Interspecific aggression declines seasonally in breeding great tits <i>Parus major</i>

During the breeding season, great tits show aggression to protect their nest from intra‐ and interspecific intruders. Aggression is a labile trait that can be plastically expressed as a result of individual differences (e.g., personality), seasonal gradients in the costs and benefits of aggression, or other environmental components (e.g., number of competitors). Competitors may try to take over great tit nests, because the number of suitable nesting sites is limited, and great tits may guard high quality territories. Taking over a great tit nest may be especially fruitful in early phenological stages (egg laying) when great tits frequent their nests less often. However, great tits may compensate for this vulnerability by being more aggressive toward intruders during early nesting stages, a pattern that has already been established in an intraspecific context. Previous studies have shown that interspecific intruders were most likely to die from great tit aggression during great tit egg laying, suggesting great tits may also be more aggressive during this phase in an interspecific context. Here, I tested this hypothesis with simulated territorial intrusions in great tit territories using taxidermized blue tits Cyanistes caeruleus (hereafter called blue tit models). Great tit aggression (number of calls and approach distance toward blue tit model) was assayed during egg laying, incubation, and chick rearing in the breeding season of 2014. Although sample size was low due to a high fraction of non‐responders (n = 44 out of 89 assays across 26 out of 35 individuals), I found that great tits showed a seasonal decline in aggressiveness, which is congruent with intraspecific results on this study species. I discuss my findings in the context of differential adjustment to climate change between interspecific competitors

Phenological sensitivity to climate change is higher in resident than in migrant bird populations among European cavity breeders

Many organisms adjust their reproductive phenology in response to climate change, but phenological sensitivity to temperature may vary between species. For example, resident and migratory birds have vastly different annual cycles, which can cause differential temperature sensitivity at the breeding grounds, and may affect competitive dynamics. Currently, however, adjustment to climate change in resident and migratory birds have been studied separately or at relatively small geographical scales with varying time series durations and methodologies. Here, we studied differential effects of temperature on resident and migratory birds using the mean egg laying initiation dates from 10 European nest box schemes between 1991 and 2015 that had data on at least one resident tit species and at least one migratory flycatcher species. We found that both tits and flycatchers advanced laying in response to spring warming, but resident tit populations advanced more strongly in relation to temperature increases than migratory flycatchers. These different temperature responses have already led to a divergence in laying dates between tits and flycatchers of on average 0.94days per decade over the current study period. Interestingly, this divergence was stronger at lower latitudes where the interval between tit and flycatcher phenology is smaller and winter conditions can be considered more favorable for resident birds. This could indicate that phenological adjustment to climate change by flycatchers is increasingly hampered by competition with resident species. Indeed, we found that tit laying date had an additional effect on flycatcher laying date after controlling for temperature, and this effect was strongest in areas with the shortest interval between both species groups. Combined, our results suggest that the differential effect of climate change on species groups with overlapping breeding ecology affects the phenological interval between them, potentially affecting interspecific interactions

Phenological sensitivity to climate change is higher in resident than in migrant bird populations among European cavity breeders

Abstract Many organisms adjust their reproductive phenology in response to climate change, but phenological sensitivity to temperature may vary between species. For example, resident and migratory birds have vastly different annual cycles, which can cause differential temperature sensitivity at the breeding grounds, and may affect competitive dynamics. Currently, however, adjustment to climate change in resident and migratory birds have been studied separately or at relatively small geographical scales with varying time series durations and methodologies. Here, we studied differential effects of temperature on resident and migratory birds using the mean egg laying initiation dates from 10 European nest box schemes between 1991 and 2015 that had data on at least one resident tit species and at least one migratory flycatcher species. We found that both tits and flycatchers advanced laying in response to spring warming, but resident tit populations advanced more strongly in relation to temperature increases than migratory flycatchers. These different temperature responses have already led to a divergence in laying dates between tits and flycatchers of on average 0.94 days per decade over the current study period. Interestingly, this divergence was stronger at lower latitudes where the interval between tit and flycatcher phenology is smaller and winter conditions can be considered more favorable for resident birds. This could indicate that phenological adjustment to climate change by flycatchers is increasingly hampered by competition with resident species. Indeed, we found that tit laying date had an additional effect on flycatcher laying date after controlling for temperature, and this effect was strongest in areas with the shortest interval between both species groups. Combined, our results suggest that the differential effect of climate change on species groups with overlapping breeding ecology affects the phenological interval between them, potentially affecting interspecific interactions

Data from: Competitor phenology as a social cue in breeding site selection

1. Predicting habitat quality is a major challenge for animals selecting a breeding patch, because it affects reproductive success. Breeding site selection may be based on previous experience, or on social information from the density and success of competitors with an earlier phenology. 2. Variation in animal breeding phenology is often correlated with variation in habitat quality. Generally, animals breed earlier in high quality habitats that allow them to reach a nutritional threshold required for breeding earlier or avoid nest predation. In addition, habitat quality may affect phenological overlap between species and thereby interspecific competition. Therefore, we hypothesized that competitor breeding phenology can be used as social cue by settling migrants to locate high quality breeding sites. 3. To test this hypothesis, we experimentally advanced and delayed hatching phenology of two resident tit species on the level of study plots and studied male and female settlement patterns of migratory pied flycatchers Ficedula hypoleuca. The manipulations were assigned at random in two consecutive years, and treatments were swapped between years in sites that were used in both years. 4. In both years, males settled in equal numbers across treatments, but later arriving females avoided pairing with males in delayed phenology plots. Moreover, male pairing probability declined strongly with arrival date on the breeding grounds. 5. Our results demonstrate that competitor phenology may be used to assess habitat quality by settling migrants, but we cannot pinpoint the exact mechanism (e.g. resource quality, predation pressure, or competition) that has given rise to this pattern. 6. In addition, we show that opposing selection pressures for arrival timing may give rise to different social information availabilities between sexes. We discuss our findings in the context of climate warming, social information use, and the evolution of protandry in migratory animals

Climate change may affect fatal competition between two bird species

Climate warming has altered phenologies of many taxa [1, 2], but the extent differs vastly between [3, 4] and within trophic levels [5-7]. Differential adjustment to climate warming within trophic levels may affect coexistence of competing species, because relative phenologies alter facilitative and competitive outcomes [8, 9], but evidence for this is scant [10, 11]. Here, we report on two mechanisms through which climate change may affect fatal interactions between two sympatric passerines, the resident great tit Parus major and the migratory pied flycatcher Ficedula hypoleuca, competing for nest sites. Spring temperature more strongly affected breeding phenology of tits than flycatchers, and tits killed more flycatchers when flycatcher arrival coincided with peak laying in the tits. Ongoing climate change may diminish this fatal competition if great tit and flycatcher phenologies diverge. However, great tit density increased after warm winters, and flycatcher mortality was elevated when tit densities were higher. Consequently, flycatcher males in synchronous and high-tit-density years suffered mortality by great tits of up to 8.9%. Interestingly, we found no population consequences of fatal competition, suggesting that mortality predominantly happened among surplus males. Indeed, late-arriving males are less likely to find a partner [12], and here we show that such late arrivers are more likely to die from competition with great tits. We conclude that our breeding population is buffered against detrimental effects of competition. Nevertheless, we expect that if buffers are diminished, population consequences of interspecific competition may become apparent, especially after warm winters that are benign to resident species. VIDEO ABSTRACT

arrival_early_late

R script in which all the results in this study can be checked and replicated, using the data files available

swaps15

Data of the 2015 tit nests used for swappin

Replication Data for: Climate change may affect fatal competition between two bird species

This data set consists of eight files providing raw data, R-code to process the data files, and metadata for interpretation of the data files