Environmental correlates and functional consequences of bill divergence in island song sparrows

Inferring the environmental selection pressures responsible for phenotypic variation is a challenge in adaptation studies as traits often have multiple functions and are shaped by complex selection regimes. We provide experimental evidence that morphology of the multifunctional avian bill is related to climate, not foraging efficiency, in song sparrows (Melospiza melodia) on the California Channel Islands. Our research builds on a study in song sparrow museum specimens that demonstrated a positive correlation between bill surface area and maximum temperature, suggesting a greater demand for dry heat dissipation in hotter, xeric environments. We sampled contemporary sparrow populations across three climatically distinct islands to test the alternate hypotheses that song sparrow bill morphology is either a product of vegetative differences with functional consequences for foraging efficiency or related to maximum temperature and, consequently, important for thermoregulation. Measurements of >500 live individuals indicated a significant, positive relationship between maximum temperature and bill surface area when correcting for body size. In contrast, maximum bite force, seed extraction time, and vegetation on breeding territories (a proxy for food resources) were not significantly associated with bill dimensions. While we cannot exclude the influence of foraging ability and diet on bill morphology, our results are consistent with the hypothesis that variation in song sparrows' need for thermoregulatory capacity across the northern Channel Islands selects for divergence in bill surface area.

The effects of habituation on boldness of urban and rural song sparrows (Melospiza melodia)

As urbanization increases globally, wildlife species are changing their behaviour in many ways. Urban animals are often bolder, or less fearful of new stimuli, than rural animals. While adaptation can drive behavioural changes in urban animals, other factors, such as learning or habituation, can also lead to behavioural modifications. To determine how repeated exposure to a non-threatening human affects song sparrow (Melospiza melodia) behaviour in urban and rural habitat, we measured boldness as flight initiation distance (FID), for 5 consecutive days. We found that urban birds had consistently lower FID’s than their rural counterparts from days 1–4, yet there was no difference in rural and urban FID by trial 5. FID decreased over 5 days of repeated trials in the rural populations, but not the urban. These results suggest that habituation can occur quickly in rural birds and may account for the greater boldness we typically see in urban populations.

Anthelmintic drugs modulate the acute phase immune response but not the microbiota in wild Song Sparrows

Co-infection with microparasites (e.g., bacteria) and macroparasites (e.g., helminths) is often the natural state for wild animals. Despite evidence that gut helminths can bias immune responses away from inflammatory processes, few field studies have examined the role that helminths, or their potential interactions with internal microbial communities, play in modulating immunity in free-living, wild birds. Here, we used anthelmintic drugs to treat wild Song Sparrows (Melospiza melodia) for helminth infections and measured markers of systemic inflammation (heterothermia and locomotor activity) in response to an immune challenge with lipopolysaccharide (LPS), a cell wall component of gram-negative bacteria. Using birds from a population that previously showed high helminth prevalence, we monitored skin temperature and activity remotely using automated radio telemetry. We also collected cloacal swabs to determine whether drug treatment was associated with changes in the cloacal microbiota, and whether cloacal microbial community structure was associated with the severity of birds’ immune responses. Because helminths can reduce the severity of inflammatory immune responses in other species, we predicted that in comparison with untreated control birds, anthelmintic-treated birds would be more lethargic and display higher fevers when challenged with LPS. Consistent with these predictions, anthelmintic-treated birds expressed higher fevers in response to immune challenge. However, all LPS-challenged birds decreased locomotor activity to a similar degree, regardless of anthelmintic treatment. Although several individual indicator bacterial taxa were strongly associated with anthelmintic treatment, this treatment did not alter overall bacterial alpha- and beta- diversity. Similarly, we did not find evidence that bacterial community diversity influenced the severity of immune responses to LPS. These results suggest that under field conditions, natural helminth infection can reduce the severity of songbirds’ thermoregulatory responses (fever) during an immune challenge, without major impacts on internal microbial communities or behavioral responses to infection.

No evidence that songbirds use odour cues to avoid malaria-infected conspecifics

Many animals have evolved mechanisms to detect and avoid parasitized conspecifics, primarily through odour cues, but whether birds are capable of odour-mediated parasite avoidance is unknown. Recently, we showed that exposing song sparrows (Melospiza melodia) to avian malaria parasites (Plasmodium sp.) alters the chemical composition of their preen oil, which is the major source of body odour in birds. Here, we presented song sparrows with preen oil from uninfected (sham-inoculated) and malaria-infected conspecifics, predicting that birds would spend more time with odour cues from uninfected than infected birds. Birds without detectable malarial infections spent about 50% more time with preen oil from uninfected than infected conspecifics, and females spent nearly twice as much time with preen oil from uninfected than infected conspecifics. However, neither difference was statistically significant. Song sparrows may be able to detect odour cues of infection, but further experiments are needed to confirm or refute this.

Song sparrows do not discriminate between their own song and stranger song

Bird song is socially learned. During song learning, the bird’s hearing its own vocalization is important for normal development of song. Whether bird’s own song is represented and recognized as a special category in adult birds, however, is unclear. If birds respond differently to their own songs when these are played back to them, this would be evidence for auditory self-recognition. To test this possibility, we presented song sparrow males (Melospiza melodia) playbacks of their own songs or stranger songs and measured aggressive responses as well as type matching. We find no evidence of behavioral discrimination of bird’s own song relative to the (non-matching) stranger song. These findings cast doubt on an earlier proposal that song sparrows display auditory self-recognition and support the common assumption in playback experiments that bird’s own song is perceived as stranger song.