Peter Rothery

Influence of Woodland Area on Breeding Success in Great Tits Parus major and Blue Tits Parus caeruleus

Reproductive performance of Great Tits and Blue Tits breeding in highly fragmented woodland was investigated in relation to woodland area in a landscape dominated by intensive arable agriculture in eastern England. Breeding performance was recorded for five years (1993-1997) for tits breeding in nestboxes in up to 43 woods ranging in size from 0.1 to 157 ha. Trends in the components of breeding success with woodland area were investigated using a linear, mixed model including both fixed and random effects, and logistic regression analysis. In both Great Tits and Blue Tits, breeding performance declined with decreasing woodland area. For Blue Tits, the decline was accounted for by the timing of breeding which was later in smaller woods. Great Tits showed a similar trend in laying date with woodland area, but over and above this effect of timing on breeding success, they also reared lighter young in smaller woods. The variation in first egg date accounted for by woodland area was about 25% for Great Tits and 33% for Blue Tits. In some years, failure at the nestling stage was frequent for Great Tits (but not Blue Tits) in small woods, but was not recorded to the same extent in large woods. The results are discussed in the context of previous studies of tit breeding success in relation to individual bird quality, habitat quality and availability, and the consequences of habitat fragmentation.

Testing for density dependence allowing for weather effects

Abstract A test for density dependence in time-series data allowing for weather effects is presented. The test is based on a discrete time autoregressive model for changes in population density with a covariate for the effects of weather. The distribution of the test statistic on the null hypothesis of density independence is obtained by parametric bootstrapping. A computer simulation exercise is used to demonstrate the gain in statistical power by allowing for weather effects. Application of the method to time-series data on three species of butterflies and two species of songbirds showed stronger evidence of density dependence than two standard tests.

Apparent lack of impact of Sparrowhawks on the breeding densities of some woodland songbirds

Counts of singing male songbirds on Bookham Common (southeast England), conducted almost every year from 1949 to 1979, covered the periods of Sparrowhawk decline and recovery. We examined these counts for evidence of an influence of Sparrowhawk predation on the breeding densities of their songbird prey. Of 13 species examined, no convincing evidence of Sparrowhawk effects was found. However, most species increased in density over the whole study period and, in four, the rate of increase slowed towards the end of the study. For two of these species (Song Thrush and Starling) it was impossible to tell whether this slowing in population growth was caused by the return of Sparrowhawks or by other factors, such as habitat effects.

AGE-RELATED TRENDS IN DIFFERENT ASPECTS OF THE BREEDING PERFORMANCE OF INDIVIDUAL FEMALE EURASIAN SPARROWHAWKS (ACCIPITER NISUS)

Abstract Age-related trends in breeding performance of female Eurasian Sparrowhawks (Accipiter nisus) were examined in south Scotland. Over a 22 year period, most individuals were banded and of known age. Previous analyses had shown that average number of young raised per breeding attempt (or year) increased progressively in successive age-groups to midlife (around five years) and declined thereafter. Here we analyze different components of breeding performance (such as clutch-size, hatching and fledging success) to find which components contribute most importantly to the overall age-related trend in productivity. Most aspects of breeding showed a similar upward–downward trend through the life span, but the downward trend in productivity in later life could be attributed mainly to poorer nestling survival. In general, in each aspect of breeding, the mean trend of the population as a whole was followed by individuals studied at different ages. Such individual trends were sufficient to account for the overall mean trend, and there was no need to postulate that high-performance or low-performance birds entered and left the breeding population at different ages. Only one aspect of breeding showed no trend with age, namely proportion of birds that laid having built a nest, but that type of failure affected only a small proportion of individuals.

Ageing and sexing Marsh Tits Poecile palustris using wing length and moult

Ageing and sexing Marsh Tits Poecile palustris presents some difficulties for ringers. Wing length has previously been found to be useful for sexing English Marsh Tits, although the replacement of all juvenile rectrices and greater coverts by first‐year birds made ageing difficult. We examined Marsh Tits of known age and sex and found that 93.5% of birds could be sexed correctly using wing lengths of ≤ 62 mm for females and ≥ 63 mm for males. Wing length was also an indicator of age: many first‐years had shorter wings, but most also retained some juvenile rectrices and greater coverts after the post‐juvenile moult, and could be readily aged using these features. We found a primary moult period of 67 days for adult Marsh Tits, with a mean onset of 27 May.

Willow Warbler Phylloscopus trochilus habitat in woods with different structure and management in southern England

Capsule Use of Light Detection and Ranging (LiDAR) data identified suitable Willow Warbler habitat based on mean vegetation height. This habitat model provided maps of distribution and occupation of suitable habitat. Aims To identify habitat associations in woods with different vegetation structure and management systems during a period of low Willow Warbler populations. Methods Locations of all Willow Warblers were mapped during the breeding season in three woods of contrasting management; recent low intervention, actively coppiced woodland and high forest with clear‐fells. Height profile models of each wood were derived from airborne LiDAR. The mean vegetation height at locations with Willow Warblers and a sample from the rest of the wood were used to produce models of optimum habitat and breadth of habitat occupied in each wood. The habitat model was then used to produce maps of suitable habitat. Results The habitat models did not differ between woods, with highest probability of Willow Warbler occurrence in mean vegetation heights of 3.7–5.3 m. Habitat of heights 6–11 m appeared less suitable, being only partly occupied. Habitat maps showed that habitat of suitable height was only occupied when it occurred as large patches; smaller patches (mostly <0.5 ha) and edges along rides and fields were not used. Conclusion The use of LiDAR derived measures of vegetation height identified areas of suitable habitat for Willow Warblers. Willow Warblers occupied areas of low mean vegetation height either as early successional or open canopy woodland in all woods. Height‐based habitat maps can identify areas of suitable habitat within larger expanses of heterogeneous woodland and are a potentially useful tool in assessing changes in extent of what are often temporary patches of habitat.

Effects of the doubly labelled water procedure on Great Tits Parus major feeding young

Capsule Female feeding rate behaviour was highly variable between individuals but no effect of the doubly labelled water (DLW) procedure on female visit rate was detected, whereas visit rates by males, which were neither trapped or manipulated, increased following trapping and manipulation of their partners. Aims To test the assumption that the subjects behaviour is normal during the measurement period when using DLW to measure energy expenditure. Methods Visits to the nestbox by parents feeding young were counted separately for females and males on the day before and the day after the female only was trapped to measure her energy expenditure using DLW. Visit rates were also counted for control pairs. Results Female visit rates did not differ before and after manipulation, or between experimental and control pairs, but bird behaviour was highly variable between individuals. In contrast, the visit rates of both experimental and control males, which were not trapped, increased on the second day. Conclusion The results for female behaviour supported the assumption of normality, but a small subset of particular individuals may be prone to adverse reactions. The response of the males may have been a reaction to disturbance at the nest.