Latitudinal differences in timing of breeding are well documented but how such differences carry over to influence timing of events in the annual cycle of migratory birds is not well understood. We examined geographical variation in timing of events throughout the year using light-level geolocator tracking data from 133 migratory tree swallows ( Tachycineta bicolor ) originating from 12 North American breeding populations. A swallow's breeding latitude influenced timing of breeding, which then carried over to affect breeding ground departure. This resulted in subsequent effects on the arrival and departure schedules at autumn stopover locations and timing of arrival at non-breeding locations. This ‘domino effect’ between timing events was no longer apparent by the time individuals departed for spring migration. Our range-wide analysis demonstrates the lasting impact breeding latitude can have on migration schedules but also highlights how such timing relationships can reset when individuals reside at non-breeding sites for extended periods of time.

Determining how migratory animals are spatially connected between breeding and non‐breeding periods is essential for predicting the effects of environmental change and for developing optimal conservation strategies. Yet, despite recent advances in tracking technology, we lack comprehensive information on the spatial structure of migratory networks across a species’ range, particularly for small‐bodied, long‐distance migratory animals. We constructed a migratory network for a songbird and used network‐based metrics to characterize the spatial structure and prioritize regions for conservation. The network was constructed using year‐round movements derived from 133 archival light‐level geolocators attached to Tree Swallows (Tachycineta bicolor) originating from 12 breeding sites across their North American breeding range. From these breeding sites, we identified 10 autumn stopover nodes (regions) in North America, 13 non‐breeding nodes located around the Gulf of Mexico, Mexico, Florida, and the Caribbean, and 136 unique edges (migratory routes) connecting nodes. We found strong migratory connectivity between breeding and autumn stopover sites and moderate migratory connectivity between the breeding and non‐breeding sites. We identified three distinct “communities” of nodes that corresponded to western, central, and eastern North American flyways. Several regions were important for maintaining network connectivity, with South Florida and Louisiana as the top ranked non‐breeding nodes and the Midwest as the top ranked stopover node. We show that migratory songbird networks can have both a high degree of mixing between seasons yet still show regionally distinct migratory flyways. Such information will be crucial for accurately predicting factors that limit and regulate migratory songbirds throughout the annual cycle. Our study highlights how network‐based metrics can be valuable for identifying overall network structure and prioritizing specific regions within a network for conserving a wide variety of migratory animals.

The differential allocation hypothesis posits that individuals should invest in the current reproductive attempt according to the attractiveness of their mate, but studies of allocation by males when female traits are manipulated to be more attractive are lacking. In the current study, we experimentally enhanced and reduced the plumage brightness of female tree swallows (Tachycineta bicolor) relative to controls to examine whether males adjust investment in parental care according to female attractiveness, while simultaneously performing a brood size manipulation. Contrary to our predictions, we found no evidence that males provisioned nestlings according to the plumage brightness of females. However, we found that nestling quality and fledging success were lowest when female plumage brightness was reduced and brood size was enlarged. This may be due to the plumage brightness treatment influencing agonistic interactions with other females, and may suggest that plumage brightness is a signal assessed by females.

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Our understanding of the annual life-cycle movements of small migratory birds has 1 advanced rapidly with the advent of light-weight geographical positioning devices (i.e., 2 geolocators), yet the effects of geolocators on reproduction and survival have not been 3 adequately quantified. We tested for impacts of attaching a 1g geolocator (using a harness 4 around the legs and back, anterior to the tail) to adult Tree Swallows (Tachycineta bicolor) on 5 parental feeding behaviour, nestling growth and size, fledging success, and return rates 6 between 2011-2012. At one breeding site, we compared feeding visits, nestling growth, and 7 nestling size between paired nest boxes where one parent was marked at the 'geolocator' box 8 with a 'control' nest box where neither parent was marked. We detected no differences 9 between geolocator and control nests in either the frequency of feeding visits to nestlings or 10 the amount of time spent at nests. Birds marked with geolocators fed nestlings as frequently 11 as their unmarked mates. Likewise, nestlings raised at geolocator nests grew at similar rates 12 to those at control nests, and had similar structural size and body mass at fledging. At three 13 widely-separated sites across the Tree Swallow breeding range in Canada, we also found that 14 fledging success was similar for geolocator and control nests. Although we found no 15 evidence for short-term negative impacts of geolocators, the return rates of geolocator-16 marked swallows tended to be significantly lower than those of unmarked control birds. 17Thus, we found little evidence for short-term impacts of geolocators on reproduction but our 18 study does suggest that long-term impacts of geolocators could be manifested in terms of 19 lower survival, higher emigration rates, or lower breeding propensity.

Despite benefits of using light-sensitive geolocators to track animal movements and describe patterns of migratory connectivity, concerns have been raised about negative effects of these devices, particularly in small species of aerial insectivore. Geolocators may act as handicaps that increase energetic expenditure, which could explain reported effects of geolocators on survival. We tested this ‘Energetic Expenditure Hypothesis’ in 12 populations of tree swallows (Tachycineta bicolor) and barn swallows (Hirundo rustica) from North America and Europe, using measurements of corticosterone from feathers (CORTf) grown after deployment of geolocators as a measure of physiology relevant to energetics. Contrary to predictions, neither among- (both species) nor within-individual (tree swallows only) levels of CORTf differed with respect to instrumentation. Thus, to the extent that CORTf reflects energetic expenditure, geolocators apparently were not a strong handicap for birds that returned post-deployment. While this physiological evidence suggests that information about migration obtained from returning geolocator-equipped swallows is unbiased with regard to levels of stress, we cannot discount the possibility that corticosterone played a role in reported effects of geolocators on survival in birds, and suggest that future studies relate corticosterone to antecedent factors, such as reproductive history, and to downstream fitness costs.

In natural populations, epidemics provide opportunities to look for intense natural selection on genes coding for life history and immune or other physiological traits. If the populations being considered are of management or conservation concern, then identifying the traits under selection (or ‘markers’) might provide insights into possible intervention strategies during epidemics. We assessed potential for selection on multiple immune and life history traits of Arctic breeding common eiders (Somateria mollissima) during annual avian cholera outbreaks (summers of 2006, 2007 & 2008). We measured prelaying body condition, immune traits, and subsequent reproductive investment (i.e., clutch size) and survival of female common eiders and whether they were infected with Pasteurella multocida, the causative agent of avian cholera. We found no clear and consistent evidence of directional selection on immune traits; however, infected birds had higher levels of haptoglobin than uninfected birds. Also, females that laid larger clutches had slightly lower immune responses during the prelaying period reflecting possible downregulation of the immune system to support higher costs of reproduction. This supports a recent study indicating that birds investing in larger clutches were more likely to die from avian cholera and points to a possible management option to maximize female survival during outbreaks.

That ornamental traits of females can act as signals of quality has gained empirical support, but whether and how such ornaments of females mediate social interactions with conspecifics remains less clear. Female Tree Swallows (Tachycineta bicolor (Vieillot, 1808)) aggressively compete for and defend nest sites, and as such, nest site intrusions by conspecific females challenging ownership of a nest may prevent territory-owning females from dishonestly displaying ornamented plumage. We tested whether plumage brightness of female Tree Swallows influences nest site retention and reproductive success by experimentally enhancing or reducing their plumage brightness relative to controls prior to breeding. Females with reduced brightness were more likely to retain their nests sites and breed relative to control females and females with experimentally enhanced brightness. Females displaying enhanced brightness also tended to initiate clutches later than females with control and reduced brightness. Overall, lower nest site retention and reproductive success for females with enhanced brightness is consistent with social costs imposed on individuals dishonestly signalling high quality. Future studies in female birds should consider whether costs of losing a nest site to intruding conspecific females, especially in species where nest sites are limited, is a mechanism that maintains the honesty of signals of quality.