Nuthatch

Abstract: The aim of this study was to investigate the effects of habitat loss, fragmentation and habitat quality on sedentary forest birds in an urban and suburban environment. The study area was situated in Stockholm, the capital of Sweden, embracing the city centre, suburbs and parts of the rural surroundings. Breeding forest birds were surveyed in 51 forested sample sites (2-700 ha) and five species of resident birds were selected for further studies: willow tit (Parus montanus), crested tit (P. cristatus) and coal tit (P. ater) representing coniferous forest and marsh tit (P. palustris) and nuthatch (Sitta europaea) representing deciduous forest. A spatial landscape analysis was made using a geographical information system (GIS). In 21 of the smaller sites (2-200 ha), a field study was conducted to examine habitat quality parameters like vegetation age, structure and composition, and human-induced disturbance. The probability of occurrence (breeding) of bird species as functions of landscape and habitat descriptors was tested using logistic regression. All investigated species of the Parus guild showed high probabilities of occurrence only in forest patches larger than 200-400 ha, and was not present in patches smaller than 10-30 ha. This meant that patches of presumably suitable habitat (coniferous vs. moist deciduous forest) were left unoccupied. The amount of standing dead and decaying trees provided additional explanation for the distribution of the willow tit. Large areas of urban open land, industrial land use and large bodies of water had a negative influence on the probability of occurrence of several species, which indicate that they were sensitive to isolation. The probability of occurrence of the marsh tit was also influenced by distance to other sample sites with marsh tits. Unlike the Parus species, the nuthatch was breeding in most of the parks and forest remnants. This species prefers mature deciduous forest, mainly oak, which habitat was common in the urban environment. The nuthatch was only absent in some of the smallest (a few ha) forest fragments, with a mean distance between forest patches in the surroundings of over 100 m. The study showed that large forest areas and a high amount of forest in the landscape are important for the investigated resident birds that are not adapted to the urban environment. Vast areas without tree-cover seemed to be poor habitat and/or restrict dispersal. Strips of high-quality habitats, including standing trees with nest-holes, were not entirely absent in the urban and suburban environment.

Abstract: Nest-holes created by woodpeckers or natural decay processes are an essential commodity for secondary hole-nesting species. Hole-making agents may strongly influence the richness and abundance of species in hole-nester communities. However, few studies have examined the characteristics and relative importance of naturally occurring holes for hole-nesters. Between 1995 and 2006, we examined 1371 excavated and non-excavated holes used by 29 bird and mammal species in central British Columbia, Canada. Excavated holes were much more abundant (85%) than non-excavated holes (15%). Red-naped sapsucker (Sphyrapicus nuchalis) and northern flicker (Colaptes auratus) excavated 52% of the holes monitored. At the community level, non-excavated holes were used for nesting less than expected based on their availability (6% of total nests), particularly among woodpeckers (2% of nests). However, secondary hole-nesters used non-excavated holes roughly in proportion to their availability (10% of nests), and some excavators used non-excavated holes for nesting (flicker and red-breasted nuthatch, Sitta canadensis, 4% of nests each; black-capped chickadee, Poecile atricapillus, 13% of nests). Although nests in non-excavated holes tended to be lower on the tree, larger internally and with larger entrances, only European starling (Sturnus vulgaris) appeared to select non-excavated holes with characteristics most similar to their preferred excavated holes. Non-excavated holes may be an alternate nesting resource for secondary hole-nesters that cannot acquire an excavated hole and, occasionally, for excavators. The use of non-excavated holes may provide an advantage for secondary hole-nesters by releasing them from the constraints of excavator nest-site preferences.

Abstract: Summary 1. Eruptions occur when a species appears in unusually high densities within and often outside of its normal range. We used 30 years of Christmas Bird Counts combined with cone/seed-crop data on boreal coniferous trees, breeding bird surveys, and weather records to test correlates of winter eruptions by 11 species of primarily boreal North American seed-eating birds. 2. Eruptions of six species in eastern (red-breasted nuthatch Sitta canadensis L., blackcapped chickadee Parus atricapillus L., evening grosbeak Hesperiphona vespertina Cooper, pine grosbeak Pinicola enucleator L., red crossbill Loxia curvirostra L. and bohemian waxwing Bombycilla garrulus L.) and five species in western North America (pine grosbeak, pine siskin Carduelis pinus Wilson, evening grosbeak, bohemian waxwing and red-breasted nuthatch) correlated with a combination of large coniferous seed crops in the previous year followed by a poor crop. Breeding population size in the year of the eruption was also correlated positively with the event in two of the species. Eruptions in these species apparently occur when large boreal seed crops resulting in high population densities (via high overwinter survivorship and/or high reproductive success) are confronted with a relatively poor seed crop the next autumn. 3. Eruptions of common redpolls Carduelis flammea L. and black-capped chickadees in the west followed only large seed crops the previous year, suggesting that high density is a more important factor leading to eruptions than seed crop failure. The opposite was true for white-winged crossbills ( Loxia leucoptera Gmelin) in the east, where eruptions correlated only with poor current year seed crops. This was the only species supporting the ‘seed-crop failure’ hypothesis as the sole cause of eruptions. 4. Purple finches Carpodacus purpureus Gmelin erupted following years when breeding population densities were high for reasons apparently unrelated to the seed crop. Eruptions of three species in both regions were uncorrelated with any of the variables tested. 5. We conclude that seed crops of boreal trees play a pivotal role in causing eruptions for a majority of boreal species, usually through a combination of a large seed crop resulting in high population densities followed by a poor seed crop rather than seed-crop failure alone. Weather conditions were not a significant factor correlating with eruptions in any of the species.