaller nestlings. This strategy is considered to be adaptive in case of unpredictable and/or poor environments in which it would guarantee that at least larger nestlings will fledge. Brood reduction is the usual outcome in asynchronously hatched broods since first-hatched nestlings are larger and get a disproportionately larger share of the food delivered by parents, often leading the youngest nestling to starve to death soon after hatching. However, we still do not know the proximate mechanisms of such brood reduction. One possibility is that the smallest nestling is not fed because larger nestlings outcompete it, which implies that nestlings control resource allocation. Alternatively, parents might actively ignore the persistent begging from their smallest nestling, which would involve that parents control food allocation. To determine whether parents or nestlings ultimately induce brood reduction in this situation, we experimentally created asynchronous broods of Eurasian blackbird (Turdus merula) nestlings and quantified food allocation by parents in two different situations: when sibling competition was allowed and, alternatively, when competition was prevented by physically separating nestlings within the nests by using wooden barriers. Our results showed that experimentally introduced smaller nestlings received less food than their larger nestmates both when competition among nestlings was allowed and when it was prevented. When adult males and females are considered separately, males fed the smallest nestling less often regardless of whether sibling competition was allowed or not, but adult females showed no differences. We can conclude that the smallest nestling starves mainly because parents actively ignore its begging. The higher competitive ability of the larger nestlings seem to have little effect given that although the smallest nestling is fed at a higher rate when physical interactions are prevented by the wooden barrier than when not, this difference is not significant. These findings suggest that parents rather than nestlings have the main control over food allocation.

Phenological Mismatch is Correlated with Fitness Outcomes and Adaptive Behavior in a Generalist Avian Predator Distributed Across North America

Climate-driven advances in the start of spring may result in a phenological mismatch between peak-prey abundance and the breeding season of secondary consumers. Phenological mismatch has been well-studied in insectivorous birds for which reproductive productivity is strongly linked to caterpillar abundance. The effects of mismatch on the productivity of dietary generalists, that forage on several types of prey, are less well-understood. Further, few studies have addressed questions about the effects of mismatch on survival, an important component of fitness that can be affected by breeding in sub-optimal conditions. We examined the relationship between phenological mismatch and fitness for a widespread generalist raptor, the American kestrel (Falco sparverius). In the first chapter, we collected productivity data from nest observations across the contiguous US and southern Canada and quantified phenological mismatch on each nest as the difference in days between the start of spring and clutch initiation. Then, we examined the relationship between mismatch, location, and productivity. Also, we investigated whether incubation behavior leading to hatching-asynchrony was related to phenological mismatch. Kestrels that laid eggs after the start of spring had fewer nestlings and higher rates of nest failure compared to kestrels that laid eggs before the start of spring. The strength of the mismatch effect depended on location. In the northeast, the number of fledglings per brood and rates of nest success were high for pairs nesting before the start of spring, but the effect of phenological mismatch was strongest here, with rapid declines in nest success associated with mismatch. Whereas, in the xi southwest, early-laying pairs had lower productivity and success relative to the northeast, but the effects of phenological mismatch were not as strong as the northeast. The effect of location is likely related to climatic constraints on the growing season and the time window for kestrel breeding that are becoming stronger in the northeast and weaker in the southwest. The timing of male incubation behavior was associated with hatching asynchrony, and males breeding after the start of spring were more likely to initiate incubation early as opposed to males breeding before the spring index date, suggesting that hatching asynchrony is a possible mechanism to cope with phenological mismatch. In the second chapter, we investigated the relationships between phenological mismatch and survival using mark-and-recapture data from two distinct, long-term study sites in Idaho and New Jersey where kestrel exhibit difference migration strategies. We created a multistate mark-recapture models to estimate the annual survival of adult (afterhatch- year) and juvenile (hatch-year or yearling) kestrels. For the multistate framework, we categorized the phenological mismatch of nests at each site “earlier” or “later” relative to the yearly median difference in days between clutch initiation date and the start-of-spring date, which was estimated at each nest box location. In addition, we included covariates for nesting success, sex, and minimum winter temperature anomaly in our survival models. Mismatch was associated with the survival of kestrels that produced young; however, the direction of this effect differed between populations. In Idaho, successful kestrels had higher survival when they bred “earlier” rather than “later.” In New Jersey, successful kestrels had higher survival when they bred “later” rather than “earlier." Differences in survival between sites may reflect differences in seasonality, climate change patterns, or consequences of migration strategies. For partially migrant xii populations (i.e, Idaho kestrels), mismatch may rapidly drive directional selection for birds to breed earlier by favoring survival and productivity, but for fully migrant populations (i.e., New Jersey) that have a limited window of time to reproduce, mismatch may create trade-offs between reproduction and survival. Mismatch did not affect the survival of adult birds with failed nests, and there was no difference in survival between hatch-year birds produced from “earlier” or “later” nests. In Idaho, males had higher survival rates than females and warmer winter temperatures positively correlated with survival in all age and sex classes. In New Jersey, sex and winter temperature did not explain survival. In sum, we found negative consequences of phenological mismatch on the fitness of American kestrels, generalist predator. For both productivity and survival, the effect of mismatch was more severe for kestrels in the northeast, where the breeding season is shorter and kestrels more migratory when compared to the west. These results demonstrate that duration of breeding season is an important factor to consider when assessing vulnerability to climate change, and that a generalist diet does not ensure resilience to phenological mismatch.

Co-evolved maternal effects selectively eliminate offspring depending on resource availability

Many plants and animals adaptively downsize the number of already-produced propagules if resources become insufficient to raise all of them. In birds, mothers often induce hatching asynchrony by incubating first eggs before last eggs are laid, creating an age/size hierarchy within broods which selectively eliminates the smallest chicks in poor food conditions. However, mothers also deposit more testosterone into late-laid eggs, which boosts competitive abilities of younger chicks, counteracts the competitive hierarchy, and ostensibly creates a paradox. Since testosterone also carries costs, we hypothesized that benefits of maternally deposited testosterone outweigh its costs in good food conditions, but that testosterone has a net detrimental effect in poor food conditions. We found experimental evidence that elevated maternal testosterone in the egg caused higher chick mortality in poor food conditions but better chick growth in good food conditions. These context-dependent effects resolve the paradox, suggesting co-evolution of two maternal effects, and explain inconsistent results of egg hormone manipulations in the literature.

Benefits of an anti-parasite treatment are influenced by within-brood size variation in Tree Swallows (Tachycineta bicolor)

In all animals, susceptibility to parasites can differ among individuals. Young, nest-bound birds are exposed to a diversity of nest-dwelling ectoparasites that typically feed on their blood. Within broods, hatching asynchrony creates size hierarchies that result in morphological and physiological variation among nest mates, and susceptibility to parasites also may vary predictably with this size hierarchy. Our objective was to use a broad-spectrum, anti-parasite drug, ivermectin (IVM), to treat individual nestling Tree Swallows (Tachycineta bicolor) and assess how nestling susceptibility to parasites varied both within and among broods. Broods were either assigned to an IVM group, where half of the nestlings in a brood received IVM injections and half received control injections of pure sesame oil, or to a control group, where all nestlings received oil injections. We found that the IVM treatment reduced parasite loads for broods as a whole, thereby benefiting all nestlings in IVM broods and suggesting our treatment resulted in herd immunity. Specifically, nestlings from IVM broods had higher hemoglobin concentrations, regardless of whether they received injections with IVM or oil, and greater fledging success, than nestlings from control broods. On the contrary, IVM treatment did not strongly affect nestling morphology, with only marginal effects on the growth rate of ninth primary feathers, and the effects of the treatment on 2 other morphological traits depending on temporal factors. Variation in size within broods, however, influenced the chance of an individual fledging, which increased with relative size within a brood, but only under lower parasite loads (i.e. IVM broods). By experimentally manipulating nestling susceptibility to parasites, we have demonstrated variation in nestling response to an anti-parasite treatment both within and among broods, and future studies should investigate the underlying mechanism for why certain nestlings along the brood size hierarchy are more susceptible to parasites.