The integration of environmental cues can be critical to the successful timing of life cycle events. Yet, our understanding of cue integration remains limited, particularly regarding migratory timing and the integration of nonphotic cues. Here, we applied a recently proposed conceptual framework that considers both cue dependency and interactions among cues to experimentally investigate the integration of social or temperature cues with changing food availability in the context of facultative migratory timing. Furthermore, we examined circulating corticosterone as a potential endocrine mechanism for cue integration. We provided pine siskins, Spinus pinus, with either ad libitum food or a restricted diet and tested whether a short-term cold exposure or socially acquired information about food availability affected behaviour indicative of a migratory response, energetic reserves or circulating corticosterone levels. Temperature and food availability cues elicited distinct behavioural responses, as cold exposure increased nocturnal activity, while food restriction increased diurnal activity. Pine siskins integrated social and food cues in an additive manner, as diurnal activity levels were highest in food-restricted birds with neighbours that also had a restricted diet. However, pine siskins showed no physiological preparations for future migratory response following exposure to any cue combination. Furthermore, we found no evidence that changes in circulating corticosterone were a mechanism underpinning cue integration. These findings highlight the diverse modes in which proximate environmental cues are integrated to time life cycle events, which may ultimately impact organismal fitness.

The final stage of migration, when animals terminate migratory movements and transition to a more sedentary state, remains the least understood phase of migration. Whereas migrants that return to the same locations each year may use mechanisms associated with locating a specific destination, migrants with low site fidelity, such as nomadic migrants, may rely on local environmental cues to determine when to cease migratory movements. Using an experiment with captive birds, we tested whether the presence of a conspecific influences the termination of migration, indicated by changes in behaviour and physiology, in a nomadic migrant (the pine siskin, Spinus pinus ). We paired migratory birds with a non-migratory individual or left migratory and non-migratory individuals unpaired. Migratory paired birds had a significant decline in nocturnal activity immediately after pairing and activity levels remained lower two weeks later, with significant declines in energetic reserves and flight muscle size also observed. By contrast, migratory unpaired birds maintained high levels of activity and energetic reserves. These results provide evidence for a role of the social environment in migratory termination decisions. Social cues may be particularly useful in nomadic migrants, such as pine siskins, to facilitate settling in high quality, but unfamiliar, habitats.

Many organisms use environmental cues to time events in their annual cycle, such as reproduction and migration, with the appropriate timing of such events impacting survival and reproduction. As the climate changes, evolved mechanisms of cue use may facilitate or limit the capacity of organisms to adjust phenology accordingly, and organisms often integrate multiple cues to fine-tune the timing of annual events. Yet, our understanding of how suites of cues are integrated to generate observed patterns of seasonal timing remains nascent. We present an overarching framework to describe variation in the process of cue integration in the context of seasonal timing. This framework incorporates both cue dependency and cue interaction. We then summarize how existing empirical findings across a range of vertebrate species and life cycle events fit into this framework. Finally, we use a theoretical model to explore how variation in modes of cue integration may impact the ability of organisms to adjust phenology adaptively in the face of climate change. Such a theoretical approach can facilitate the exploration of complex scenarios that present challenges to study in vivo but capture important complexity of the natural world.

Physiological preparations for migration generally reflect migratory strategy. Migrant birds fuel long-distance flight primarily with lipids, but carrying excess fuel is costly; thus, the amount of fat deposited prior to departure often reflects the anticipated flight duration or distance between refueling bouts. Seasonal pre-migratory deposition of fat is well documented in regular seasonal migrants, but is less described for more facultative species. We analyze fat deposits of free-living birds across several taxa of facultative migrants in the songbird subfamily Carduelinae, including house finches (Haemorhous mexicanus), American goldfinches (Spinus tristis), pine siskins (Spinus pinus) and four different North American ecotypes of red crossbills (Loxia curvirostra), to evaluate seasonal fat deposition during facultative migratory periods. Our data suggest that the extent of seasonal fat deposits corresponds with migratory tendency in these facultative taxa. Specifically, nomadic red crossbills with a seasonally predictable annual movement demonstrated relatively large seasonal fat deposits coincident with the migratory periods. In contrast, pine siskins, thought to be more variable in timing and initiation of nomadic movements, had smaller peaks in fat deposits during the migratory season, and the partial migrant American goldfinch and the resident house finch showed no peaks coincident with migratory periods. Within the red crossbills, those ecotypes that are closely associated with pine habitats showed larger peaks in fat deposits coincident with autumn migratory periods and had higher wing loading, whereas those ecotypes associated with spruces, Douglas-fir and hemlocks showed larger peaks coincident with spring migratory periods and lower wing loading. We conclude that population averages of fat deposits do reflect facultative migration strategies in these species, as well as the winter thermogenic challenges at the study locations. A difference in seasonal fattening and wing loading among red crossbill ecotypes is consistent with the possibility that they differ in their migratory biology, and we discuss these differences in light of crossbill reproductive schedules and phenologies of different conifer species.

Like many organisms, birds exhibit daily (circadian) and seasonal biological rhythms, and within populations both daily and seasonal timing often vary among individuals. Because photoperiod interacts with the circadian rhythms of many organisms to induce seasonal changes in behaviour and physiology, it is hypothesized that differences in daily timing, called chronotypes, underpin differences among individuals in the timing of seasonal events. For seasonal events stimulated by increasing daylength, this hypothesis predicts a positive relationship between the timing of daily and seasonal activities of individuals, with advanced chronotypes expressing events earlier in the year. The few previous tests of this hypothesis have focused on seasonal reproductive timing in birds. However, the hypothesis predicts that this relationship should extend to other photoinduced seasonal events. Therefore, we tested whether variation in chronotype was associated with variation in spring migratory timing in a captive songbird model, the pine siskin (Spinus pinus). We found that pine siskins expressing migratory restlessness exhibited repeatable chronotypes in their timing of nocturnal activity. Further, chronotype was significantly associated with the onset date of migratory behaviour, consistent with the hypothesized relationship between chronotype and seasonal timing.

While obligate migrants time their movements to respond to predictable changes in the environment, facultative migration is characterized by more variable movements that are driven by unpredictable changes in resource availability. The proximate cues that trigger facultative migrations and the endocrine mechanisms involved in these responses remain poorly understood, though corticosterone may be a key mediator of facultative migration due to its effects on activity and metabolic processes. We conducted experiments in the fall and spring to examine the response of pine siskins (Spinus pinus), a facultative migrant, to a two-stage food restriction. Our goals were to examine whether declining food availability stimulated behavioral and/or physiological changes consistent with a migratory response, whether anticipatory behavioral and physiological adjustments occurred when birds were initially presented with changing food availability, and if observed changes corresponded to changes in circulating corticosterone levels. We found no evidence of preparatory physiological changes for migration, but food-restricted birds in the spring had increased daytime activity indicative of a migratory response. Corticosterone increased at each stage of the restriction and the change in corticosterone corresponded to the magnitude of decline in body condition. Increased corticosterone was also correlated with larger increases in activity during the initial stage of food restriction, but only during the spring, when birds also displayed higher levels of initial body condition. These results suggest that initial energetic state and corticosterone response may interact to determine an individual's behavioral and physiological response to declining food availability and ultimately the facultative migratory response.

In many monogamous species, females behave aggressively toward other females, as they may threaten their exclusive access to paternal resources. However, in species with a high degree of breeding asynchrony not all females are true reproductive rivals. Female ornamentation that advertises sexual receptivity is a possible mechanism whereby parental females could assess the potential threat of rival females and attack only those which could challenge their mating status. Convict cichlids (Amatitlania siquia) are sexually dichromatic, monogamous fish that exhibit biparental care. Females develop gold ventral coloration when reproductively receptive and actively court males. We presented breeding pairs of convict cichlids with confined conspecific females to investigate whether parental response was affected by the intruder's reproductive status. We also investigated whether differences in partner quality within breeding pairs mediated the response to intruders, as mate quality influences both intra- and intersexual dynamics in many monogamous species, We found parental females responded more aggressively to reproductive intruders and that parental females mated to high quality males decreased the aggression they directed at general brood predators during the reproductive intruder presentation. Contrary to our predictions, we also found that males behaved aggressively toward reproductive intruders, particularly when paired with small females. Our results indicate that both parents engage in pair-bond defense and that differences in partner quality determine the level of aggression directed at extra-pair reproductive females. These findings suggest that when bipa rental care greatly increases offspring survival, reproductive success for both sexes may be maximized by cooperation and coordination, rather than conflict.

The transition to a migratory state involves coordinated changes in physiology and behaviour. In species with regular, predictable (obligate) migrations, increasing day length triggers the expression of a spring migratory state and androgens play an important role in stimulating its development. By contrast, we know little about the environmental cues and endocrine mechanisms that regulate migration in species with less predictable (facultative) migrations. Here, we tested whether photoperiod stimulates a migratory state in a facultative nomadic migrant, the pine siskin (Spinus pinus). We exposed wintering birds to either a naturally increasing or short-day photoperiod and measured physiological and behavioural changes indicative of a migratory state. We also examined changes in circulating hormones that may play a role in the migratory transition. Natural-day, but not short-day, birds displayed physiological preparations for migration, including increases in fat deposition, and showed increased levels of migratory restlessness. We found no evidence for a role of corticosterone in the migratory transition, but testosterone may be important. This study is the first experimental test of the role of photoperiod in regulating facultative migration and demonstrates that the predictive cue used by many obligate migrants to time spring migration is also important in a facultative migrant.

Positive differential allocation predicts that females will increase their parental investment when mated to high-quality males due to the benefits that these males provide to offspring. Differential allocation predicts trade-offs between current and future reproduction, but investment trade-offs can also occur within a breeding event for species that provide parental care. However, differential allocation studies that have examined investment in both gametic effort and parental care remain rare and have only been conducted in birds. Fish may have more conservative investment strategies due to the large effect that current reproductive investment has on future reproductive potential. We examined gametic effort and parental care in the convict cichlid, Amatitlania siquia, a biparental fish, in response to male size, a proxy for quality. We randomly mated females to either large or small males and measured fecundity and parental behaviour of both parents. Females mated to large males did not increase their fecundity, but did provide more parental care to offspring during the larval stage, which is when male desertion in natural populations is most likely. Male size did not predict the amount of paternal care provided to offspring during the larval stage. Female condition at the end of the breeding event was negatively associated with higher fecundity and level of maternal care provided. The observed pattern of maternal investment suggests that females limit costs to future reproduction by limiting their increased investment to only one component of their reproductive effort. The increase in maternal investment during the larval stage may also serve to stabilize the pair bond and ensure continued male care, which ultimately increases the reproductive success of both parents. These results highlight important taxonomic and parental care pattern differences and broaden our understanding of the factors that influence differential allocation in diverse taxa.

The vast majority of what is known about migration comes from studies of obligate migration-regular and predictable "to-and-fro" seasonal movements. Much less is known about facultative migration, which occurs less predictably in space and time. Here, we use pine siskins (Spinus pinus), nomadic and irruptive finches, as a captive model to advance our understanding of facultative migration. Building on approaches used to study obligate migration, we investigate whether pine siskins exhibit migratory restlessness in captivity, similar to obligate migrants, and whether pine siskins express a seasonal pattern of migratory behavior. Pine siskins exhibited nocturnal migratory restlessness that was remarkably similar to that in obligate migrants; it was characterized by jumping and wing whirring behavior that was preceded by a period of quiescence. This behavior was expressed seasonally, peaking in the spring, when free-living pine siskins frequently migrate. As in many obligate migrants, migratory restlessness in pine siskins coincided with elevated body condition and showed minimal overlap with body molt. Although pine siskins are both nocturnal and diurnal migrants, patterns of diurnal activity did not mirror patterns of nocturnal activity and were more difficult to interpret. Free-living pine siskins frequently migrate in the fall, but in captivity showed no clear pattern of fall migratory restlessness. We suggest that this could reflect a relatively predictable and programmed spring migration and a more facultative fall migration.Significance statementMigrations are undertaken by a diverse array of animals and are important to the functioning of many ecosystems. The goal of this study was to shed light on an important, but understudied form of migration: facultative migration. Facultative migrations occur less predictably than the better-studied "to-and-fro" seasonal migrations of obligate migrants. Using a songbird, the pine siskin, as a model, we sought to determine if pine siskins express a migratory state in captivity and whether pine siskins exhibit a seasonal pattern of migratory behavior. We found that pine siskins showed behavioral and physiological changes associated with a migratory state, which were remarkably similar to those in obligate migrants. Pine siskins showed seasonal changes in migratory behavior in the study that suggest that this species has a predictable and programmed spring migratory period and a much more flexible fall migration.