In common with many other raptors, female peregrine falcons are about 50% heavier than males. Their sexual dimorphism is thought to allow breeding pairs to exploit a wider range of prey through a division of labor: the male being able to catch more maneuverable prey species; the female capable of carrying larger ones. Given the difficulty of assessing the catch success and load carrying capacity of both sexes of falcon in the field, we here adopt a novel approach to test the division-of-labor theory by using a detailed physics-based flight simulator of birds. We study attacks by male and female peregrines on prey species ranging from small passerines to large ducks, testing how catch success relates to the flight performance of predator and prey. Males prove to be better than females at catching highly maneuverable prey in level flight, but the catch success of both sexes improves and becomes more similar when diving, because of the higher aerodynamic forces that are available to both sexes for maneuvering in high-speed flight. The higher maximum roll acceleration of the male peregrine explains its edge over the female in catching maneuverable prey in level flight. Overall, catch success is more strongly influenced by the differences in maneuverability that exist between different species of prey than between the different sexes of falcon. On the other hand, the female can carry up to 50% greater loads than the male. More generally, our detailed simulation approach highlights the importance of several previously overlooked features of attack and escape. In particular, we find that it is not the prey's instantaneous maximum centripetal acceleration but the prey's ability to sustain a high centripetal acceleration for an extended period of time that is the primary driver of the variation in catch success across species.

Infectious diseases can cause host mortality through direct or indirect mechanisms, including altered behavior. Diminished anti-predator behavior is among the most-studied causes of indirect mortality during infection, particularly for systems in which a parasite's life-cycle requires transmission from prey to predator. Significantly less work has examined whether directly-transmitted parasites and pathogens also reduce anti-predator behaviors. Here we test whether the directly-transmitted bacterial pathogen, (MG), reduces responses to predation-related stimuli in house finches (). MG causes conjunctivitis and reduces survival among free-living finches, but rarely causes mortality in captivity, suggesting a role for indirect mechanisms. Wild-caught finches were individually housed in captivity and exposed to the following treatments: 1) visual presence of a stuffed, mounted predator (a Cooper's Hawk ()) or control object (a vase or a stuffed, mounted mallard duck ()), 2) vocalizations of the same predator and non-predator, 3) approach of a researcher to enclosures, and 4) simulated predator attack (capture by hand). MG infection reduced anti-predator responses during visual exposure to a mounted predator and simulated predator attack, even for birds without detectable visual obstruction from conjunctivitis. However, MG infection did not significantly alter responses during human approach or audio playback. These results are consistent with the hypothesis that predation plays a role in MG-induced mortality in the wild, with reduced locomotion, a common form of sickness behavior for many taxa, as a likely mechanism. Our results therefore suggest that additional research on the role of sickness behaviors in predation could prove illuminating.

Most birds rely on cooperation between pair partners for breeding. In long-term monogamous species, pair bonds are considered the basic units of social organization, albeit these birds often form foraging, roosting or breeding groups in which they repeatedly interact with numerous conspecifics. Focusing on jackdaws , we here investigated 1) the interplay between pair bond and group dynamics in several social contexts and 2) how pair partners differ in individual effort of pair bond maintenance. Based on long-term data on free-flying birds, we quantified social interactions between group members within three positive contexts (spatial proximity, feeding and sociopositive interactions) for different periods of the year (non-breeding, pre-breeding, parental care). On the group level, we found that the number of interaction partners was highest in the spatial proximity context while in the feeding and sociopositive contexts the number of interaction partners was low and moderately low, respectively. Interactions were reciprocated within almost all contexts and periods. Investigating subgrouping within the flock, results showed that interactions were preferentially directed towards the respective pair partner compared to unmated adults. When determining pair partner effort, both sexes similarly invested most into mutual proximity during late winter, thereby refreshing their bond before the onset of breeding. Paired males fed their mates over the entire year at similar rates while paired females hardly fed their mates at all but engaged in sociopositive behaviors instead. We conclude that jackdaws actively seek out positive social ties to flock members (close proximity, sociopositive behavior), at certain times of the year. Thus, the group functions as a dynamic social unit, nested within are highly cooperative pair bonds. Both sexes invested into the bond with different social behaviors and different levels of effort, yet these are likely male and female proximate mechanisms aimed at maintaining and perpetuating the pair bond.

We examined variation at MHC Class IIB genes in a recently established population of dark-eyed juncos (Junco hyemalis) in a coastal urban environment in southern California, USA relative to an ancestral-range population from a nearby species-typical montane environment. The founding population is estimated to have been quite small, but we predicted that variation at the major histocompatibility complex (MHC) among the founders would nevertheless be preserved owing to the high functional significance of MHC. Previous studies of MHC in songbirds have had varying degrees of success in isolating loci, as passerines show extensive MHC gene duplication. In order to compare diversity in the two populations, we employed two published approaches to sequencing MHC Class II exon 2: direct sequencing with exon-based primers, and traditional cloning and sequencing with intron-based primers. Results from both methods show that the colonist population has maintained high levels of variation. Our results also indicate varying numbers of alleles across individuals, corroborating evidence for gene duplication in songbird MHC. While future studies in songbirds may need to take a genomic approach to fully understand the structure of MHC in this lineage, our results show that it is possible to use traditional methods to reveal functional variation across populations.

Investment in signalling is subject to multiple trade-offs that vary with life-stage, leading to a complex relationship between survival and trait expression. We show a negative relationship between survival and song rate in response to simulated territorial intrusion in male banded wrens (), and test several explanations for this association. (1) Male age failed to explain the association: though age affected song rate in a cross-sectional analysis, longitudinal analysis showed that individuals did not increase their song rate as they got older. Reconciling these results suggests differential selection against young males that respond to intrusion with low song rates. (2) Mortality costs of high song rates did not appear to explain the negative relationship between song rate and survival because, though song rate in response to playback was condition-dependent, high song rates in a different context did not appear to impose mortality costs. (3) High levels of territorial pressure may have increased mortality, but were not associated with high song rates in response to playback. (4) Since song rates did not increase with age, but tended to increase only in the last year of life, we tentatively suggest that the negative relationship between song rate and survival could represent a terminal investment in territorial defence by males in their final breeding season, though further work is needed to confirm this conclusion.