Food abundance and clutch size of Tree Swallows Tachycineta bicolor

We studied Tree Swallows Tachycineta bicolor during eight breeding seasons (1977-84) at three sites that differed in food availability, which we measured as an index of the biomass of flying insects derived from daily catches in suspended nets. Mean clutch size differed significantly between sites (6.2, 5.8, and 5.6 eggs) and was related to average food abundance at the site. Within two sites, clutch sizes were significantly larger in years when food was more abundant during egg-laying ( r = 0.83 and 0.97 at the high and low food abundance sites, respectively; r = 0.63, not significant, at the medium abundance site). Both clutch size and laying season food abundance were positively correlated with food abundance during the nestling stage at the 'high' site, but not at the other two sites. Our results support the hypothesis that female Tree Swallows respond proximately to abundant food during egg-laying by laying more eggs, but they do not elucidate the ultimate evolutionary factors involved in determining clutch size.     

Age-Related Changes in Signalling of Need by Nestling Tree Swallows (Tachycineta bicolor)

Despite a large literature on the ontogeny of behaviour, few studies have examined how the function of juvenile behaviour changes during development. One of the most widespread and important juvenile behaviours is begging, the display used by young animals to solicit food from their parents. Begging signals generally vary reliably with offspring need for food and have served as models for understanding the evolution of honest signalling. Little is known, however, about whether the relationship between begging and need varies over the period of rapid juvenile development. Here, we examine whether tree swallow, Tachycineta bicolor, begging calls consistently reflect hunger levels across the 20 d nestling period. We recorded begging calls at 5, 10 and 15 d post‐hatch, during an hour of food deprivation, and related call features to time without food (i.e. hunger) at each age. The overall correlation between call structure and hunger, as measured by canonical correlation, was consistent across ages. The particular features that correlated with hunger varied, however. Call rate and length increased with hunger at all ages, but call amplitude and frequency range increased with hunger at days 10 and 15 only. The results of our study suggest that begging calls consistently convey information about offspring hunger throughout the nestling period, with the number of call features encoding hunger increasing with nestling age. This change may enhance the ability of parents to assess offspring hunger levels by adding redundancy to the signal.     

How Nestling Tree Swallows (Tachycineta bicolor) Integrate their Responses to Hunger and Signalling by Nestmates

Young animals in a broad range of taxa solicit care from their parents with begging displays, which are used at least partly for competition among brood or litter mates. The effect of other begging offspring on an individual’s own begging display varies across studies, however, increasing its intensity in some, but not changing, or even decreasing it, in others. One possible reason for this discrepancy is that the potential pay‐off for more intense begging depends not only on how intensely an individual’s brood or littermates are begging, but also on how long that individual has been without food. Surprisingly, however, no studies have focused on how begging responses vary when both factors are varied simultaneously. We therefore examined how nestling tree swallows, Tachycineta bicolor, respond to nestmates in relation to both their own hunger levels and the begging intensity of nestmates. During a period of food deprivation, we played focal nestlings parental contact calls either alone (control) or with the begging calls of a nestling deprived of food for 30–50 (low intensity) or 100–110 min (high intensity). Nestlings called for longer in response to the low‐intensity playback, but, surprisingly, not in the high‐intensity playback, in which they instead delayed the onset of their calling. All these responses to nestmates were independent of how long the responding nestling had been deprived of food. Thus, even in the seemingly intensely competitive environment of a passerine brood, offspring do not necessarily respond to nestmates with escalation. This may be because de‐escalation is the best competitive option in some circumstances, or because begging has other functions besides advertisement of individual need and competition over food allocation. Certainly, the results illustrate the need for studies of how nestmate interactions vary across a broad range of contexts.     

Growth rate, size, and sex ratio of last-laid, last-hatched offspring in the tree swallow Tachycineta bicolor

In tree swallows Tachycineta bicolor, last‐laid eggs typically hatch one to two days after the other eggs in the clutch hatch, putting last‐hatched offspring at a disadvantage when competing for food delivered by parents. We studied the biology of last‐laid, last‐hatched tree swallow offspring over two years in a Wyoming, USA, population. Our first objective was to compare the growth of last‐hatched offspring to that of their earlier‐hatched nestmates. One previous study had suggested that last‐hatched, competitively disadvantaged offspring grow feathers faster than senior nestmates, even at the expense of other aspects of growth. This may allow last‐hatched offspring to fledge with senior nestmates and avoid abandonment by parents. A second objective was to determine the sex of nestlings from last‐laid eggs. If last‐laid eggs typically produce undersized, weak adults that are poor competitors for resources, and if the fitness costs of being undersized/weak are more severe for males than for females, then selection may favour having offspring from last‐laid eggs to be female. In this study, last‐laid eggs hatched in 63 of 66 (94%) nests and hatched last in 93% of cases. At hatching, offspring from last‐laid eggs weighed, on average, 63% as much as their three heaviest nestmates (range: 26–107%). Offspring from last‐laid eggs fledged from 71% of the nests that produced at least one fledgling and apparently starved to death in remaining nests. Last‐hatched offspring who were presumably at a substantial competitive disadvantage (those whose mass at hatching was no more than about 75% of the mean mass of their three heaviest nestmates), gained mass more slowly than their senior nestmates but they eventually attained the same peak mass before fledging. Last‐hatched offspring grew primary feathers more slowly than their senior nestmates although the difference in growth rate was slight (0.2 mm/d) and only marginally significant. As a group, offspring from last‐laid eggs did not differ from offspring from all other eggs in either maximum mass attained before fledging or tarsus length at fledging. This is atypical for species with asynchronous hatching and is possibly the result of another unusual trait: the tendency of parent tree swallows to distribute food equally among young within broods. The sex ratio of offspring from last‐laid eggs did not deviate from 1:1 (22 males, 21 females). Given that last‐hatched eggs do not routinely produce undersized/weak individuals in our study population, there should be little selection on parent females to bias the sex ratio of last‐laid offspring towards females.     

Reproductive effort reduces long-term immune function in breeding tree swallows (Tachycineta bicolor)

We examined whether strategies of reproductive allocation may reduce long-term immunocompetence through the effects of manipulated effort on secondary or acquired immunity. We tested whether increased reproductive effort leads to reduced immune function and survival by manipulating brood size in tree swallows (Tachycineta bicolor) and exposing breeding females to a primary and secondary exposure of sheep red blood cells to elicit a humoral immune response. Females raising enlarged broods produced fewer secondary antibodies than did females raising control or reduced broods. Most importantly, individuals with high secondary responses were more likely to survive to breed 3 years after brood manipulations, suggesting that differences in disease susceptibility may be caused by trade-offs in reproductive allocation. We also found that individual quality, measured by clutch initiation date, mediated the effects of brood manipulations, with higher-quality birds showing a greater ability to deal with increases in effort.     

Dietary calcium limits size and growth of nestling tree swallows Tachycineta bicolor in a non-acidified landscape

Much previous research has focussed on the role of food supply in determining the growth and the survival of avian offspring. More recently, acid deposition in some ecosystems has demonstrated that in addition to energy, birds also need to acquire sufficient nutrients such as calcium to be successful. Whether procurement of adequate levels of calcium can limit reproductive success in areas that have not been impacted by acid rain remains equivocal. We tested whether calcium affected reproductive success of tree swallows Tachycineta bicolor by feeding extra calcium to nestlings during the brood-rearing period. Our manipulation did not enhance the survival of offspring, however, provisioning of extra calcium resulted in nestlings showing enhanced rates of growth of mass (all nests) and of ninth primary flight feathers (nests with after-second year female parents), compared to control nestlings. Calcium supplementation also resulted in nestlings having longer feathers and tarsi at age 16 days, and there was evidence that some nestlings receiving extra calcium were heavier at 16 days old. As offspring that have faster growth, or that are in good condition at fledging, often survive better after leaving the nest, these results suggest that calcium availability can limit fitness. Our results are noteworthy because our experiment was conducted in an area with abundant soil calcium where acid deposition has not occurred. The role of calcium in limiting the reproductive performance of avian species may therefore be more pervasive than previously thought.     

Population decline in tree swallows ( Tachycineta bicolor ) linked to climate change and inclement weather on the breeding ground

Population decline and the threat of extinction are realities currently facing many species. Yet, in most cases, the detailed demographic data necessary to identify causes of population decline are unavailable. Using 43 years (1975−2017) of data from a box-nesting population of tree swallows (Tachycineta bicolor), we identified reduced survival of offspring as a probable demographic cause of population decline. Poor fledging success was associated with increased predation and poor weather conditions during early nestling development. Low juvenile survival and subsequent recruitment was linked to poor weather conditions during the post-fledging period and may also be linked to conditions on the wintering grounds. Regional weather conditions during critical stages of breeding (early nestling and post-fledging) have become progressively worse over the 43-year study period. None of the other factors linked to offspring survival have similarly deteriorated. Overall, our results suggest tree swallows should be added to the growing list of species challenged by climate change, and that other species of aerial insect specialists may face similar impacts of climate change.     

Tree swallows (Tachycineta bicolor) nesting on wetlands impacted by oil sands mining are highly parasitized by the bird blow fly Protocalliphora spp

Oil sands mining is steadily expanding in Alberta, Canada. Major companies are planning reclamation strategies for mine tailings, in which wetlands will be used for the bioremediation of water and sediments contaminated with polycyclic aromatic hydrocarbons and naphthenic acids during the extraction process. A series of experimental wetlands were built on companies' leases to assess the feasibility of this approach, and tree swallows (Tachycineta bicolor) were designated as upper trophic biological sentinels. From May to July 2004, prevalence and intensity of infestation with bird blow flies Protocalliphora spp. (Diptera: Calliphoridae) were measured in nests on oil sands reclaimed wetlands and compared with those on a reference site. Nestling growth and survival also were monitored. Prevalence of infestation was surprisingly high for a small cavity nester; 100% of the 38 nests examined were infested. Nests on wetlands containing oil sands waste materials harbored on average from 60% to 72% more blow fly larvae than those on the reference site. Nestlings on reclaimed sites suffered mean parasitic burdens about twice that of those on the reference site; and for comparable parasitic load, they exhibited greater pathologic effects (e.g., decreased body mass) than control nestlings. The heavy blow fly infestation on oil sands-impacted wetlands suggests that oil sands mining disturbs several components of the local ecosystem, including habitat characteristics, blow fly predators, and host resistance to parasites.     

Temperature effects on food supply and chick mortality in tree swallows (Tachycineta bicolor)

Tree swallow (Tachycineta bicolor) breeding success in Ithaca, NY, USA, over the past quarter century has shown generally healthy fledgling production punctuated by years of high nestling mortality. This study tested the potential effects that temperature may have on the food supply and breeding success of swallows. Data from 17 years of daily insect samples were used to relate flying insect abundances to daily maximum temperatures and to define “cold snaps” as strings of consecutive days when the maximum temperatures did not exceed critical temperatures. The distributions of cold snaps and chick mortality events were investigated both through detailed reconstructions of the fates and fate dates of individual chicks, focused on the three breeding seasons of lowest fledging success, and with less detailed brood-level analyses of a larger 11-year dataset including years of more moderate mortality. Mark–recapture analyses of daily brood survival rate (DSR) reveal very strong support for the effects of cold temperatures on brood survival rates, and all the top models agree on a critical temperature of 18.5 °C for insect flight activity in Ithaca. The individual-level analyses, focused on years of higher mortality, favored a 3-day cold snap definition as the most predictive of DSR effects, whereas the larger-scale brood-level analyses revealed 1- and 2-day cold snaps as having the most significant effects on DSR. Regardless, all analyses reveal that, in an age of generally warmer climates, the largest effect of weather on swallow fledgling production is from cold temperatures.     

Perceived risk of ectoparasitism reduces primary reproductive investment in tree swallows Tachycineta bicolor

Many birds have been shown to reduce their reproductive investment in response to infestation of nest sites by ectoparasites. Nest-dwelling parasite populations increase throughout the breeding season, and can reduce the condition and future survival of both breeding adults and their offspring. Thus, avian hosts should be capable of assessing early cues that predict future ectoparasitism risk, and should be able to facultatively adjust their primary reproductive investment in response to anticipated future costs of parasites. We tested this hypothesis in the tree swallow Tachycineta bicolor , a cavity nesting passerine, by presenting a visual cue of avian fleas on the outer surface of nest boxes. This treatment manipulated perceived ectoparasitism risk without exposing birds to parasites, thereby allowing us to examine facultative responses in the absence of early physiological effects of parasites on female reproductive investment. During one of the study years, birds preferentially occupied control boxes, however, across both years, birds nesting in treatment boxes produced significantly smaller clutches, resulting in smaller broods at hatching, relative to those in control boxes. This difference in clutch size could not be explained by differences in phenotypic quality of breeding birds, indicating that for cavity nesting birds such as tree swallows, the perception of future ectoparasitism risk may be sufficient to induce a facultative reduction in reproductive investment early in the breeding season, before nest-dwelling parasite populations have grown very large.     

Timing of copulations and the pattern of paternity in relation to laying order in tree swallows Tachycineta bicolor

It is well established that extra-pair young are present to varying degrees in nests of most avian species. However, the timing of extra-pair copulations during a female's fertile cycle is not well known, nor is how these copulations translate into the distribution of extra-pair young within the laying sequence. We determined the rates of successful within-pair copulations where at least one cloacal contact was achieved, and the number of extra-pair copulations/attempts in 17 pairs of tree swallows Tachycineta bicolor during the mornings encompassing the female's fertile period. We also examined nestling paternity with respect to laying order in eighteen mixed-paternity tree swallow broods. The rate of successful within-pair copulations peaked three days prior to a female laying her first egg, and decreased steadily throughout the laying period. We therefore predicted that extra-pair young would be more likely to hatch out of later-laid than earlier-laid eggs within a female's brood. We found no evidence of an increased frequency of extra-pair young with laying order. Instead, extra-pair young appear to be randomly distributed within the laying sequence of tree swallows.     

Experimental addition of greenery reduces flea loads in nests of a non-greenery using species, the tree swallow Tachycineta bicolor

Several bird species, including cavity-nesters such as European starlings Sturnus vulgaris , add to their nests green sprigs of plants such as yarrow Achillea millefolium that are rich in volatile compounds. In this field study on another cavity-nester, tree swallows Tachycineta bicolor , we tested whether yarrow reduced ectoparasite loads (the nest protection hypothesis), stimulated nestling immune systems (the drug hypothesis), or had other consequences for nestling growth or parental reproductive success (predicted by both preceding hypotheses). Tree swallows do not naturally add greenery to their nests, and thus offer several advantages in testing for effects of greenery independent of other potentially confounding explanations for the behaviour. We placed fresh yarrow in 23 swallow nests on the day the first egg was laid, replenishing every two days until clutch completion (=three times), and at 44 control nests, nesting material was simply touched. At 12 days of age, we measured nestling body size and mass, and took blood smears to do differential white blood cell counts. We subsequently determined the number and proportion of young fledging from nests and the number of fleas remaining after fledging. Higher humidity was associated with higher flea numbers whereas number of feathers in the nest was not. Our most significant finding was that an average of 773 fleas Ceratophyllus idius was found in control nests, versus 419 in yarrow nests. Possibly, parents compensate for blood that nestlings lose to ectoparasites by increasing food delivery, because we detected no differences between treatments in nestling mass, nestling leukocyte profiles, or proportion of young fledging, or relative to flea numbers. Our results provide no support for the drug hypothesis and strong support for the nest protection hypothesis.     

Individual quality and food availability determine yolk and egg mass and egg composition in tree swallows Tachycineta bicolor

Variation in egg size and composition can have important consequences for the quality of offspring. We investigated the factors influencing the yolk mass and egg mass of tree swallows breeding in Ithaca, NY. Using a nondestructive technique to estimate yolk mass via standardized digital-candler photographs, we compared yolk size and egg size of tree swallows Tachycineta bicolor in response to variation in food availability and individual quality. Insect availability one to three days prior to laying, but not four to six days, predicted yolk mass, while insect availability two to three days prior to laying predicted total egg mass. This suggests that, while eggs are formed over longer periods, food availability closer to time of laying has the greatest influence on egg size. These results were supported by collected eggs, as yolk rings revealed that tree swallow eggs are formed over 5–6 days. There was an influence of female quality as well, with early laying birds, independent of food availability, laying larger eggs. Eggs laid later in the laying sequence had larger yolks and greater egg mass. Overall, variation in egg quality appears to be due to a combination of environmental conditions, reflected in food resources, individual quality, and allocation tradeoffs during the laying period.     

Polygyny in the tree swallow Tachycineta bicolor: a result of the cost of searching for an unmated male

The costs imposed by parental care duties on an individual's future survival and reproduction generate conflicts because parents should attempt to minimize their investment in the present brood, and exploit the parental care of the other parent. This conflict is likely to contribute to cases of both polygamy and desertion. Here, we study the costs of polygyny in the tree swallow Tachycineta bicolor , using observations on 52 nests that were attended by polygynous males over 14 y. Both females mated to polygynous males paid reproductive costs at several stages of the nesting cycle. Clutches laid by social mates of polygynous versus monogamous males did not differ in size. However, initial brood sizes for polygynously mated females were lower because a higher proportion of their eggs failed to hatch. Likewise, fledging success was lower and nest predation rates were higher, perhaps reflecting the direct or indirect effects of reduced male attention. These results demonstrate that females pay a productivity cost when breeding with reduced male parental care. In contrast, polygynous males fledge on average more young than monogamous ones and clearly benefit from the association. We suggest that a mate-search cost is leading to the few cases of polygamous males: although females are likely evaluating males for their prospective dedication to the breeding attempt, in a short-lived bird with a short breeding season, the cost to females of searching for a more dedicated male is the risk of not breeding at all.     

Egg size, first instar behaviour and the ecology of Lepidoptera

There is striking variation in egg size among Lepidoptera. Part of the explanation could be a link between egg size and larval feeding ecology.
The relationship between absolute egg size and aspects of feeding ecology for different Lepidoptera families from different temperate regions is examined. Species that overwinter in the egg stage have larger eggs. There are significant differences in egg size with respect to feeding specificity but different families show different patterns. Woody plant feeders have larger eggs than herb feeders. There is little effect of proximity of the egg to the plant part that is eaten.
Patterns in the behaviour and survival of newly hatched larvae of 42 spp. of British Lepidoptera and their relationship to egg and larval size and to food plant characteristics are examined. Patterns in egg size with respect to feeding ecology are similar to those described above. There is a strong correlation between egg size and the size of newly hatched larvae. Newly hatched larvae survived for a mean of 1–20 days without food. Survival is not correlated with larval weight. Grass feeders survive longer than herb and woody plant feeders; the species surviving the longest feeds on lichens. Newly hatched larvae moved at a mean speed of 0.7-267.8 cm h^-1. Speed is not correlated with larval weight or survival time. Grass feeders move faster than woody plant feeders which, in turn, move faster than herb feeders. Woody plant feeders tend to move upwards, grass feeders downwards and herb feeders both upwards and downwards. The proportion of larvae silking is negatively correlated with larval weight.
The strong links between egg size and larval feeding ecology and between feeding ecology and larval behaviour are discussed. It is surprising that larval body size does not appear to constrain the speed of movement, nor tolerance to starvation.     

Sociality, ecology, and relative brain size in lemurs

The social brain hypothesis proposes that haplorhine primates have evolved relatively large brains for their body size primarily as an adaptation for living in complex social groups. Studies that support this hypothesis have shown a strong relationship between relative brain size and group size in these taxa. Recent reports suggest that this pattern is unique to haplorhine primates; many nonprimate taxa do not show a relationship between group size and relative brain size. Rather, pairbonded social monogamy appears to be a better predictor of a large relative brain size in many nonprimate taxa. It has been suggested that haplorhine primates may have expanded the pairbonded relationship beyond simple dyads towards the evolution of complex social groups. We examined the relationship between group size, pairbonding, and relative brain size in a sample of 19 lemurs; strepsirrhine primates that last share a common ancestor with monkeys and apes approximately 75 Ma. First, we evaluated the social brain hypothesis, which predicts that species with larger social groups will have relatively larger brains. Secondly, we tested the pairbonded hypothesis, which predicts that species with a pairbonded social organization will have relatively larger brains than non-pairbonded species. We found no relationship between group size or pairbonding and relative brain size in lemurs. We conducted two further analyses to test for possible relationships between two nonsocial variables, activity pattern and diet, and relative brain size. Both diet and activity pattern are significantly associated with relative brain size in our sample. Specifically, frugivorous species have relatively larger brains than folivorous species, and cathemeral species have relatively larger brains than diurnal, but not nocturnal species. These findings highlight meaningful differences between Malagasy strepsirrhines and haplorhines, and between Malagasy strepsirrhines and nonprimate taxa, regarding the social and ecological factors associated with increases in relative brain size. The results suggest that factors such as foraging complexity and flexibility of activity patterns may have driven selection for increases in brain size in lemurs.     

The price of insulation: costs and benefits of feather delivery to nests for male tree swallows Tachycineta bicolor

Many species of birds line their nests with feathers, and it has been hypothesized that this functions to provide a thermally stable microenvironment for the development of eggs and nestlings. Feathers in the nest may also function as a mechanism for parasite control, providing a physical barrier that protects nestlings from ectoparasites. We tested these hypotheses by performing a feather removal and addition experiment in tree swallows Tachycineta bicolor, a species well‐known for lining their nests with feathers. While we found no evidence that quantity of feathers in nests influenced the ability of females to produce and incubate eggs, offspring in well‐feathered nests had longer flight feathers and were structurally larger just prior to fledging that those in nests with fewer feathers. Furthermore, we also demonstrated a positive correlation between feathers and the abundance of larval blow flies Protocalliphora spp. in nests, a result opposite to that predicted by the anti‐parasite hypothesis. While our study provides strong support for the insulation hypothesis, we also discuss the possibility that devoting time to feather gathering may result in males losing paternity in their nests, although manipulative studies will be necessary to fully evaluate this idea.     

Cross‐fostering reveals sources of variation in innate immunity and hematocrit in nestling tree swallows Tachycineta bicolor

Innate immunity plays an important role in protecting birds early in development. The strength of innate immunity in nestlings appears to be affected by heritable and early maternal effects, as well as later environmental effects. To explore factors affecting innate immunity, we conducted a partial cross‐fostering experiment in nestling tree swallows Tachycineta bicolor. The ability of nestling blood plasma to kill Escherichia coli in vitro was influenced by heritable or early maternal effects and was unrelated to body condition of nestlings or brood size of the foster nest. The relative importance of heritable and early maternal effects versus environmental effects on nestling hematocrit (packed volume of red blood cells) and body condition were also examined. Hematocrit was significantly influenced by the nest in which nestlings were reared, though it was not related to nestling body condition. Body condition was significantly influenced by the nest in which nestlings were reared, though it was also weakly influenced by heritable or early maternal effects. In summary, heritable and early maternal effects are stronger predictors than rearing conditions of innate immunity in nestling tree swallows. Furthermore, hematocrit is largely environmentally determined but is not related to body condition or innate immunity, as measured in this study.     

Feeding ecology of Saguinus bicolor bicolor (Callitrichidae: Primates) in a relict forest in Manaus, Brazilian Amazonia.

This study is part of a long-term ecological study of habitat and dietary requirements of the pied bare-face tamarin (Saguinus bicolor bicolor). One group was studied for 11 months in an area of secondary forest in a suburb of Manaus, Amazonas, Brazil. Three main vegetation types occurred inside the group's home range (12 ha): capoeira, older secondary forest and campinarana (white sand forest). The tamarins ate fruits (21 species), flowers (1 species), exudates (4 species), and arthropods (insects and spiders). They spent 14.3% of total activity time seeking and eating animal prey, and 9.9% feeding on plant material, mostly fruits. In general, fruits consumed were ripe, small and succulent. Trees used for feeding were low and had small crown diameters. Three plant species (Protium aracouchinni, Myrcia cf. fallax, and Couma utilis) were used intensively during the three seasons covered by the study period. The concentrated use of 3 fruit species, each for an extended period (one fruiting species per season), provided the tamarins with a regular food supply. Tamarins consumed exudates from holes in the bark of trees of the families Anacardiaceae and Vochysiaceae, as well as gum exuded from seed pods of Mimosaceae. Exudates were exploited during the dry season and at the beginning of the wet season. Group travel was primarily based on routes connecting the fruiting trees exploited, with foraging for animal prey occurring during travel. Tamarins searched for arthropods on trunks, branches and leaves and in trunk holes. The foraging and feeding tactics displayed by S. b. bicolor are closely linked to morphological characteristics (small size and weight, claw-like nails) that allowed access to energy-rich resources (arthropods and plant exudates) in different strata of the vegetation.