Carla S. Pimentel

Variation in clutch size in relation to nest size in birds

Nests are structures built to support and protect eggs and/or offspring from predators, parasites, and adverse weather conditions. Nests are mainly constructed prior to egg laying, meaning that parent birds must make decisions about nest site choice and nest building behavior before the start of egg-laying. Parent birds should be selected to choose nest sites and to build optimally sized nests, yet our current understanding of clutch size-nest size relationships is limited to small-scale studies performed over short time periods. Here, we quantified the relationship between clutch size and nest size, using an exhaustive database of 116 slope estimates based on 17,472 nests of 21 species of hole and non-hole-nesting birds. There was a significant, positive relationship between clutch size and the base area of the nest box or the nest, and this relationship did not differ significantly between open nesting and hole-nesting species. The slope of the relationship showed significant intraspecific and interspecific heterogeneity among four species of secondary hole-nesting species, but also among all 116 slope estimates. The estimated relationship between clutch size and nest box base area in study sites with more than a single size of nest box was not significantly different from the relationship using studies with only a single size of nest box. The slope of the relationship between clutch size and nest base area in different species of birds was significantly negatively related to minimum base area, and less so to maximum base area in a given study. These findings are consistent with the hypothesis that bird species have a general reaction norm reflecting the relationship between nest size and clutch size. Further, they suggest that scientists may influence the clutch size decisions of hole-nesting birds through the provisioning of nest boxes of varying sizes.

Clutch-size variation in Western Palaearctic secondary hole-nesting passerine birds in relation to nest box design

Secondary hole-nesting birds that do not construct nest holes themselves and hence regularly breed in nest boxes constitute important model systems for field studies in many biological disciplines with hundreds of scientists and amateurs involved. Those research groups are spread over wide geographic areas that experience considerable variation in environmental conditions, and researchers provide nest boxes of varying designs that may inadvertently introduce spatial and temporal variation in reproductive parameters. We quantified the relationship between mean clutch size and nest box size and material after controlling for a range of environmental variables in four of the most widely used model species in the Western Palaearctic: great tit Parus major, blue tit Cyanistes caeruleus, pied flycatcher Ficedula hypoleuca and collared flycatcher F.albicollis from 365 populations and 79610 clutches. Nest floor area and nest box material varied non-randomly across latitudes and longitudes, showing that scientists did not adopt a random box design. Clutch size increased with nest floor area in great tits, but not in blue tits and flycatchers. Clutch size of blue tits was larger in wooden than in concrete nest boxes. These findings demonstrate that the size of nest boxes and material used to construct nest boxes can differentially affect clutch size in different species. The findings also suggest that the nest box design may affect not only focal species, but also indirectly other species through the effects of nest box design on productivity and therefore potentially population density and hence interspecific competition.

Impact of climatic variation on populations of pine processionary moth Thaumetopoea pityocampa in a core area of its distribution

1 There is growing appreciation of climatic effects on insect population dynamics at the margins of distribution limits. Climatic effects might be less important and/or involve different drivers and processes near the centre of distributions. 2 We evaluated the effects of interannual variation in temperatures, radiation and precipitation on populations of pine processionary moth Thaumetopoea pityocampa in Central–South Portugal, a low altitude area with a relatively mild Mediterranean climate near the centre of the north–south range of the species. 3 We tested for effects of climate on mortality of young larvae, growth rates, final larval mass and fecundity. 4 Results indicated high mortality of early instars associated with low minimum and maximum daily temperatures and low precipitation. Low minimum temperatures were further associated with high parasitism by the larval parasitoid Phryxe caudata (Rondani) (Diptera, Tachinidae). Furthermore, larval growth rates were higher with high solar radiation during December and January, which was itself negatively related to precipitation and air temperature. Slow larval growth rates led to lower final mass at pupation in the spring, and smaller egg masses and smaller initial colony sizes during the next autumn. 5 Thus climatic factors, and temperature in particular, apparently contributed to population dynamics of T. pityocampa in the core of its distribution, as well as at its northern limits. The most specific climatic parameters of importance, however, and the connections between climate, physiology and insect demographics in the core area were clearly different from northern areas.

Interspecific variation in the relationship between clutch size, laying date and intensity of urbanization in four species of hole-nesting birds

Abstract The increase in size of human populations in urban and agricultural areas has resulted in considerable habitat conversion globally. Such anthropogenic areas have specific environmental characteristics, which influence the physiology, life history, and population dynamics of plants and animals. For example, the date of bud burst is advanced in urban compared to nearby natural areas. In some birds, breeding success is determined by synchrony between timing of breeding and peak food abundance. Pertinently, caterpillars are an important food source for the nestlings of many bird species, and their abundance is influenced by environmental factors such as temperature and date of bud burst. Higher temperatures and advanced date of bud burst in urban areas could advance peak caterpillar abundance and thus affect breeding phenology of birds. In order to test whether laying date advance and clutch sizes decrease with the intensity of urbanization, we analyzed the timing of breeding and clutch size in relation to intensity of urbanization as a measure of human impact in 199 nest box plots across Europe, North Africa, and the Middle East (i.e., the Western Palearctic) for four species of hole‐nesters: blue tits (Cyanistes caeruleus), great tits (Parus major), collared flycatchers (Ficedula albicollis), and pied flycatchers (Ficedula hypoleuca). Meanwhile, we estimated the intensity of urbanization as the density of buildings surrounding study plots measured on orthophotographs. For the four study species, the intensity of urbanization was not correlated with laying date. Clutch size in blue and great tits does not seem affected by the intensity of urbanization, while in collared and pied flycatchers it decreased with increasing intensity of urbanization. This is the first large‐scale study showing a species‐specific major correlation between intensity of urbanization and the ecology of breeding. The underlying mechanisms for the relationships between life history and urbanization remain to be determined. We propose that effects of food abundance or quality, temperature, noise, pollution, or disturbance by humans may on their own or in combination affect laying date and/or clutch size.

Response of great tits Parus major to an irruption of a pine processionary moth Thaumetopoea pityocampa population with a shifted phenology

Since 1997, a population of the Pine Processionary Moth Thaumetopoea pityocampa exhibited a shifted life cycle in a restricted area of a managed pine forest, at the central west coast of Portugal. Rather than during the regular winter period, larval development of this novel population occurs during the summer. We indicate the populations accordingly as the Winter Population (WP) and the Summer Population (SP). We quantified the numerical response of Great Tits Parus major to irruptions of the SP. In the years following an irruption, Great Tits were more abundant in the SP area, than in two similar non-infested forest areas. This was particularly pronounced during summer and autumn, at the time when moths, eggs and larvae of the SP were available for bird predation. A nestbox study allowed us to verify that the SP moth emergence period coincided with egg laying and raising of second broods in Great Tits, and higher reproductive rates were recorded in the SP than in the WP area. To our knowledge, this is the first study to document a response of a bird predator to a local T. pityocampa irruption, which was even more special as it concerned an insect population with a large shift in its life cycle. Our results show that the Great Tit is able to respond to local outbreaks of forest defoliator insects, and we suggest a potential impact of Great Tit predation on the population dynamics of the SP.

Geographical variation in seasonality and life history of pine sawyer beetles Monochamus spp: its relationship with phoresy by the pinewood nematode Bursaphelenchus xylophilus.

Bursaphelenchus xylophilus (Steiner & Buhrer) (Nematoda: Aphelenchoididae), the pinewood nematode and the causal agent of the pine wilt disease, is a globally important invasive pathogen of pine forests. It is phoretic in woodborer beetles of the genus Monochamus (Megerle) (Coleoptera, Cerambycidae) and has been able to exploit novel indigenous species of Monochamus (but only Monochamus) in newly‐invaded areas. North America (NA) is the continent of origin for the B. xylophilus/Monochamus spp. phoretic system. NA also contains the largest number of Monochamus species known to act as vectors for B. xylophilus. Understanding this phoretic system in its native geographical area helps to explain the evolutionary ecology of pine wilt disease. In the present study, we measured the flight phenology, size, sex ratios and species identity of Monochamus species in five geographically distant forests in NA. We also measured phoresy by B. xylophilus. We found the nematode to be abundant across eastern NA but rare or absent in western NA. In eastern forests, nematode phoresy was highest on the Monochamus species that flew earliest in the year. However, in the southeast, where Monochamus is most likely multivoltine with a long flight season, we found vectors with high nematode loads throughout the season, indicating that B. xylophilus can be transmitted to new hosts during most part of the year. The frequency distribution of nematode dauers on Monochamus was highly aggregated. Bursaphelenchus xylophilus in NA appears to be able to use all available Monochamus species as vectors. In native NA pine forests, the pinewood nematode appears to have an ecology that is sufficiently flexible to exploit different species (and both genders) of Monochamus, and disperse at different times of the year. This flexibility may contribute to its recent success in invading Eurasian pine forests.

Latitudinal patterns in temperature-dependent growth rates of a forest pathogen

Direct effects of temperature on plant pathogens can be crucial to determine the onset and epidemiology of disease. The pinewood nematode Bursaphelenchus xylophilus, the causal agent of the pine wilt disease (PWD), has a wide geographical distribution in Eurasia and East Asia, and local temperatures are considered determinant for the onset of the PWD. However, direct effects of temperature on this pathogen are never considered when forecasting its distribution and impact. In the present study we assessed: 1) at which temperatures is the development of wild populations of the pinewood nematode optimized; 2) if there is niche divergence on different populations from its wide distribution area. For this we studied two populations originated from different latitudes in eastern North America (NA), and used multi-model inference to evaluate the contributions of temperature, diet, and nematode population toward the growth rates of B. xylophilus. Although population origin had some effect on the predictive models, there seems to be niche conservatism, with temperatures of 28-29°C maximizing the growth rates of the pathogen. Thus, the use of the number of days in the summer with temperatures between 25 and 31°C would probably help to improve models forecasting B. xylophilus dispersion and PWD. The present work highlight the importance of considering adaptations to temperatures in forest pathogens with large geographical distributions, when building models forecasting the impact of climate on these organisms.

Spatial patterns at host and forest stand scale and population regulation of the pine processionary moth Thaumetopoea pityocampa

Different spatial processes are likely to generate variability at different scales. Thus, the explanation of patterns may be facilitated by knowledge about the spatial scales where variation in patterns occurs. In the present study, the link between spatial patterns of the pine processionary moth Thaumetopoea pityocampa Den. & Schiff. (Lepidoptera, Notodontidae) at two different scales, as well as its population dynamics, was assessed. First, the spatial patterns of T. pityocampa population were assessed at the forest stand scale by surveying the distribution of larval colonies within the pine forest, indicative of adult dispersion. Second, egg‐laying patterns across individual host‐plants, indicative of female oviposition choices, were assessed, as well as their impact on the survival of immature stages. It was found that T. pityocampa presents a gregarious distribution in homogeneous pine forests, over distances of a few dozen metres. At the pine tree scale, females tend to aggregate their egg batches, whereas aggregation increases with population density. Natural enemies such as parasitoids do not appear to play an important role in regulating the population dynamics of the species. At the same time, aggregation in individual hosts leads to increased mortality in the period from hatching to the third instar, which is notable at high population densities. A patchy distribution in the landscape over short distances has a potentially positive effect on population dynamics of the species, avoiding the Allee effect, and allowing for a rapid increase in population, even where initial numbers are low, leading to localized outbreaks. At the same time, increased mortality as a result of egg‐batch clumping at high population densities is a potential density‐dependent mechanism of population regulation.