Treecreeper Certhia Familiaris Research Articles

The Eurasian Treecreeper (Certhia familiaris) has an effective camouflage against mammalian but not avian vision in boreal forests

A well‐known example of visual camouflage in birds is the plumage coloration of the Eurasian Treecreeper Certhia familiaris, yet this species’ camouflage has never been objectively quantified. Here, we quantify Treecreeper camouflage in its boreal forest habitat, test whether Treecreepers better match tree backgrounds at nest‐site, territory or habitat spatial scales, and explore which common tree species provide the best background match. Using photographic data of the birds and forested backgrounds, we test their background match using human, ferret and avian vision modelling. We found that a Treecreeper’s wing and mantle provided the closest background matching, whereas the wing stripe and tail were more conspicuous against tree trunks. Vision modelling also suggests that Treecreeper camouflage provides a better protection against mammalian than against avian vision. The matching was not significantly different across spatial scales in local spruce forests. However, the background match was better on conifers than on deciduous trees. Our results support the long‐standing conjecture of Treecreeper camouflage but also suggest that the camouflage is an adaptation especially against mammalian predators, which are important nest predators of Treecreepers.

Single dead trees matter: Small-scale canopy gaps increase the species richness, diversity and abundance of birds breeding in a temperate deciduous forest

Canopy gaps produced by natural disturbances are an important source of structural and functional heterogeneity within forest ecosystems. However, the effect of natural, small-scale canopy gaps, caused by the mortality of a single tree or a group of trees, on bird assemblages has not been adequately addressed in temperate deciduous forests. We studied the effects of small-scale canopy gaps on the breeding birds of old-growth deciduous forests in the Carpathians (SE Poland). Our hypothesis was that such small gaps and the adjacent closed-canopy forest would differ in bird richness, diversity and relative abundance. Breeding bird assemblages were surveyed using point counts in 30 canopy gaps (mean area 301 m2 ± 228 SD, range: 37–970 m2) and in 30 control plots situated in the adjacent closed-canopy forest. The species richness, diversity and relative abundance of birds were significantly higher in the gaps than in the control plots and were correlated with the number of large snags. Collared Flycatcher Ficedula albicollis and White-backed Woodpecker Dendrocopos leucotos occurred more frequently in the gaps than in the control plots. Eurasian Treecreeper Certhia familiaris, Blackcap Sylvia atricapilla, Song Thrush Turdus philomelos and Eurasian Nuthatch Sitta europaea were more abundant in the gaps than in the control plots. The species richness of cavity- and ground-nesters was higher and these birds were more abundant in the gaps than in the control plots. The presence/absence of a gap and the number of large snags recorded in it explained 35.4% and 52.7% of the deviance in the total number of species and the total number of individual birds, respectively. Our results indicate that even small-scale canopy gaps, in combination with the large snags recorded in them resulting from the death of a single tree or group of trees, affect bird assemblages in deciduous forests as measured by the occurrence or abundance of individual species or community indices. We suggest that treefall gaps, which are heterogeneous structures, mediate the spatial distribution of bird territories within a stand and are crucial for the biodiversity of temperate deciduous forests.

Interactions between sibling species of treecreepers Certhia familiaris and C. brachydactyla in the Pyrenees and the mistaken identity hypothesis

ABSTRACT Capsule Aggressive interactions between two treecreeper species were probably the result from misidentification of heterospecifics rather than resource competition. Aims To test the alternative hypotheses that aggressive interactions between two sibling species, the Eurasian Treecreeper Certhia familiaris and the Short-toed Treecreeper Certhia brachydactyla, were the result of resource competition or misidentification of the species involved. Methods Interspecific interactions were investigated by direct observation and territory mapping of the two species during 12 years in a 25 ha plot in the Pyrenees. In addition, we performed playback experiments to test the behavioural response of both species to the heterospecific song in three sympatric areas of the Pyrenees, and in an adjacent area where only the Short-toed Treecreeper occurred. Results Territories largely overlapped between the two species. Subsequently, distance between the two species’ nearest nests was significantly shorter than distance between the same species’ nearest nests. Overall, the Short-toed Treecreeper was more aggressive towards the sibling species than the Eurasian Treecreeper. However, in contrast to what might be expected according to the resource-competition hypothesis, it was not significantly more aggressive in sympatry than in allopatry. Conclusion These results suggest that the aggressive behaviour of the Short-toed Treecreeper was mainly caused by the birds mistaking the identity of Eurasian Treecreepers, and had little impact on the cohabitation between the two species.

How generalist are these forest specialists? What Sweden’s avian indicators indicate

AbstractMonitoring of forest biodiversity and habitats is an important part of forest conservation, but due to the impossible task of monitoring all species, indicator species are frequently used. However, reliance on an incorrect indicator of valuable habitat can reduce the efficiency of conservation efforts. Birds are often used as indicators as they are charismatic, relatively easy to survey, and because we often have knowledge of their habitat and resource requirements. In the Swedish government’s environmental quality goals, there are a number of bird species identified as being associated with ‘older’ and ‘high natural value’ forests. Here we evaluate the occurrence of four of these indicator species using data from 91 production forest stands and 10 forest reserves in southern Sweden. The bird species assessed are willow tit Poecile montanus, coal tit Periparus ater, European crested tit Lophophanes cristatus and Eurasian treecreeper Certhia familiaris. For the production stands assessed, these indicator species exhibited no significant preferences regarding forest composition and structure, indicating a wider range of habitat associations than expected. These species frequently showed territorial behavior in forest stands <60 and even 40 years of age; much younger than the 120‐year threshold for ‘older forest’ as defined by governmental environmental goals. As almost 80% of the production stands ≥10 years old included at least one of the four indicator species, this raises questions regarding the suitability of these species as indictors of forests of high conservational value in southern Sweden. Notably, besides the four species assessed here, none of the additional indicator taxa identified by the government, were recorded in the 10 reserves. This outcome may reflect the difficulties involved in finding bird indicator species indicative of high natural values in this region. Our results highlight the importance of coupling bird surveys with quantified assessments of proximate vegetation cover.

The impact of landslide stabilization on birds: Insights from an Alpine valley

Landslides are a common issue in mountains and other areas with slopes, and their frequency is increasing because of climate change. Interventions aiming at stabilizing slopes are thus common worldwide. The environmental monitoring of consequences of such interventions rarely consider the potential impacts on wild species, and especially on animal taxa. Birds are widely adopted as biological indicators thanks to their ecology and high sensitivity to environmental changes, and could represent an ideal subject also for monitoring the impacts of landslide stabilization. By monitoring birds for 8 years in a complex restoration intervention in the Italian Alps, I investigate their potential use for understanding the impacts of landslide rehabilitation at different scales, in a 350 ha-area. A BACI protocol was adopted, with bird data collected by means of point counts. Generalized linear mixed models were used to evaluate the potential impacts of restoration phases and intervention sites; analyses were developed for the breeding period and for the whole year, and for the local vs. large scale.I found evidence for different types of impacts, including the local impact of construction sites (e.g. negative for chaffinch Fringilla coelebs), of in operam phase (negative for mistle thrush Turdus viscivorus, Eurasian treecreeper Certhia familiaris and crested tit Lophophanes cristatus, positive for grey wagtail Motacilla cinerea and rock bunting Emberiza cia), as well as for a disturbance effect on species' detectability and for independent trends. The overall species richness was not (or very scarcely) affected by restoration works. This 8 yr-work provides an example of the potential efficacy of birds as indicators of the environmental impacts caused by landslide rehabilitation, which will likely become increasingly common in the next decades. Avian monitoring could helpfully be integrated within standard monitoring of environmental impacts of landslide stabilization/restoration.

Use of ‘tree tin’ nesting sites by the Eurasian Treecreeper Certhia familiaris, with notes on nest composition

ABSTRACT A corrugated bituminised roofing felt ‘tree tin’ originally provided to serve as a bat roost was adopted on a regular basis by Eurasian Treecreepers as nest sites. Data on nest construction under ‘tree tins’ are discussed.

Factors affecting the distribution of the sibling species of treecreepers Certhia familiaris and C. brachydactyla in the Pyrenees

We investigated the main environmental variables determining the occurrence of the Eurasian Treecreeper Certhia familiaris and Short-toed Treecreeper C. brachydactyla within the mountain range of the Pyrenees. Presence/absence data were obtained, using the response to the playback of each species’ typical song; they were collected during the springs of 2004–2012 over 487 sites, within 33 areas of mature forest distributed over the mountain range. The analysis, based upon generalised linear models, showed that altitude, forest type, location within the mountain range and the presence of the sibling species influenced the occurrence of both treecreepers. Probability of occurrence of the Eurasian Treecreeper increased with altitude except in mountain pine forest, whereas at a given altitude it was lower in Scots pines than in other forest types. In the Short-toed Treecreeper, whose probability of occurrence was enhanced in oak forest, the altitude had a negative influence except in mountain pine forest. In the Eurasian Treecreeper, all other things being equal, probability of occurrence was higher in the forested areas experiencing an oceanic climate than those experiencing a Mediterranean-continental climate. The opposite was found in the Short-toed Treecreeper, whose probability of occurrence on the Pyrenees’ southern slope increased from the western to the eastern end of the mountain range. Finally, the presence of the sibling species decreased the probability of occurrence of both treecreepers at low and medium altitude, suggesting a restricted form of interspecific competition.

Modified forest rotation lengths: Long-term effects on landscape-scale habitat availability for specialized species

We evaluated the long-term implications from modifying rotation lengths in production forests for four forest-reliant species with different habitat requirements. By combining simulations of forest development with habitat models, and accounting both for stand and landscape scale influences, we projected habitat availability over 150 years in a large Swedish landscape, using rotation lengths which are longer (+22% and +50%) and shorter (−22%) compared to current practices. In terms of mean habitat availability through time, species requiring older forest were affected positively by extended rotations, and negatively by shortened rotations. For example, the mean habitat area for the treecreeper Certhia familiaris (a bird preferring forest with larger trees) increased by 31% when rotations were increased by 22%, at a 5% cost to net present value (NPV) and a 7% decrease in harvested volume. Extending rotation lengths by 50% provided more habitat for this species compared to a 22% extension, but at a much higher marginal cost. In contrast, the beetle Hadreule elongatula, which is dependent on sun-exposed dead wood, benefited from shortened rather than prolonged rotations. Due to an uneven distribution of stand-ages within the landscape, the relative amounts of habitat provided by different rotation length scenarios for a given species were not always consistent through time during the simulation period. If implemented as a conservation measure, prolonging rotations will require long-term strategic planning to avoid future bottlenecks in habitat availability, and will need to be accompanied by complementary measures accounting for the diversity of habitats necessary for the conservation of forest biodiversity.

Density and fluctuations of a nest-pocket breeding population of the Treecreeper Certhia familiaris over a 28-year period

The number of breeding Treecreeper pairs was estimated from 1982 to 2009 in a 2.7 km2 study area located in south-western Sweden (57°39'N; 12°4'E). Most of the area, which was provided with 205 man-made nest pockets, is covered by broad-leafed forest. The number of first clutches varied between 5 and 21 with an annual average of 14±4.2 (SD) breeding pairs (CV 30%). The density of breeding Treecreepers varied from 1.9 to 7.8 pairs/km2 with a mean of 5.1±1.86 pairs/km2. The population did not show any statistically significant density trend over the 28 years. The between-year variation in the return rate of ringed adult females that bred after wintering was significantly negatively related to the temperature and precipitation means of the preceding winter. Thus, fewer females returned after milder winters with higher precipitation. The statistical tests pertaining to the variation in the whole breeding population indicate that the species can cope with moderate fluctuations in winter weather, thus preventing significant changes in the number of breeders in the study area.

Bird ringing and nest recording in Britain and Ireland in 2011

This is the 75th annual report of the British Trust for Ornithology's Ringing Scheme, covering data received and work carried out in 2011. A review of priorities for the Scheme identified the need to increase submission of nest records through the Nest Record Scheme and to extend the network of Retrapping Adults for Survival (RAS) studies. Collection of different demographic parameters from the same site (eg nest records, numbers of breeding pairs, adult survival, etc) can provide a powerful tool to identify reasons for population change. Constant Effort Sites (CES) data showed long-term declines in abundance for four migrants (Willow Warbler Phylloscopus trochilus, Lesser Whitethroat Sylvia curruca, Sedge Warbler Acrocephalus schoenobaenus and Reed Warbler Acrocephalus scirpaceus) and three resident species (Willow Tit Poecile montana, Song Thrush Turdus philomelos and Reed Bunting Emberiza schoeniclus). Robin Erithacus rubecula and Reed Bunting had the lowest survival rates since CES began and survival-rate estimates were also significantly below average for Long-tailed Tit Aegithalos caudatus and Wren Troglodytes troglodytes, two small resident species that are particularly vulnerable to cold weather. Abundance of Blue Tit Cyanistes caeruleus, Great Tit Parus major, Goldfinch Carduelis carduelis and Bullfinch Pyrrhula pyrrhula recorded by CES increased. Nine of the 16 resident species covered by CES also had higher-than-average productivity, in particular the highest ever recorded for Dunnock Prunella modularis, Chaffinch Fringilla coelebs and Bullfinch. The number of Constant Effort Sites increased in 2011, with a higher percentage of reedbed habitat covered by the scheme, but fewer dry and wet scrub sites. In 2011 the number of RAS projects increased by 34 to 158. Survival trends were generated for 46 species and the use of colour marks has been promoted to increase the resighting probability of trap-shy species such as House Sparrow Passer domesticus and Starling Sturnus vulgaris, and for species with low recapture probability such as seabirds. As in 2010, the total number of birds ringed in 2011 (1,151,665) was the highest ever. It included 540,696 first-year birds, 383,136 adults (birds in their second calendar year or later) and 27,495 unaged birds. All ringing data for 2011 were computerised. In addition, there were 166,061 recaptures of ringed birds at or near the ringing site. The increased numbers of fully grown birds ringed is probably due to three main factors: high survival of some species, as suggested by CES results, combined with high productivity in 2011 and the increasing number of ringers in the Scheme catching higher numbers of birds. In total 20,702 BTO-ringed birds were recovered (found dead or recaptured away from the ringing site) in 2011, the highest ever as a result of the increased number of birds ringed in the past decade, combined with efforts to increase reports of ringed birds by the public, an increasing number of colour-ringing projects and associated resightings, and a change in the way recoveries are processed. Recoveries of 17 BTO- and 13 foreign-ringed birds that have extended our existing knowledge of movements of a species, are unusual movements, or are a new longevity record are also presented. In 2011 40,845 nest records were submitted (over 70% electronically), the highest total since 1998. Of these, 82% originated from England, 9% from Wales and 8% from Scotland, with the remaining 1% divided equally between Northern Ireland and the Republic of Ireland. Long-term nest productivity trends (1966–2010) were significant for 36 of the 71 species for which sufficient data were available, and seven of these (Nightjar Caprimulgus europaeus, Tree Pipit Anthus trivialis, Willow Warbler, Bullfinch, Coal Tit Periparus ater, Treecreeper Certhia familiaris and Chaffinch) showed a significant decline over time. The 2011 breeding season was characterised by an early start compared to the previous five years, possibly due to the warmest April on record.

Parental and genetic factors rather than nesting forest patch size affect physiological traits of offspring in an area‐sensitive passerine: an experimental study

Our understanding of the effects of habitat loss on individual performance is limited due to a lack of experimental studies that take the potential genetic and parental effects producing phenotypic variation into consideration. To assess the relative role of habitat loss on offspring phenotype while controlling for the confounding effects of genetic and parental variation we performed a partial cross‐fostering experiment using the Eurasian Treecreeper Certhia familiaris. The experiment was carried out by swapping half the nestlings in a brood between small and large nesting forest patches to determine the effect of nesting forest patch size on five nestling traits reflecting morphological size, body condition, physiological stress and inflammation status. There was no effect of nesting forest patch size on the offspring traits examined. Instead, we found evidence of genetic and early parental effects on all traits except inflammation status, as well as parental effects after cross‐fostering for all of the measured offspring traits. Our results suggest that genetic and parental effects should be taken into account when making inferences about species’ responses to habitat loss.

The efficiency of three-visit square surveys vs. one-visit line transects in censusing sparsely distributed birds in managed forest landscapes

SummaryWe conducted three-visit surveys of 1-km2plots and traditional Finnish single-visit line transects (considering only the 50 m wide main belt) to evaluate these methods in censusing of a predetermined set of 23 target species known to prefer old forests in three regions in Finland. The efficiency of the two methods was compared on the basis of the number of territories recorded per hour. An attempt was made to find indicators of the occurrence of suitable habitats for species preferring old forest in general, including the rarest ones, and so also largely indicating total diversity of forest bird fauna of the study area. The total number of pairs observed per hour and the abundance of sedentary bird species were significantly higher in the square surveys than in the main belt of the line transects. There were significant positive relationships between the densities of relatively abundant (density > 1.0 pairs km−2) and less abundant target species. There emerged five common forest bird species that seemed to form a suitable set of indicators of the occurrence of habitats for birds preferring old forest in the northern boreal zone: Great Spotted WoodpeckerDendrocopos major, Mistle ThrushTurdus viscivorus, Willow TitParus montanus, Eurasian TreecreeperCerthia familiaris, and Eurasian BullfinchPyrrhula pyrrhula. We concluded that sedentary species preferring old forest are good candidates for indicators to characterize some threatened aspects of forest bird diversity.

Bird ringing in Britain and Ireland in 2009

This is the 73rd annual report of the British Trust for Ornithology's Ringing Scheme, covering data received and work carried out in 2009. Much of the research carried out during the year looked at ways to improve the analysis of ringing data to understand population change. This included producing survival rates from the Constant Effort Sites (CES) scheme for the first time and further developing methods of analysing our demographic data sets in an integrated manner. Research was also undertaken on the biometric data to investigate how individual condition responds to long‐term environmental change. CES data suggested that the low productivity in 2007 and 2008, combined with one of the coldest winters since 1995/96 for residents, contributed to reduced adult numbers in 2009 in many species. There were significant decreases in Wren Troglodytes troglodytes, Sedge Warbler Acrocephalus schoenobaenus, Reed Warbler A. scirpaceus, Blackcap Sylvia atricapilla, Lesser Whitethroat S. curruca, Willow Warbler Phylloscopus trochilus, Long‐tailed Tit Aegithalos caudatus, Blue Tit Cyanistes caeruleus, Great Tit Parus major, Treecreeper Certhia familiaris, Chaffinch Fringilla coelebs, Greenfinch Carduelis chloris and Reed Bunting Emberiza schoeniclus. However, a good breeding season, for both residents and migrants, resulted in significantly higher productivity for 15 species, but significantly lower for Willow Tit Poecile montana, compared to the long‐term mean (1983–2007). Data for 104 Retrapping Adults for Survival studies were submitted, covering 39 species, three quarters of which are Birds of Conservation Concern. The total number of birds ringed (935,867) was the highest ever recorded, and was over 10% higher than the mean of the previous five years. However, the total number of pulli ringed in 2009 (179,712) was only 5% higher than the preceding five‐year mean, while that of fully grown birds (756,155) was over 13% higher. The recovery total (14,643) was also high. However, this is partly due to changes in the way that records of colour‐ringed and other specially marked birds are stored. The number of recoveries of foreign‐ringed birds (1,314) was higher than the mean of the preceding five years, although this figure is influenced by varying response times of different ringing schemes. Recovery details for 250 individual birds are given in the final section of the report. They include movements that confirm suspected or known migration patterns, unexpected movements and longevity records.

First, second and replacement broods in the breeding biology of a Treecreeper Certhia familiaris population

First, second and replacement broods in the breeding biology of a Treecreeper Certhia familiaris population

First, second and replacement broods in the breeding biology of a Treecreeper Certhia familiaris population

The breeding behaviour of a Treecreeper population that bred in nest pockets and in which all females were colour-ringed was followed over ten seasons. On average, 50% of the first breeding attempts in spring failed. Between half and two thirds of the successful females were double-brooded. The remaining females were single- brooded. These two breeding categories were identical in terms of initial clutch size and egg volume. The double-brooded females invested less in their second than their first clutch. Most of the females that failed laid replacement clutches of similar size as in the first attempt but with larger eggs. A replacement brood where young successfully fledged was never followed by a second breeding attempt. The average distance between the first and the repeat nest was the same for double-brooded and replacement-breeding females. The egg volumes of the first and the repeat clutches were strongly correlated. The double-brooded pairs were the most common breeding category and reared 50% of the population’s total output of fledged broods, while the single-brooded and replacement breeding pairs each fledged ca. 25%. The adaptive values of the three breeding categories are compared.

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

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

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.

Where in Europe should we look for sources of the cutaneous trematode Collyriclum faba infections in migrating birds?

Cutaneous cysts with trematodes of Collyriclum faba have been found in birds during their spring and post-breeding migrations in the Czech Republic. During spring migrations, C. faba was found in one dunnock Prunella modularis, two European robins Erithacus rubecula, three common nightingales Luscinia megarhynchos, one song thrush Turdus philomelos and one great reed warbler Acrocephalus arundinaceus. During post-breeding migration, the same parasite was found in one garden warbler Sylvia borin, one whitehroat S. communis, three goldcrests Regulus regulus and one Eurasian treecreeper Certhia familiaris. The newly identified hosts of C. faba are dunnock, common nightingale, song thrush, great reed warbler and Eurasian treecreeper. Geographical areas of the birds' infection were identified from an analysis of reports on ringed birds of the same species, the time necessary for the development of cutaneous cysts with C. faba and the time of their survival, and hitherto known geographical areas of endemic occurrence of C. faba. It is presumed that birds trapped during spring migrations were infected in some montane and submontane regions in south-western Europe (the Alps, the Apennines). Birds infected during autumn migration or post-breeding vagrancy could have been infected in the Central European Carpathians, the region of C. faba endemic occurrence. For migrating birds, the impact of C. faba infections has not been hitherto assessed.

Background matching and evolution of cryptic colours of selected passerines in deciduous woodlands

Most drab plumage colours are probably cryptic. Crypsis (camouflage) occurs when the colour of a significant part of the plumage is similar to the colour of a significant part of the background against which the prey bird may be detected by a potential predator. In this study we compare back colours of tits and associated species with colour backgrounds in their habitat during a four-month period in winter. We test the hypothesis that in some of the species back colour is similar to one of the background colours. In addition to colour backgrounds, microhabitats and tree species were also recorded. Great Tit Parus major, Nuthatch Sitta europea and Treecreeper Certhia familiaris showed distinct preferences for different colour backgrounds, reflecting their choice of microhabitats and tree species. The data suggest that in the Great Tit the olive-moss green back colour has evolved as crypsis for foraging close to the base of tree trunks, where most of the moss is found. The bluish-grey back colour of the Nuthatch is suggested to have evolved as crypsis against greyish beech trunks and branches. Similarly the brown back colour of the Treecreeper is suggested to have evolved as crypsis against pale brown- greyish brown bark surfaces. The possibility that the grey back colour of the Marsh Tit Parus palustris has evolved as crypsis against greyish branches is discussed. For the Blue Tit Parus caeruleus it is suggested that the green back colour has evolved as crypsis against leaves during summer; thus no background matching of the back occurs in winter. For the black back colour of the Longtailed Tit Aegithalos caudatos no background matching can be suggested. The study demonstrates that it is possible to quantify background colours for fouraging birds in the wild as seen by a human observer on the ground. It is proposed that these observations are also representative for a hunting Sparrowhawk Accipiter nisus, even though differences in observing height and possible differences in spectral visual range may present difficulties.

Weights of yolk body and hatchling in relation to the egg weight in the Treecreeper Certhia familiaris

The weights of fresh eggs, yolk bodies, and hatchlings were measured during the period 1998—2000 in a Treecreeper Certhia familiaris population in a deciduous forest in south-western Sweden. The weight of the hard-boiled yolk body was positively and significantly correlated with the fresh egg weight among and within clutches. The same significant inter- and intra-clutch relations were also found between eggs and young nestlings (average age 12 hours) or fresh hatchlings, both with regression coefficients of a magnitude surpassing those of other investigated passerine species. The proportion of yolk weight tended to decrease with increasing egg weight, whereas the proportional weight of the hatchlings increased, showing a significant linear regression upon egg weight. The pronounced weight increase of hatchlings from heavier eggs strongly suggests a possible adaptive significance of the likewise pronounced intra-clutch egg weight hierarchy in the Treecreeper population studied.