Short-toed treecreeper

The Short-toed treecreeper (Certhia brachydactyla) is a small passerine bird in the family Certhiidae, measuring 12–13 cm in length and weighing 7.5–11 g, with cryptic brown upperparts streaked in white, pale greyish underparts, a long thin downcurved bill adapted for probing bark, and stiffened tail feathers that aid in climbing.^[1] This resident species forages primarily on insects and spiders extracted from tree crevices, spiraling upward along trunks in a distinctive manner, and is best distinguished from the similar Eurasian treecreeper by its penetrating "tut" call and subtle features like a longer bill and shorter hind claw.^[1]^[2] Native to temperate and Mediterranean regions, the Short-toed treecreeper has a broad distribution spanning approximately 8,570,000 km² across western and southern Europe—from the Iberian Peninsula eastward to western Turkey and Greece—northwest Africa (Morocco, Algeria, and Tunisia), and locally in the Middle East including Cyprus and the Caucasus.^[3] It recognizes five subspecies, such as C. b. megarhynchos in central and western Europe and C. b. mauritanica in North Africa, reflecting regional variations in plumage and size.^[1] The bird is largely non-migratory, with some individuals undertaking short post-breeding dispersals, and it occupies elevations from sea level up to 2,520 m.^[3] It prefers mature deciduous and mixed woodlands featuring rough-barked trees like oaks and pines, but adapts well to coniferous forests, orchards, parks, gardens, and even suburban areas with sufficient tree cover.^[3]^[1] During the breeding season from late March to mid-June, pairs construct nests in tree crevices, behind loose bark, or occasionally in nest boxes and building walls, laying 4–9 eggs (typically 5–6) per clutch and potentially raising two broods annually.^[1] In winter, its diet supplements invertebrates with seeds when arthropods are scarce.^[1] The global population is estimated at 11.2–18.7 million mature individuals (as of 2018) and appears stable or increasing, classified as Least Concern on the IUCN Red List due to its wide range and adaptability, though it faces localized threats from intensive forestry and loss of old-growth forests.^[3]

Taxonomy and systematics

Classification and etymology

The short-toed treecreeper (Certhia brachydactyla) is a species of small passerine bird classified in the order Passeriformes and the family Certhiidae, which comprises the treecreepers. The genus Certhia is part of a broader Holarctic radiation of treecreepers, encompassing species distributed across northern temperate regions of Eurasia and North America, with two primary European lineages represented by the western C. brachydactyla and the eastern C. familiaris (Eurasian treecreeper).^[4] First described by German ornithologist Christian Ludwig Brehm in 1820 from specimens collected in Germany, the species has long been recognized within the genus Certhia, though its taxonomic boundaries have been debated.^[5] Historically, C. brachydactyla was occasionally lumped as a subspecies of C. familiaris due to morphological similarities, but separation as a distinct species was confirmed in the 1990s through studies revealing consistent genetic and vocal differences between the two, including distinct mitochondrial DNA haplotypes and song dialects that limit interbreeding in contact zones.^[6] The etymology of the binomial reflects its morphology and historical nomenclature. The genus name Certhia derives from the Ancient Greek kerthios (κερθίος), a term used by Aristotle to describe a small, tree-dwelling, insectivorous bird, likely referring to a treecreeper-like species.^[7] The specific epithet brachydactyla combines the Greek brachys (βραχύς, meaning "short") and daktylos (δάκτυλος, meaning "toe" or "finger"), alluding to the bird's notably shorter toes compared to other Certhia species.^[8]

Subspecies

The Short-toed treecreeper (Certhia brachydactyla) is currently recognized as comprising five subspecies, differentiated by subtle morphological variations such as plumage tone and bill size, as well as regional song dialects, with genetic analyses supporting their isolation due to limited gene flow across geographic barriers.^[9]^[10]^[11] The nominate subspecies, C. b. brachydactyla, inhabits central and western continental Europe east of the range of megarhyncha, serving as the baseline for the species with moderately long bill and standard brownish upperparts streaked with whitish.^[9] C. b. megarhyncha occupies the Iberian Peninsula, extending north to northwest Spain, Portugal, western Germany, the Netherlands, Belgium, northern France, and the Channel Islands; it features a larger bill relative to the nominate form and paler, warmer brown plumage.^[9]^[1] In northwest Africa, C. b. mauritanica ranges from Morocco south to the Middle and High Atlas, Algeria south to the Saharan Atlas and Aurès Range, and northwest Tunisia; this subspecies exhibits paler overall plumage adapted to arid conditions, along with a lower-pitched song dialect characterized by verses with a larger frequency span (up to 1.5 kHz) and more elements than in European populations.^[9]^[1]^[10] C. b. dorotheae is distributed in southeast Europe, including Crete, Cyprus, and extending to Turkey, displaying browner tones in plumage and a slightly longer bill; genetic studies using ddRAD-seq on island populations like those in Crete and Cyprus reveal significant divergence, indicating reduced gene flow and potential for further taxonomic revision.^[9]^[1]^[11] Finally, C. b. harterti occurs from eastern Turkey to the Caucasus Mountains, showing intermediate traits between the nominate and eastern forms, including blended plumage tones and song patterns that intergrade with neighboring subspecies in areas of overlap.^[9]^[10]

Subspecies	Distribution	Key Distinguishing Traits
C. b. brachydactyla (nominate)	Central/western continental Europe	Standard bill length (~10-11 mm), brownish upperparts with whitish streaks
C. b. megarhyncha	Iberian Peninsula to W Germany	Larger bill, paler/warmer brown plumage
C. b. mauritanica	Northwest Africa (Morocco, Algeria, Tunisia)	Paler plumage, lower-pitched song with broader frequency span
C. b. dorotheae	Southeast Europe (Crete, Cyprus) to Turkey	Browner tones, longer bill; genetically distinct island populations
C. b. harterti	Eastern Turkey to Caucasus	Intermediate plumage and song; intergrades with others

Description

Morphology

The Short-toed treecreeper (Certhia brachydactyla) is a small passerine bird with an average body length of 12.5 cm, a weight ranging from 7.5 to 11 g, and a wingspan of 17–21 cm.^[9]^[12] Its slender body is well-suited to an arboreal lifestyle, characterized by a long, gently decurved bill averaging 15–18 mm in length, which is longer than that of the closely related Eurasian treecreeper (C. familiaris).^[2] The hind claw is notably short, measuring under 13 mm (typically around 7–8 mm), facilitating precise grip on tree bark during vertical ascent.^[13]^[14] Key anatomical adaptations include stiff tail feathers that act as a supportive prop against tree trunks, enabling the bird to lean back while foraging or climbing.^[15] Strong, flexible feet with sharp claws further enhance its climbing ability, allowing it to spiral upward on rough bark surfaces without slipping.^[15] These features collectively support efficient navigation in vertical habitats. Sexual dimorphism is minimal, with males averaging slightly larger in overall size and bill length than females, though overlap is substantial.^[16] Juveniles exhibit a similar body structure to adults but possess softer, less worn plumage and marginally shorter bills during their initial months.^[16]

Plumage and variation

The adult Short-toed treecreeper exhibits cryptic plumage adapted for blending with tree bark, featuring mottled brown upperparts intricately patterned with black, buff, and white streaks, including narrow off-white shaft streaks on the crown and mantle. The underparts are creamy-white to pale grey, with the throat white and the flanks washed buff to rufous-brown, appearing browner than those of the closely related Eurasian treecreeper; the rump is cinnamon-rufous with whitish centers. A prominent white wingbar, formed by pale buff tips on the greater coverts and even-stepped white fringes on the primaries and secondaries, is visible on the folded wing. This overall coloration, supported by the species' curved bill and stiffened tail feathers, enhances camouflage on tree trunks.^[1]^[2]^[17] Plumage shows seasonal wear, appearing fresher and brighter in autumn after the complete post-breeding moult in late summer, with feathers retaining crisp edges and vivid tones through winter; by summer, the plumage becomes abraded and faded, particularly on the upperparts.^[17]^[9] Juveniles resemble adults but display duller, more uniformly brown upperparts with shorter white spots rather than distinct streaks, resulting in a less patterned appearance; the underparts feature small brownish or grey spots on the breast, flanks, and belly, and the wingbar is incomplete or less prominent until the first partial moult in autumn.^[9]^[1]^[17] Geographic variation in plumage is subtle and primarily involves tone and intensity rather than structure, with the nominate subspecies C. b. brachydactyla (central and southern Europe) showing typical warm brown upperparts; the western European C. b. megarhynchos is slightly paler and warmer-toned overall, while the North African C. b. mauritanica has darker, colder brown upperparts, narrower crown streaks, and more extensively buff-washed underparts and flanks. Other subspecies, such as C. b. dorotheae in the eastern Mediterranean, exhibit marginally duller greyish-brown tones above.^[1]^[9] The Short-toed treecreeper's whiter underparts, uniform buff wash on the flanks without prominent dark spots, and even-stepped wingbar distinguish it from the Eurasian treecreeper, which typically shows paler flanks with subtle spotting and a more irregular wingbar pattern.^[2]^[1]^[17]

Voice

The Short-toed treecreeper produces a variety of high-pitched vocalizations, primarily consisting of calls and songs that are essential for communication and species identification, particularly in regions of overlap with the Eurasian treecreeper (Certhia familiaris). Its calls are typically quiet, thin, and repetitive, often rendered phonetically as "tyt...tyt" or "tsi-tsi," with a sharp, even quality that contrasts with the more modulated and descending notes of the Eurasian treecreeper's calls.^[4]^[18] These calls last about 0.2 seconds each, feature L-shaped spectrograms, and span frequencies from approximately 5.3 to 6.9 kHz, making them penetrating at lower frequencies compared to the higher-pitched equivalents in C. familiaris.^[4] The song is a short, accelerating phrase of high-pitched "tsi-tsi-tsi" notes, usually lasting 1.4 to 2–3 seconds and comprising around 7–8 elements that rise in frequency from about 4.3 kHz to 7.5 kHz before ending with a frequency-modulated "sreeh" element.^[4]^[19] Delivered by males from high perches throughout the year but most frequently during the breeding season, the song serves territorial functions and exhibits low intra-individual variation, with phrases repeating every 10–15 seconds in active singing bouts.^[4]^[18] Subspecies show subtle dialects; for instance, the western European subspecies C. b. megarhynchos often produces slower, extra-long verses up to 1.7 seconds, while C. b. mauritanica in North Africa has longer, more jumbled phrases with a wider frequency span and lower pitch.^[4] These variations, including distinct final elements in eastern subspecies like C. b. dorotheae on Cyprus, contribute to regional dialects that aid in distinguishing from C. familiaris in sympatric zones.^[4]^[20] Contact calls, such as the repetitive "tjit" or incisive "siiih," are used year-round for maintaining pair bonds and signaling during foraging or movement, while songs intensify for territorial defense in spring.^[19] In field recordings, songs frequently overlap with these calls, creating continuous vocalizations that are crucial for auditory identification, especially where visual cues are limited by dense woodland habitats.^[20] The overall vocal repertoire's simplicity and persistence—compared to the more complex, jingling song of C. familiaris—further facilitates separation in overlap areas across central Europe.^[18]

Distribution and habitat

Geographic range

The Short-toed treecreeper (Certhia brachydactyla) has a breeding range spanning western and southern Europe, from the Iberian Peninsula through France, central Europe, the Balkans, and into Asia Minor (Turkey), as well as northwest Africa from Morocco to Tunisia.^[9]^[3] It is absent from most of Scandinavia and Britain as a breeder, though it has recently colonized parts of Denmark.^[9]^[21] The species occupies a broad extent of occurrence estimated at 8,570,000 km² across these regions.^[3] Several subspecies define regional variations within this range. The nominate C. b. brachydactyla occurs in central and southeastern Europe, from southeast Spain to Turkey.^[9]^[19] C. b. megarhynchos is found in western Europe, including Portugal, northwest Spain, France, Belgium, the Netherlands, and western Germany, extending to the Channel Islands.^[9] In northwest Africa, C. b. mauritanica inhabits the Atlas Mountains of Morocco, the Saharan Atlas and Aurès Range of Algeria, and northwest Tunisia.^[9]^[19] Eastern populations include C. b. dorotheae on Crete and Cyprus, and C. b. harterti in Asia Minor and the Caucasus Mountains.^[9]^[19] The species is largely resident year-round within its breeding range, with limited post-breeding dispersal and few long-distance movements recorded.^[9]^[19] Vagrant individuals have been documented outside this core area, including in the United Kingdom (primarily the Channel Islands, with rare records on the mainland), Sweden, England, northwest Belarus, northwest Russia, Lithuania, the Balearic Islands, and Syria in the Middle East.^[9]^[3]^[5] Historically, the short-toed treecreeper has shown northward expansion in parts of its range, including colonization of eastern Poland during the 20th century and Denmark, where it first bred in 1940 and reached approximately 300 pairs by the 1980s.^[9] In Denmark, breeding is now established in southern and eastern Jutland and on nearby islands, reflecting ongoing range stability elsewhere in Europe and Africa.^[9]^[21]

Habitat preferences

The Short-toed treecreeper primarily inhabits mature deciduous and mixed woodlands, with a strong preference for areas featuring large, old trees such as oaks (Quercus spp.) and beeches (Fagus spp.), as well as edge habitats along woodland borders.^[3] It also occupies riverine corridors with willows (Salix spp.) and poplars (Populus spp.), and extends into more open landscapes like copses, orchards, parks, well-wooded farmlands, and suburban gardens provided suitable mature trees are present.^[3] While it can utilize pine (Pinus spp.) forests in southern portions of its range and plantations of rough-barked exotic conifers, it generally avoids dense, pure coniferous stands, favoring open-structured woods over closed-canopy forests.^[22]^[3] This species occurs from sea level up to 2,500 m in elevation, though it is typically found below 1,000 m in northern parts of its range and shows a preference for lower altitudes overall.^[22]^[3] At the microhabitat level, it relies heavily on trees with rough or fissured bark for foraging and nesting, often selecting crevices behind loose bark or in trunk splits up to 16 m above ground.^[3] The bird is sensitive to habitat fragmentation caused by intensive forestry and conversion to conifer monocultures, which reduce availability of mature broadleaf trees; however, it can persist in smaller woodland patches of 5–10 ha and regularly moves between isolated fragments separated by 30–200 m of urban or built-up areas.^[3]^[23] Compared to the Eurasian treecreeper (Certhia familiaris), the Short-toed treecreeper is more tolerant of open, warmer habitats, including Mediterranean maquis and farmlands with scattered trees, while showing a greater affinity for deciduous-dominated woodlands at lower elevations in areas of range overlap.^[22] Breeding often occurs in these preferred woodland edges or park-like settings, where suitable nest sites in bark crevices are abundant.^[3]

Behaviour and ecology

Breeding

The breeding season of the short-toed treecreeper (Certhia brachydactyla) typically spans from late March or early April to mid-June in much of its range, though it may extend to July in warmer regions such as Cyprus. Pairs often produce one to two broods per year, with clutch sizes ranging from 4–9 eggs, most commonly 5–6. The eggs are white with fine reddish-brown speckles and measure about 1.5 cm in length.^[3]^[24]^[25]^[26] Nests are constructed in natural or artificial cavities, such as tree crevices, behind loose bark, old woodpecker holes, building gaps, or stone walls, often up to 16 m above the ground. The nest foundation consists of twigs, conifer needles, grass, bark, plant fibers, and occasionally cloth or paper, lined with softer materials including feathers, hair, down, rootlets, moss, and lichen. Both sexes contribute to nest building, with the male assisting the female.^[3]^[26]^[27] The female alone incubates the eggs for 13–15 days, beginning after the last egg is laid. Upon hatching, both parents feed the nestlings, which remain in the nest for 14–18 days before fledging; the female primarily broods the young early on, while the male increasingly handles provisioning as the chicks age. For second broods, the female may start a new nest while the male continues feeding the first brood. Territorial defense during breeding relies on song, with limited courtship displays beyond mutual preening between mates.^[25]^[24]^[26]^[27]

Feeding

The Short-toed treecreeper primarily feeds on arthropods extracted from tree bark, with its diet dominated by insect larvae and pupae, supplemented by spiders and occasionally ants and beetles.^[22]^[28] In colder months, when arthropod availability declines, it supplements its diet with some seeds.^[22] Foraging occurs mainly on the trunks and larger branches of trees, where the bird spirals upward in a jerky, hopping motion, using its stiffened tail feathers for support while probing crevices with its slender, decurved bill.^[22] Upon reaching higher branches, it typically descends via a short flight to the base of a nearby tree to resume searching.^[22] This species covers bark at a rate of 10–20 m² per minute, with its curved bill well-adapted for extracting hidden prey from fissures.^[22] Seasonally, foraging shifts toward the ground in winter, especially when snow obscures lower tree trunks, allowing access to fallen arthropods or exposed crevices.^[22] Juveniles develop proficiency in these techniques by observing and mimicking adult foraging patterns shortly after fledging.^[22] Foraging efficiency increases on rough-barked trees, which harbor greater arthropod densities in their crevices compared to smoother surfaces.^[29] Interspecific competition remains low overall, though the dominant Eurasian nuthatch (Sitta europaea) can displace it from preferred foraging sites in overlapping habitats, prompting shifts in height or substrate use.^[30] The Short-toed treecreeper typically occurs solitarily or in pairs throughout the year, maintaining a low profile through its cryptic plumage that blends seamlessly with tree bark, rendering it inconspicuous to predators and observers alike.^[31]^[32] These birds are active from dawn until dusk, methodically climbing tree trunks in search of invertebrate prey, and exhibit a generally quiet demeanor with subdued movements that minimize detection.^[19] At night, they roost individually in narrow crevices of tree bark, rock faces, or buildings, though small communal groups of up to several individuals form in winter, particularly during cold spells, to conserve heat through huddling.^[33] In terms of social structure, the species is territorial during the breeding season, with males using song to defend areas and assert dominance, showing minimal overt aggression toward conspecifics but occasionally displacing the sympatric Eurasian treecreeper (Certhia familiaris) via heightened responses to heterospecific songs in overlapping territories.^[34] Outside breeding, social interactions remain sparse, with rare formation of loose winter flocks; cooperative breeding has not been recorded.^[31] The Short-toed treecreeper is largely sedentary across its range, undertaking only minor altitudinal shifts in response to seasonal conditions and exhibiting limited post-breeding dispersal, often confined to movements between nearby habitat patches spanning tens to hundreds of meters.^[9]^[29] Juveniles show somewhat greater mobility during natal dispersal, though distances remain short relative to many passerines, supporting philopatry in stable woodland environments. Individuals may live up to 7 years in the wild, based on ringing recoveries.^[35] Brief contact calls facilitate coordination during occasional pair or group activities.^[31]

Conservation status

Population and trends

The global population of the Short-toed treecreeper (Certhia brachydactyla) is estimated at 11.2–18.7 million mature individuals, with a breeding range spanning approximately 8.57 million km² across Europe and parts of North Africa.^[3] In optimal habitats such as mature deciduous woodlands, the species prefers areas with abundant tree cover.^[9] Regionally, the species is abundant in western Europe, with an estimated 2.15–2.74 million breeding pairs in Spain and 0.9–1.6 million in France, comprising a significant portion of the European total of 5.06–8.39 million breeding pairs.^[36] In contrast, populations are scarce in peripheral areas like the Caucasus, where Georgia supports only 210–2,100 breeding pairs, equating to fewer than 10,000 individuals.^[36] Overall, the population has remained stable or shown moderate increases since the 1980s, with Europe-wide trends indicating a slight uptick over the past three generations (approximately 10 years).^[3] Monitoring through systematic breeding bird surveys across Europe reveals slight declines in fragmented habitats, balanced by expansions in northern regions; for example, the species colonized Denmark in 1946 and grew to around 300 breeding pairs by the 1980s, representing substantial range extension.^[9]^[36] The International Union for Conservation of Nature (IUCN) assesses the Short-toed treecreeper as Least Concern as of 2024, owing to its large range size and lack of major population declines.^[3]

Threats and conservation measures

The Short-toed treecreeper faces primary threats from habitat fragmentation driven by deforestation and urbanization, which reduce the availability of suitable mature woodlands essential for foraging and nesting.^[3] Intensive forestry practices, including the replacement of deciduous woodlands with conifer plantations, exacerbate these losses by altering forest structure and patchiness, negatively impacting population distribution.^[1] Additionally, agricultural intensification, including pesticide use, indirectly affects the species by diminishing insect prey abundance in adjacent habitats.^[37] Climate change influences the species variably; warmer winters have facilitated northern range expansion by benefiting resident populations, though southern habitats may experience stress from associated shifts in woodland conditions.^[38] Locally, the bird is rare in Britain and the Channel Islands due to geographic isolation, limiting colonization.^[5] In overlap zones with the Eurasian treecreeper, interspecific competition occurs, with the Short-toed treecreeper exhibiting greater aggression toward its sibling species, potentially affecting resource partitioning.^[34] Conservation efforts include legal protections under the EU Birds Directive and Appendix II of the Bern Convention, which provide habitat safeguards across Europe.^[3] Recommended measures focus on woodland restoration to counteract fragmentation, low-intensity forest management to preserve mature deciduous stands, and retention of deadwood to support nesting cavities for this hole-nesting species.^[39] Pan-European monitoring through breeding bird surveys indicates a stable to increasing population trend overall, highlighting the species' resilience to some pressures.^[3] Future actions should prioritize expanding mature woodland cover to offset ongoing habitat losses and enhance connectivity in fragmented landscapes.^[40]