Will Cresswell

The phenology mismatch hypothesis: are declines of migrant birds linked to uneven global climate change?

1. Migrant bird populations are declining and have been linked to anthropogenic climate change. The phenology mismatch hypothesis predicts that migrant birds, which experience a greater rate of warming in their breeding grounds compared to their wintering grounds, are more likely to be in decline, because their migration will occur later and they may then miss the early stages of the breeding season. Population trends will also be negatively correlated with distance, because the chances of phenology mismatch increase with number of staging sites. 2. Population trends from the Palaearctic (1990-2000) and Nearctic (1980-2006) were collated for 193 spatially separate migrant bird populations, along with temperature trends for the wintering and breeding areas. An index of phenology mismatch was calculated as the difference between wintering and breeding temperature trends. 3. In the Nearctic, phenology mismatch was correlated with population declines as predicted, but in the Palaearctic, distance was more important. This suggests that differential global climate change may be responsible for contributing to some migrant species declines, but its effects may be more important in the Nearctic. 4. Differences in geography and so average migration distance, migrant species composition and history of anthropogenic change in the two areas may account for the differences in the strength of the importance of phenology mismatch on migrant declines in the Nearctic and Palaearctic.

Personality predicts individual responsiveness to the risks of starvation and predation

Theory suggests that individual personality is tightly linked to individual life histories and to environmental variation. The reactive–proactive axis, for example, is thought to reflect whether individuals prioritize productivity or survival, mutually exclusive options that can be caused by conflicts between foraging and anti-predation behaviour. Evidence for this trade-off hypothesis, however, is limited. Here, we tested experimentally whether exploration behaviour (EB), an assay of proactivity, could explain how great tits (Parus major) respond to changes in starvation and predation risk. Individuals were presented with two feeders, holding good or poor quality food, which interchanged between safe and dangerous positions 10 m apart, across two 24 h treatments. Starvation risk was assumed to be highest in the morning and lowest in the afternoon. The proportion of time spent feeding on good quality food (PTG) rather than poor quality food was repeatable within treatments, but individuals varied in how PTG changed with respect to predation- and starvation-risk across treatments. This individual plasticity variation in foraging behaviour was linked to EB, as predicted by the reactive–proactive axis, but only among individuals in dominant social classes. Our results support the trade-off hypothesis at the level of individuals in a wild population, and suggest that fine-scale temporal and spatial variation may play important roles in the evolution of personality.

Good foragers can also be good at detecting predators

The degree to which foraging and vigilance are mutually exclusive is crucial to understanding the management of the predation and starvation risk trade–off in animals. We tested whether wild–caught captive chaffinches that feed at a higher rate do so at the expense of their speed in responding to a model sparrowhawk flying nearby, and whether consistently good foragers will therefore tend to respond more slowly on average. First, we confirmed that the time taken to respond to the approaching predator depended on the rate of scanning: as head–up rate increased so chaffinches responded more quickly. However, against predictions, as peck rate increased so head–up rate increased and mean length of head–up and head–down periods decreased. Head–up rate was probably dependent on peck rate because almost every time a seed was found, a bird raised its head to handle it. Therefore chaffinches with higher peck rates responded more quickly. Individual chaffinches showed consistent durations of both their head–down and head–up periods and, therefore, individuals that were good foragers were also good detectors of predators. In relation to the broad range of species that have a similar foraging mode to chaffinches, our results have two major implications for predation/starvation risk trade–offs: (i) feeding rate can determine vigilance scanning patterns; and (ii) the best foragers can also be the best at detecting predators. We discuss how our results can be explained in mechanistic terms relating to fundamental differences in how the probabilities of detecting food rather than a predator are affected by time. In addition, our results offer a plausible explanation for the widely observed effect that vigilance continues to decline with group size even when there is no further benefit to reducing vigilance.

Predator‐hunting success and prey vulnerability: quantifying the spatial scale over which lethal and non‐lethal effects of predation occur

1. The shape of the function linking predator-attack success rate with distance to predator-concealing cover, or prey refuge, will affect population dynamics, distribution patterns and community trophic structure. Theory predicts that predator-attack success should decline exponentially with distance from predator-concealing cover, resulting in a threshold distance value above which there is little change in risk. Animals should then completely avoid areas of otherwise suitable habitat below this threshold, except when starvation risk exceeds predation risk. 2. We measured the shape of the function linking attack success with distance from cover in a system of Eurasian Sparrowhawks Accipiter nisus attacking (n = 445) and killing (n = 71) Redshanks Tringa totanus. We then determined if there was a threshold value and whether redshanks avoided areas below this threshold. 3. Sparrowhawk success rate with distance to predator-concealing cover declined exponentially with a threshold value of approximately 30 m. Redshanks used habitat above the threshold according to profitability and only fed below it, on average, in cold weather when starvation risk can be imminently high. Above about 5 degrees C, 26% of available habitat was avoided. 4. Our data support the hypothesis that predators create discrete areas with respect to cover that are avoided by prey. Large areas of suitable habitat may be unused, except in times of high starvation risk, when such areas may provide a foraging reserve, with large implications for population distribution and dynamics. 5. Our results are generated from a system in which predators attack their prey from concealing cover. But in the theoretically identical reverse scenario where the prey animals distance from protective cover determines predation risk, such non-lethal effects will be equally important, especially in heavily fragmented landscapes.

A preliminary assessment of some factors influencing the density and distribution of palearctic passerine migrants wintering in the Sahel zone of West Africa

Population densities of Palearctic migrant and African birds were estimated from 444 point counts, that estimated minimum density, at 10 sites in the Sahel zone of northern Nigeria during December–January 1993/94. In total, 15 Palearctic species were recorded during point counts averaging 5 species per site, and 76 African species averaging 24 species per site. The densities of Palearctic migrants varied from 1 bird ha-1 in semi-desert and highly degraded woodland, up to 8 birds ha-1 in dense Sahelian woodland. Common Whitethroats Sylvia communis were most abundant (up to 0.7 birds ha-1) in sites rich in Piliostigma reticulata trees; Subalpine Warblers S. cantillans were most abundant (up to 5.9 birds ha-1) in sites rich in Acacia spp. and Cassia sieberiana; Lesser Whitethroats S. curruca and Redstarts Phoenicurus phoenicurus were associated with high overall tree densities (both up to 0.6 birds ha-1); Northern Wheatears Oenanthe oenanthe occurred at densities of about 1 bird ha-1 at very low tree density...

Does an opportunistic predator preferentially attack nonvigilant prey

The dilution effect as an antipredation behaviour is the main theoretical reason for grouping in animals and states that all individuals in a group have an equal risk of being predated if equally spaced from each other and the predator. Stalking predators, however, increase their chance of attack success by preferentially targeting nonvigilant individuals, potentially making relative vigilance rates in a group relatively important in determining predation compared with the dilution effect. Many predators, however, attack opportunistically without stalking, when targeting of nonvigilant individuals may be less likely, so that the dilution effect will then be a relatively more important antipredation reason for grouping. We tested whether an opportunistically hunting predator, the sparrowhawk, Accipiter nisus, preferentially attacked vigilant or feeding prey models presented in pairs. We found that sparrowhawks attacked vigilant and feeding mounts at similar frequencies. Our results suggest that individuals should prioritize maximizing group size or individual vigilance dependent on the type of predator from which they are at risk. When the most likely predator is a stalker, individuals should aim to have the highest vigilance levels in a group, and there may be relatively little selective advantage to being in the largest group. In contrast, if the most likely predator is an opportunist, then individuals should simply aim to be in the largest group and can also spend more time foraging without compromising predation risk. For most natural systems this will mean a trade-off between the two strategies dependent on the frequency of attack of each predator type.

Predation in bird populations

One of the classic ecological questions is how predators affect population size. This is often assessed by measuring how many individuals are killed by a predator, yet such direct effects may only be a relatively minor part of population dynamics. Predators frequently affect prey populations indirectly, with the fear of predation resulting in costly behavioural compensation that has the potential to lead to large population and community effects. Large observable lethal effects may then just represent the most easily observed “special” cases of the effects of predation on populations, with the costs of non-lethal effects being ubiquitous and usually dominant. This review explores these two ideas: that both cases where there are no population effects due to predation and those where lethal effects dominate are unusual and involve special circumstances. First, systems in which predation effects appear not to arise include (1) complete avoidance of predators by prey; (2) when other environmental factors limit populations so that predation is not additive to mortality; (3) when there are other more vulnerable prey for a predator; (4) when predators interact; (5) because the relationship of perceived predation risk with predator abundance is usually a non-linear function; (6) for the simple reason that non-lethal effects have not been considered. Second, lethal effects tend to dominate over non-lethal effects when (1) there is a high cost of compensating for predation risk associated with either a resource constraint or a particularly vulnerable niche or life-history stage (e.g. the nest stage generally for birds); (2) prey are the most popular prey of a predator or linear trophic chains operate; (3) there is evolutionary lag, such as introduced predators and naïve prey populations; (4) there are several predator species hunting the same prey in diverse ways. The presence of predators may or may not affect the size of a bird population at any particular life-history stage, although in most cases it will do so through non-lethal effects and, occasionally, through lethal effects. However, the presence of predators will always affect intra- and interspecific competition and so will always affect population dynamics. Studies that wish to fully demonstrate that predation has no effect on bird populations must show that lethal effects and the costs of non-lethal compensation by the prey do not significantly change its density and, consequently, the level of competition.

How climate change might influence the starvation-predation risk trade-off response

Climate change within the UK will affect winter starvation risk because higher temperatures reduce energy budgets and are likely to increase the quality of the foraging environment. Mass regulation in birds is a consequence of the starvation–predation risk trade-off: decreasing starvation risk because of climate change should decrease mass, but this will be countered by the effects of predation risk, because high predation risk has a negative effect on mass when foraging conditions are poor and a positive effect on mass when foraging conditions are good. We tested whether mass regulation in great tits (Parus major) across the UK was related to temporal changes in starvation risk (winter temperature 1995–2005) and spatial changes in predation risk (sparrowhawk Accipiter nisus abundance). As predicted, great tits carried less mass during later, warmer, winters, demonstrating that starvation risk overall has decreased. Also, the effects of predation risk interacted with the effects of temperature (as an index of foraging conditions), so that in colder winters higher sparrowhawk abundance led to lower mass, whereas in warmer, later, winters higher sparrowhawk abundance led to higher mass. Mass regulation in a small bird species may therefore provide an index of how environmental change is affecting the foraging environment.

Low migratory connectivity is common in long-distance migrant birds

Estimating how much long-distance migrant populations spread out and mix during the non-breeding season (migratory connectivity) is essential for understanding and predicting population dynamics in the face of global change. We quantify variation in population spread and inter-population mixing in long-distance, terrestrial migrant land-bird populations (712 individuals from 98 populations of 45 species, from tagging studies in the Neotropic and Afro-Palearctic flyways). We evaluate the Mantel test as a metric of migratory connectivity, and explore the extent to which variance in population spread can be explained simply by geography. The mean distance between two individuals from the same population during the non-breeding season was 743xa0km, covering 10-20% of the maximum width of Africa/South America. Individuals from different breeding populations tended to mix during the non-breeding season, although spatial segregation was maintained in species with relatively large non-breeding ranges (and, to a lesser extent, those with low population-level spread). A substantial amount of between-population variation in population spread was predicted simply by geography, with populations using non-breeding zones with limited land availability (e.g. Central America compared to South America) showing lower population spread. The high levels of population spread suggest that deterministic migration tactics are not generally adaptive; this makes sense in the context of the recent evolution of the systems, and the spatial and temporal unpredictability of non-breeding habitat. The conservation implications of generally low connectivity are that the loss (or protection) of any non-breeding site will have a diffuse but widespread effect on many breeding populations. Although low connectivity should engender population resilience to shifts in habitat (e.g. due to climate change), we suggest it may increase susceptibility to habitat loss. We hypothesize that, because a migrant species cannot adapt to both simultaneously, migrants generally may be more susceptible to population declines in the face of concurrent anthropogenic habitat and climate change.

Changes in densities of Sahelian bird species in response to recent habitat degradation

Habitat loss in the Sahel region of West Africa has been pronounced, due to anthropogenic effects and (potentially) climate change. Although strong links have been found between conditions in the Sahel and subsequent breeding populations of certain Palaearctic migrants, the effects of these fluctuations upon Afrotropical species are unknown. We repeated bird censuses (Dec 1993 to Jan 1994, Dec 2001 and Dec 2002) at Watucal Forest Reserve, northern Nigeria, an area of rapidly degrading Sahelian woodland. We predicted declines in the abundance of woodland bird species with deforestation. For the purpose of setting up an experimental control, we also repeated bird censuses in adjacent farmland habitats that had already been deforested by the first census period: we predicted no change in abundance of farmland bird species. Tree density at Watucal decreased significantly by 82% over the eight-year period. The number of birds counted per point, the total number of species recorded per point, and the Shannon diversity index, all declined significantly in Watucal, but there was no significant change in adjacent farmland. Of those species that were reasonably abundant in either census period, 22% (n = 41) at Watucal showed a significant change in abundance between periods: all of these species showed a decline. In contrast, only 4% (n = 23) showed a significant change in abundance (a decline) on farmland. Of the seven species that disappeared from Watucal between periods, 100% were classified as broadly woodland species and, of the eight species that were only recorded in 2001/02 at Watucal, 63% were classified as scrub or open species. Given that Watucal is a protected area, the degree of deforestation and concomitant changes in bird abundance suggest that the Sahel is likely to be undergoing major changes in bird diversity.