Age‐related prenatal maternal effects and postnatal breeding experience have different influences on nestling development in an altricial passerine

Reproductive success and nestling performance are related to the age of parents across several vertebrate taxa. However, because breeding experience and prenatal maternal investment in reproduction often covary, the source of these age‐related differences can be difficult to determine. In this study, we evaluated the influence of prenatal maternal effects and postnatal breeding experience on the performance of nestling tree swallows Tachycineta bicolor by conducting a carefully controlled partial cross‐fostering experiment. We swapped half‐broods of nestlings between the nest of a young first‐time breeding female and the nest of a female known to have previously raised and fledged young. Our manipulation did not influence the within‐brood nestling hierarchies, and controlled for the effects of egg laying order. We found that nestlings of older females were heavier just prior to fledging regardless of the breeding experience of the attending female. In addition, fledglings raised by experienced females grew their flight feathers faster, and had greater probability of fledging. Our study demonstrates that prenatal investment in reproduction by older females can have long‐term consequences on nestling mass, and suggests limited potential for compensatory mass gains prior to fledging. Because our analyses controlled for feeding rates, our results also suggest that foraging quantity and quality are not the only benefits nestlings gain by being raised by an experienced female.     

Rainfall during parental care reduces reproductive and survival components of fitness in a passerine bird

Adverse weather conditions during parental care may have direct consequences for offspring production, but longer‐term effects on juvenile and parental survival are less well known. We used long‐term data on reproductive output, recruitment, and parental survival in northern wheatears (Oenanthe oenanthe) to investigate the effects of rainfall during parental care on fledging success, recruitment success (juvenile survival), and parental survival, and how these effects related to nestling age, breeding time, habitat quality, and parental nest visitation rates. While accounting for effects of temperature, fledging success was negatively related to rainfall (days > 10 mm) in the second half of the nestling period, with the magnitude of this effect being greater for breeding attempts early in the season. Recruitment success was, however, more sensitive to the number of rain days in the first half of the nestling period. Rainfall effects on parental survival differed between the sexes; males were more sensitive to rain during the nestling period than females. We demonstrate a probable mechanism driving the rainfall effects on reproductive output: Parental nest visitation rates decline with increasing amounts of daily rainfall, with this effect becoming stronger after consecutive rain days. Our study shows that rain during the nestling stage not only relates to fledging success but also has longer‐term effects on recruitment and subsequent parental survival. Thus, if we want to understand or predict population responses to future climate change, we need to consider the potential impacts of changing rainfall patterns in addition to temperature, and how these will affect target species' vital rates.     

Different responses to cold weather in two pied flycatcher populations

We compared how breeding parameters differ according to prevailing weather conditions between a marginal, subarctic (69°N) and temperate (61°N) population of the pied flycatcher Ficedula hypoleuca, a small migratory insectivorous passerine. We predicted that the effects of weather on breeding performance (clutch size, hatching success, nestling growth, fledging success) would be greater at northern latitudes, where the weather conditions are more extreme and unpredictable. We found that the breeding parameters, except clutch size, were not, however, inferior in the north. Northern birds, unlike the southern ones, responded to colder conditions by laying smaller clutches and maintaining a larger energy reserve (indicated by higher female body mass and higher levels of subcutaneous fat). If a cold spell occurred during the nestling period, southern flycatchers had 5–10% lower fledging success than the northern ones. Our results indicate that in the north, the breeding individuals coped with cold and variable weather better than the individuals in the southern population. This could be adaptive, because at high latitudes there is a higher probability of cold weather at the time of breeding.     

Effects of transmitters on the reproductive success of Tufted Puffins

ABSTRACT.   Although radiotelemetry is useful for monitoring nest attendance and the foraging ranges and distribution of breeding birds, attachment of transmitters may affect reproductive behavior. In 2003, we captured 25 adult Tufted Puffins ( Fratercula cirrhata ) at two colonies in Chiniak Bay, Kodiak Island, Alaska, and fitted them with subcutaneously anchored radiotransmitters (<1.2% of body mass). We determined the presence of radio-marked birds at each study site using automated, remote radiotelemetry systems, and compared rates of nestling growth, fledging mass, and fledging success for chicks with and without (control group) a radio-marked parent. Although most radio-marked adults continued to attend nests after capture and attachment of transmitters, nestlings with a radio-marked parent had lower mean growth rates (6.9 g/d vs. 14.4 g/d) and fledging success (33% vs. 84%) than control chicks. These results suggest that colony attendance by adult puffins fitted with transmitters declined sharply or completely and this led to high nestling mortality. Given the negative effects of transmitters on Tufted Puffins in our study and in other studies of alcids, we suggest delaying the attachment of transmitters until well after the brooding period. In addition, we recommend pilot studies be undertaken to distinguish the possible effects of capture and handling from those of actually carrying the device.     

The effect of egg mass on the growth and survival of blackbirds: a field experiment

A number of studies on birds have shown a positive correlation between egg mass and the growth (or survival) of chicks. However, a correlation based on non-experimental data does not demonstrate that egg mass affects growth, because it could be confounded by parental or territory quality. One way to see if pre-hatching attributes affect growth or survival is to swap hatchlings between nests, so that parental or territory quality do not confound correlations. I conducted such a fostering experiment on the blackbird, Turdus merula . Apart from very light eggs, that did not hatch, egg mass did not affect hatching success. Heavier eggs produced both heavier and larger nestlings. Nestlings raised by their own parents showed a positive correlation between hatchling mass and mass and size both early (day-4) and late (day-8) in the nestling period. The mass and size of fostered nestlings correlated with the mean mass of their natural parents' hatchlings, with higher coefficients early rather than late in the nestling period. By contrast, early in the nestling period there were no significant correlations of nestling mass or size and mean hatchling mass of the foster parents, but there were significant correlations late in the nestling period. Thus pre-hatching attributes of the egg do affect nestling size but environmental effects, including parental or territory quality, have an affect late in the nestling period. Direct manipulations of components of egg mass are required before one can conclude that egg mass affects growth, rather than some correlated pre-hatching attribute. There was no clear effect of egg mass on the probability that a hatching bird would survive until two weeks after fledging (shortly before nutritional independence), despite the fact that nestling mass does correlate with fledgling survival. I suggest that egg mass affects the 'size' component of mass and that juvenile survival depends on the 'condition' component of mass.     

Laying date,body mass and tick infestation of nestling tropical Roseate Terns Sterna dougallii predict fledging success,first‐year survival and age at first return to the natal colony

Both intrinsic and extrinsic factors recorded at individual nests can predict offspring fitness and survival but few studies have examined these effects in the tropics. We recorded nestling survival, post‐fledging survival and age at first return of Roseate Terns breeding at Aride Island, Seychelles, over a 12‐year period (1998–2009). Nest data recorded at the egg, nestling and fledging stages were collected during six breeding seasons (1998, 2001–2005) and a capture‐mark‐recapture dataset of six cohorts of fledglings was obtained from 2001–2009. Logistic regression models were used to assess the predictive effect of reproductive variables on fledging success, while multistate capture‐mark‐recapture models were used to estimate post‐fledging survival and return–recruitment probabilities to the natal site. Nestling survival probability increased with earliness of laying and was negatively affected by tick infestation during the growth period (0–23 days). Fledging probability was also positively related to chick body condition, whereas other pre‐fledging reproductive parameters such as clutch size and egg size were not influential. A multistate modelling of age‐specific survival and return–recruitment (transition) rates found that first‐year survival differed between cohorts and was also negatively affected by tick infestation. Annual survival stabilized from age 2 onwards at 0.83 ± 0.02. Transition rates were positively related to body condition at fledging, with heavier individuals returning for the first time to the natal colony at a younger age compared with lighter individuals. These results highlight the importance of local conditions encountered by tropical seabirds during the breeding season in shaping demographic parameters.     

The effect of body condition on the timing and success of breeding in Little Penguins Eudyptula minor

We studied the effect of parental body condition on the breeding biology of Little Penguins Eudyptula minor . Daily attendance patterns and body mass were recorded using an Automated Penguin Monitoring System, which collected arrival and departure masses for approximately 200 breeding birds over the 2000 and 2001 breeding seasons. Breeding success varied between the two years; 2000 was a year of average breeding success (fledging 1.07 chicks per pair), and 2001 a year of poor breeding success (fledging 0.53 chicks per pair). In both years, adult body condition (body mass divided by flipper length) increased significantly prior to laying. The laying period began over a month later in 2000 than in 2001, and birds in 2000 exhibited significantly better body condition at laying. However, the mean laying dates in 2000 were less variable than in the 2001 breeding season. Body condition appeared to influence both the time of breeding and breeding synchrony in Little Penguins. Breeding success was correlated with adult body condition at incubation in 2000 but not in 2001, indicating that success was not solely influenced by adult body condition at incubation.     

Comparing food limitation among three stages of nesting: supplementation experiments with the burrowing owl

Food availability is an important limiting factor for avian reproduction. In altricial birds, food limitation is assumed to be more severe during the nestling stage than during laying or incubation, but this has yet to be adequately tested. Using food‐supplementation experiments over a 5‐year period, we determined the degree and timing of food limitation for burrowing owls (Athene cunicularia) breeding in Canada. Burrowing owls are an endangered species and food limitation during the nestling stage could influence reproductive performance of this species at the northern extent of their range. Supplemented pairs fledged on average 47% more owlets than unfed pairs, except during a year when natural food was not limiting (i.e., a prey irruption year). The difference in fledgling production resulted from high nestling mortality in unfed broods, with 96% of all nestling deaths being attributed to food shortage. Supplemental feeding during the nestling period also increased fledgling structural size. Pairs fed from the start of laying produced the same number of hatchlings as pairs that received no supplemental food before hatch. Furthermore, pairs supplemented from egg laying to fledging and pairs supplemented during the nestling period alone had the same patterns of nestling survival, equal numbers of fledglings, and similar fledgling mass and structural size. Our results provide empirical support for the hypothesis that the nestling period is the most food‐limited phase of the breeding cycle. The experimental design we introduce here could be used with other altricial species to examine how the timing of food limitation differs among birds with a variety of life‐history strategies. For burrowing owls, and other species with similar life histories, long‐term, large‐scale, and appropriately timed habitat management increasing prey abundance or availability is critical for conservation.     

Avoidance,tolerance, and resistance to ectoparasites in nestling and adult tree swallows

We examined avoidance, tolerance, and resistance strategies of nestling and adult tree swallows Tachycineta bicolor in response to ectoparasitic blowflies Protocalliphora sialia. Tree swallows avoided settling in north‐facing nest boxes early in the breeding season. These boxes were more likely to be parasitized later in the season, suggesting that box selection may facilitate blowfly avoidance. After experimentally manipulating blowfly intensity, we found that nestlings were generally tolerant of parasitism. Parasites significantly reduced nestling blood hemoglobin but had no effect on nestling body mass, primary feather growth, age at fledging, or fledging success. Parents of parasitized nestlings did not increase their food provisioning rate to promote nestling tolerance. Adult female tree swallows demonstrated both tolerance and resistance: blowfly parasitism had no effect on adult hemoglobin and body mass, and those with higher P. sialia‐binding antibody levels had fewer blowfly larvae in their nests. Nestling antibodies were unrelated to blowfly intensity. Despite considerable variation among years, our results suggest that the costs of blowfly parasitism to nestling and adult tree swallows are modest, and limited to blood loss in nestlings. Future work should examine the effects of reduced blood hemoglobin on fledgling survival and the importance of parasite‐specific antibodies.     

Seasonal variation in reproductive success and post-nuptial moult of blue tits in southern Europe: an experimental study

Post-nuptial moult and reproductive success were studied in relation to timing of breeding in blue tits, Parus caeruleus, breeding in southern Europe. A group of experimentally delayed pairs was created by removing first clutches, thereby inducing late repeat clutches. Reproductive success and post-nuptial moult of delayed pairs were compared with both control pairs that bred early and unmanipulated late-breeding pairs. Delayed pairs fledged fewer young and with a lower body mass than control pairs. However, the number of fledged young and fledgling mass did not differ between delayed and late-breeding pairs. These results were more consistent with the date hypothesis, and it is concluded that the timing of breeding and reproductive success may be causally related in the blue tit. This study reveals a harmful effect of relaying on female body mass at the end of the nestling period. Therefore, females apparently pay the costs of relaying, since a reduction in body mass during the nestling period may be accompanied by a lowered survival probability. Delayed and late-breeding males often began moulting while still feeding young, but neither control males nor females from the three study groups did so. These results support the view that timing-related energy constraints on breeding may be important causes of a seasonal decline in reproductive success at different latitudes. Received: 15 March 1999 / Accepted: 19 July 1999     

2. SWIFTS AND OTHER BIRDS NESTING IN BUILDINGS

The nesting success of Marabou Storks Leptoptilos crumeniferus breeding in north-eastern Swaziland is closely associated with rainfall, with nests started late in the season exposed to higher rainfall and showing lower success. This may be related to lower food intake and slower growth of the chicks. This study set out to determine whether hatching date and sequence of laying affected the growth rate of chicks. Chicks were also sexed, as Marabou Storks show sexual size dimorphism—males are on average 20% larger—and this trait is often associated with differing patterns of growth between sexes. Nestlings were measured weekly from hatching until they either died or fledged. Nestling development is described in detail and photographs of different-aged chicks are presented. The nestling period was significantly shorter for female chicks, at 94 d, than for male chicks at 104 d. Male and female chicks differed in growth rate and asymptote for both mass and wing length. Unusually, females showed higher instantaneous growth rates for much of the nestling period. Chicks surpassed adult mass before fledging. Date of hatching had an effect on growth rates, with chicks at late nests having slower growth, consistent with a decline in food availability. Marabou Storks appear to be slower growing than expected for the Ciconiidae, the taxonomic family to which they belong.     

Rates and consequences of relaying in little auks Alle alle breeding in the High Arctic an experimental study with egg removal

Most seabirds have a small clutch size. Thus, replacement of a clutch after loss can make important contributions to an individual’s lifetime reproductive success. However, in the condition of short polar summer, relaying propensity may be time‐constrained. In this study, we investigated rates and consequences of relaying in a small High Arctic seabird, the little auk Alle alle. We performed an experiment in which we removed the single egg from 20 nests of early‐laying breeders. We measured relaying rates, and compared chick body mass and breeding success between the experimental and control nests. Despite the narrow window of the Arctic summer and the closely synchronized breeding, 75% of females produced a replacement egg just 2.7% smaller in volume than the first egg. This indicates that in little auks, the demographic effects of disruptions to breeding attempts (by predators, adverse weather or human activity) may be mitigated to some extent by replacement clutches. However, peak body mass and fledging body mass were lower in the experimental than the control chicks. This effect was rather a consequence of late hatching – chicks from replacement clutches followed seasonal decline in peak body mass and fledging mass. Finally, breeding success and chick survival up to 20 d in the experimental nests were respectively 34 and 37% lower than in the control nests. Thus, the quality and post‐fledging survival of chicks from the replacement clutches were probably lower compared to the chicks hatched from the first‐laid eggs.     

Early life events carry over to influence pre-migratory condition in a free-living songbird

Conditions experienced during development can have long-term consequences for individual success. In migratory songbirds, the proximate mechanisms linking early life events and survival are not well understood because tracking individuals across stages of the annual cycle can be extremely challenging. In this paper, we first use a 13 year dataset to demonstrate a positive relationship between 1(st) year survival and nestling mass in migratory Savannah sparrows (Passerculus sandwichensis). We also use a brood manipulation experiment to show that nestlings from smaller broods have higher mass in the nest relative to individuals from larger broods. Having established these relationships, we then use three years of field data involving multiple captures of individuals throughout the pre-migratory period and a multi-level path model to examine the hypothesis that conditions during development limit survival during migration by affecting an individual's ability to accumulate sufficient lean tissue and fat mass prior to migration. We found a positive relationship between fat mass during the pre-migratory period (Sept-Oct) and nestling mass and a negative indirect relationship between pre-migratory fat mass and fledging date. Our results provide the first evidence that conditions during development limit survival during migration through their effect on fat stores. These results are particularly important given recent evidence showing that body condition of songbirds at fledging is affected by climate change and anthropogenic changes to landscape structure.     

Behavior of adult and young grassland songbirds at fledging

The behavior of adults and young at the time of fledging is one of the least understood aspects of the breeding ecology of birds. Current hypotheses propose that fledging occurs either as a result of parent‐offspring conflict or nestling choice. We used video recordings to monitor the behavior of nestling and adult grassland songbirds at the time of fledging. We observed 525 nestlings from 166 nests of 15 bird species nesting in grasslands of Alberta, Canada, and Wisconsin, USA. Overall, 78% of nestlings used terrestrial locomotion for fledging and 22% used wing‐assisted locomotion. Species varied in propensity for using wing‐assisted locomotion when fledging, with nestling Grasshopper Sparrows (Ammodramus savannarum) and Henslow's Sparrows (Centronyx henslowii) often doing so (47% of fledgings) and nestling Song Sparrows (Melospiza melodia), Common Yellowthroats (Geothlypis trichas), and Chestnut‐collared Longspurs (Calcarius ornatus) rarely doing so (3.5% of fledgings). For 390 fledging events at 127 nests, camera placement allowed adults near nests to be observed. Of these, most young fledged (81.5%) when no adult was present at nests. Of 72 fledging events that occurred when an adult was either at or approaching a nest, 49 (68.1%) involved feeding. Of those 49 fledgings, 30 (62.1%) occurred when one or more nestlings jumped or ran from nests to be fed as an adult approached nests. The low probability of nestlings fledging while an adult was at nests, and the tendency of young to jump or run from nests when adults did approach nests with food minimize opportunities for parents to withhold food to motivate nestlings to fledge. These results suggest that the nestling choice hypothesis best explains fledging by nestlings of ground‐nesting grassland songbirds, and fledging results in families shifting from being place‐based to being mobile and spatially dispersed.     

Selection for synchronous breeding in the European starling

Colonial birds often demonstrate considerable breeding synchrony. In southern Sweden the semi-colonial European starling initiated the vast majority of clutches within one week. Laying dates were positively skewed so that many birds initiated clutches at similar dates early in the season. Breeding was further synchronised by a particularly strong clutch-size reduction equivalent to one third of an egg per day during the first part of the breeding season. The decline in clutch size with season also held true for separate age-classes of females, for individual females laying at different times at different years and for individual females laying at different times the same year. Trends in breeding success during nestling rearing were unlikely to explain the high degree of breeding synchrony or the seasonal decline in clutch size; nestling survival and growth were weakly related or unrelated to reproductive timing. In contrast recruitment success of fledged offspring declined sharply with season. Even within the synchronous laying period, defined as clutches initiated during the first week each year, local recruitment success declined. It is suggested that the early seasonal decline is caused by selection for synchronous fledging permitting the immediate formation of flocks after fledging, whereas the late seasonal trends may be caused by either population differences in female quality or deteriorating conditions for raising young.     

Brood provisioning rates and fledgling behavior of Cordilleran Flycatchers in southwestern Colorado

The behavior of young songbirds after fledging is one of the least understood phases of the breeding cycle, although parental provisioning rates and movement of fledglings are key to understanding life history evolution. We studied Cordilleran Flycatchers (Empidonax occidentalis) at two sites in southwestern Colorado, USA, from 2012 to 2017. We banded and sexed breeding adults to determine the relative contributions of males and females to nestling and fledgling care, and attached radio‐transmitters to nestlings to facilitate observations of brood behavior after fledging. Females made 60% and 78% of total observed feedings of nestlings and fledglings, respectively. Parental provisioning rates increased with nestling age, and per‐nestling provisioning rates increased with brood size. Parental provisioning rates declined just before fledging, then increased just after fledging. Fledging times of individuals in broods were asynchronous and concentrated during the late afternoon and early evening. Males stopped caring for fledglings before females even though this species is single‐brooded, with some late‐season broods being abandoned by males. Broods spent the first three weeks after fledging within 400 m of nests, after which they began to disperse. Most aspects of the breeding biology of Cordilleran Flycatchers in our study, including the duration of nestling and fledging periods, female‐dominated provisioning, and movement patterns of fledglings, were similar to those of other Empidonax species. However, the times when young fledged were not concentrated in the morning as reported in most other songbirds, and this result warrants additional study of the timing of fledging in ecologically and taxonomically similar species. The increased per‐nestling provisioning rate with increasing brood size was unexpected, and additional study is needed to determine if this increase results from a trade‐off between adult annual survival and productivity favoring increased provisioning of young in larger broods, or from the existence of high‐quality individuals where larger clutches and higher provisioning rates are linked.     

Effects of temperature and nest heat exposure on nestling growth,dehydration and survival in a Mediterranean hole‐nesting passerine

Variable environments impose constraints on adaptation by modifying selection gradients unpredictably. Optimal bird development requires an adequate thermal range, outside which temperatures can alter nestling physiology, condition and survival. We studied the effect of temperature and nest heat exposure on the reproductive success of a population of double‐brooded Spotless Starlings Sturnus unicolor breeding in a nestbox colony in central Spain with a marked intra‐seasonal variation in temperature. We assessed whether the effect of temperature differed between first and second broods, thus constraining optimal nest‐site choice. Ambient temperature changed greatly during the chick‐rearing period and had a strong influence on nestling mass and all body size measures we recorded, although patterns of clutch size or nestling mortality were not influenced. This effect differed between first and second broods: nestlings were found to have longer wings and bills with increasing temperature in first broods, whereas the effect was the opposite in second broods. Ambient temperature was not related to nestling body mass or tarsus‐length in first broods, but in second broods, nestlings were lighter and had smaller tarsi with higher ambient temperatures. The exposure of nestboxes to heat influenced nestling morphology: heat exposure index was negatively related to nestling body mass and wing‐length in second broods, but not in first broods. Furthermore, there was a positive relationship between nest heat exposure and nestling dehydration. Our results suggest that optimal nest choice is constrained by varying environmental conditions in birds breeding over prolonged periods, and that there should be selection for parents to switch from sun‐exposed to sun‐protected nest‐sites as the season progresses. However, nest‐site availability and competition for sites are likely to impose constraints on this choice.     

High blowfly parasitic loads affect breeding success in a Mediterranean population of blue tits

The detrimental effects of ectoparasites on the breeding success of birds have been especially well demonstrated in the case of ectoparasites that affect both chicks and their parents. Since blowfly larvae of the genus Protocalliphora attack only nestling birds, they represent a good model for testing the consequences of parasitism on nestlings. A Corsican population of blue tit suffers extremely high rates of infestation by blowflies, which are suspected to negatively affect young birds. Comparing experimentally deparasitized (treated) and naturally infested (control) broods, we showed that the attack by Protocalliphora causes anaemia and an important disturbance to the chicks. Therefore, we expected that these effects would have a negative impact on body condition and survival in the infested broods. Although we did not find any effect of treatment on fledging success, our predictions were confirmed by lower growth rate, body mass at fledging and tarsus length at fledging in the control compared with the treated group. This suggests that in this population, blowflies decrease the probability of recruitment of young blue tits. Received: 6 May 1997 / Accepted: 14 July 1997     

The breeding biology of the Pacific Swallow Hirundo tahitica in Malaysia

The breeding biology of the Pacific Swallow was studied in Malaysia. Time-budgets and preliminary energy-budgets were calculated for different stages of the breeding cycle. Breeding was seasonal, the first eggs were laid in the first week of March from 1978 to 1982. The nest was a mud cup built under an overhang, often on man-made structures over water. The mean clutch size was 2.98 eggs. So more than two successful clutches were ever raised, although up to four sets of eggs can be laid if clutches are lost. The incubation period was 16.2 days and the nestling period 19–21 days. Nestling weights showed the typical hirundine recession before fledging. The mean brood size was 2.32 and there was no evidence of undernourishment in larger broods. Nesting success was lower than for temperate hirundines but high for a tropical passerine. Incubation duties were carried out by the female alone but both parents fed the brood. The male never delivered more than 44% of the total feeds. Daily energy expenditure varied from 3.44–4.9×BMR depending upon the stage of the breeding cycle.     

Within territory abundance of red wood ants Formica rufa is associated with the body condition of nestlings in the Eurasian treecreeper Certhia familiaris

Studies on individual reproductive success in relation to interspecific competition between distantly related taxa are scarce. We studied whether the abundance of red wood ants Formica rufa -group is related to the breeding habitat selection, fecundity and offspring quality in the Eurasian treecreeper Certhia familiaris, an old-growth forest passerine. The nest-box occupancy data were gathered over a five-year study period, whereas the breeding performance analyses were based on a two-year data set. The abundance of wood ants, measured within 50  m around the nest-boxes, was not related to nest-box occupancy rate, fecundity or the physiological stress of nestlings. In contrast, the abundance of wood ants was negatively related to the offspring quality and life-history traits, such as lowered body mass, subcutaneous fat reserves, and tarsus length at fledging. Our results suggest that exploitative competition between distantly related taxa may have considerable and adverse influences on nestling quality as measured by body mass, subcutaneous, fat and tarsus length. Red wood ants may decrease the fitness of treecreepers as the lower body condition of nestlings has the potential to impair recruitment into the breeding population and, additionally, impair the future reproductive effort.