512 SHORT COMMUNICATIONS IBIS 122

unlikely that pairs would form during the short stay in Panama. Zahavi (1971), however, observed the formation and dissolution of short-term pair bonds among migratory White Wagtails defending winter territories. The higher proportion of pairs of Canada Warblers in spring migration than during autumn migration is consistent with the idea that breeding pairs remain together during the non-breeding season. A greater proportion of immature birds might be expected in autumn migration if young incur greater mortality.

If these pairs are actually permanent monogamous associations, Canada Warblers should be observed in pairs in other areas in migration. Some Canada Warblers should also arrive as pairs on breeding territories, rather than the males preceding the females as is typical of parulids (Nolan 1979). As far as we know, such observations do not exist.

We would like to thank Richard Gordon for sharing his observations, and S. Farabaugh, H. Howe, K. Milton and T. Poulson for reading drafts of the manuscript. Field work was supported by Frank M. Chapman Memorial Fund, National Science Foundation dissertation improvement grant, President’s Undergraduate Fellowship, University of California at Berkeley, Phi Beta Kappa, Smithsonian Tropical Research Institute and John Tinker Foundation through the Center for Latin American Studies, University of California at Berkeley. We appreciate logistical support from the Smithsonian Tropical Research Institute.

REFERENCES GRADWOHL, J. & GREENBERC, R. 1980. In press. The formation of antwren flocks on Barro Colorado

Island, Panama. Auk. GRISCOM, L. & SPRUNT, A. Jr. 1957. The warblers of America. New York: Devin-Adair Company. HAMILTON, T. H. 1961. On the functions and causes of sexual dimorphism in breeding plumage

characters of North American species of warblers and orioles. Am. Nat. 95: 121-124. JONES, S. E. 1977. Coexistence in mixed species antwren flocks. Oikos 29: 366-375. LECK, C. F. 1972. The impact of some North American migrants on fruiting trees in Panama. Auk

MILLER, A. H. 1963. Seasonal activity and ecology of the avifauna of an American equatorial cloud forest. Univ. Calif. Publ. 2001. 66: 1-78.

MORTON, E. S. 1980. Adaptations to seasonal changes by migrant land birds in the Panama Canal Zone. In Keast, A. & Morton, E. S. (eds.), Migrant birds in the neotropics: ecology, behavior, distribution and conservation. Washington, D.C. : Smithsonian Institution Press.

NOLAN, V. 1979. The ecology and behavior of the Prairie Warbler. A.O.U. Ornithol. Monogr. 26. RIDGELY, R. S. 1976. A guide to the birds of Panama. Princeton, N.J.: Princeton University Press. WILLIS, E. 0. 1966. The role of migrant birds at swarms of army ants. Living Bird 3 : 187-231. WILLIS, E. 0. 1972. The behavior of the Spotted Antbird. A.O.U. Ornithol. Monogr. 10. ZAHAVI, A. 1971. The social behavior of the White Wagtail Motacilla alba alba wintering in Israel.

89: 842-850.

Ibis 113: 203-212.

Museum of Vertebrate Zoology,

Berkeley, CA 94720, U.S.A. University of California,

20 July 1979

RUSSELL S. GREENBERG JUDY A. GRADWOHL

THE CONTRIBUTION OF NEST SITE CHARACTERISTICS TO BREEDINGSUCCESS AMONG BLACKBIRDS TURDUS MERULA

It has been shown that increased breeding-success can be achieved in urban areas through predator reduction (Tomialojd & Profus 1977, Tomialojf 1978) and that vul- nerability to predation is related to nest site characteristics (e.g., Caccamise 1977, Best 1978, Brown & Goertz 1978). Our observations on the Blackbird Turdus merulu suggest that nesting-success and human activity are related even within the urban environment.

The data were collected in 1977 during a study of the Blackbird population of 45 ha around the central Campus of the University of Exeter. Daily censuses were carried out, normally in the evening, from March until 6 July. Nest site characteristics were measured immediately a new nest was found and active nests were revisited every two or three days.

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1980 SHORT COMMUNICATIONS 513

At each site we recorded: the species of plant or type of structure supporting the nest; the manner of attachment to the supporting structure; nest height; and nest cover. The distance of nests from the nearest path or building was measured from a map and used as a description of the proximity of human influence. Two categories were used in the analy- sis; ‘near’ nests were those placed less than 7 m from paths or buildings whilst the remain- der were ‘far’ nests.

Cover was the only factor for which measurement was not straightforward. It was desirable to have a measure which could assess the exposure of the nest to unfavourable weather and predation, and be the type of measurement which the bird could have made itself. To satisfy these requirements, an apparatus was designed to measure cover as the difference between diffuse light intensity at a nest site and that in the open. A full account of this apparatus is deposited at the Edward Grey Institute library. The index of cover used here is essentially the negative logarithm of the percentage daylight at nest sites; values range from 0 in the open to about 14 in the dark.

The statistical procedures adopted in the analyses follow Sokal & Rohlf (1969) and Gilbert (1973).

NESTING OUTCOME

Of 108 nesting attempts, 33 % produced at least one fledged chick. Predation accounted for 71 % of failures, desertion for 27% and bad weather for 2%. The main predators were semi-feral cats, Grey Squirrels Neosciurus carolinensis and corvids (Magpie Pica pica, Carrion Crow COYW corone and Jay Garrulus glandarius).

The survivorship curve for eggs and nestlings (Fig. 1) forms a straight line on an arithmetic plot and differs from the negatively skewed curves of Caccamise (1976), Young

21c

180

f ISC

s 12c

9c

6 C 5 10 15 20 25

Tm in nest (doys)

FIGURE 1 . Survivorship curve for Blackbird eggs and nestlings from the first day of incubation. The initial drop represents losses through infertility and chilling. Regression equation y = 198*9-5.25~, 9 = 0.99.

5 14 SHORT COMMUNICATIONS IBIS 122

(1963), Robertson (1972), Horn (1968) or Holcombe (1972). This straight line indicates an increasing death rate (but a constant number of individuals dying in each age-interval) and is of a form very rarely recorded in natural populations (Collier et al. 1973). Its significance here is unknown and the curve may not apply to Blackbirds elsewhere.

NEST SITE CHARACTERISTICS AND BREEDING-SUCCESS

No differential mortality was attributable to the 59 plant species used for nesting; these are not considered further. Significant differences in height, cover and the manner in which nests were attached to their supporting plants were found between nests grouped according to outcome. Correlations between these site factors were statistically non- significant. The pattern of mortality was different for the nests in the ‘near’ and ‘far’ categories (Table 1) and subsequent analysis was carried out separately on the two.

TABLE 1

Probability of outcome of Blackbird nests at dtgerent proximities to paths or buildings

Outcome Proximity

Success 0.35 0.23 Predation 0.38 0.63 Desertion 0.26 0.14

Note: 1G = 5.54, df = 2, 0.1 > P > 0.05. For successful v. predated nests only, C = 3.82, df = 1, P = 0.05

‘Far’ nests Successful nests were placed significantly higher and in darker situations than those

suffering predation (Table 2). Differences for both height and cover did not arise because of correlation between them ( x = -0.09, n = P > 0-4). Thus height and cover act independently in influencing nesting outcome.

Deserted ‘far’ nests were high but in open situations. Thus nest height was not statistically different for deserted and successful nests but there was a difference in cover (U = 10, n1 = 8, n2 = 5, P < 0.01). This combination of being high and in the open suggests vulnerability to unfavourable weather. Of the five nests in this category, the date of desertion was known for three cases: two failed two days after the heaviest rainfall of the month (in April and May) and the third was deserted during the hottest period in July. All three deserted in the egg-stage.

TABLE 2

Mean diflerence in height and cover +s.e. in ‘far’ Blackbird nests with dt&rent outcomes

Height1 (m) (n) Cover2 n

Successful 1.94 + 0.1 5 (8) 6.1 5 + 0.62 Deserted 1.96 z0.30 ( S j Predated 1.30 & 0.12 (22)

Notes: 1 For successful v. predated nests U = 150.5, P < 0.01. 2 For successful v. predated nests U = 127.5, P < 0.05 and successful v. deserted nests U = 10, P < 0.01.

1980 SHORT COMMUNICATIONS 515

5 .

0

‘Near’ nests Unlike the ‘far’ nests, there was no difference in either height or cover at nests with

different outcomes when less than 7 m from paths or buildings (Table 3). Th’ is was true despite the distributions of height and cover being the same in both proximities (height: U = 1080, nl = 65, n2 = 35, P > 0.5; cover: U = 1077.5, nl = 65, n2 = 35, P > 0.5). There was, however, a difference in the number of side attachments (Fig. 2) between successful and unsuccessful nests (G = 5.1, df = 2, 0.1 > P > 0.05). This is enhanced

. . . . . . . . 1 . r

TABLE 3

Mean diflerence in height and cover s.e. in ‘near’ Blackbird nests with diflerent outcomes

Height’(rn) n Cover2 n

Successful 1.75 f 0.1 7 (23) Deserted 1.59f0.17 (17) Predated 1.42f0.18 (25)

5.09 f 0.39 (22) 4.54f0.39 (17) 4.89 f 0.32 (26)

Notes: For successful v. predated nests U = 361, P > 0.1. For successful v. predated nests U = 303, P > 0.5 and suc-

cessful v. deserted nests U = 208, P > 0.5.

by not considering wall and tree-bole contacts (G = 6.71, df = 1, P < 0.01). Successful nests had more side attachments than those suffering predation. Nest on walls or against tree-boles (excluded above) were all in climbers or creepers. They had only two contacts on average. Eight out of 13 ‘near’ climber nests were successful and seven of the eight were on walls; only two of the five failed creeper nests were on walls. There is evidence, however, that nest-height may have been the important factor rather than the creepers in which the nests were placed; successful nests in creepers were higher than non-creeper nests (V = 95.5, nl = 15, nz = 8, 0.05 > P > 0.02).

Of the eight deserted ‘near’ nests of known dates, two failed in wet periods; the remain- der failed in average weather conditions. Two of the last six nests were deserted during the nestling stage.

,Ot r l

I II

Number of side attachments

FIGURE 2. Number of side attachments between the Blackbird nest and its supporting plant (n = 108).

516 SHORT COMMUNICATIONS IBIS 122

DISCUSSION

Nests at different distances from paths and buildings had the same site-characteristics and yet had different outcomes. For ‘far’ nests success was determined by height and cover. In the ‘near’ nests however, predation was less frequent and nests were successful irrespective of height or cover. Furthermore, ‘near’ nests were deserted for reasons other than inclement weather. We suggest the presence of man affects the outcome of these nests. This within-habitat difference in outcome is analogous to the situation reported by Tomialojd & Profus (1977) and Tomialojd (1978) for different habitats. The effectiveness of this protection by man is probably determined by the frequency, intensity and pre- dictability of human presence. Strawiliski (1963) has suggested that the composition of bird communities in parks may be determined by the frequency of human visits.

Why should the number of side-attachments be important in nesting outcome? Pos- sibly it is a question of the time available to plunder a nest before being disturbed by man; a mass of twigs may take longer to negotiate than one or two branches.

If breeding-success is higher for ‘near’ nesting Blackbirds tameness is likely to have selective advantage. This has probably contributed to the urbanization of Blackbirds over the last 150 years; nesting-success is now lowest in the original woodland habitat (Snow 1958, Snow & Mayer-Gross 1967, Simms 1978). Havlin (1965) however, stated that high abundance of food in winter and early spring produced the high breeding-density in urban habitats.

Whilst tameness may have selective advantage, the ability to choose a favourable nest site could become less important for urban Blackbirds. A wider range of sites is used in towns than in rural habitats (pers. obs.). Graczyk (1963) has suggested that urban and rural populations have diverged to the extent that the differences between them are genetic, but there is little evidence to support this view. Furthermore, it is doubtful whether differential breeding-success could be sustained indefinitely through tameness. The increase in occurrence of predators in towns is an indication that the advantages of being tame are only temporary.

We are grateful to Drs J. R. Krebs, C. M. Perrins and D. W. Snow for their criticisms of drafts of this manuscript. Tim Mountain and Dorothy Appleton gave valuable assistance in the field. The gardening and security staff allowed us free access to the grounds of the University of Exeter; to them we are especially grateful.

REFERENCES BEST, L. B. 1978. Field Sparrow reproductive success and nesting ecology. Auk 95 : 9-22. BROWN, B. T. & GOERTZ, J. W. 1978. Reproduction and nest site selection by Red-winged Blackbirds

CACCAMISE, D. F. 1976. Nesting mortality in the Red-winged Blackbird. Auk 93: 517-534. CACCAMISE, D. F. 1977. Breeding success and nest site characteristics of the Red-winged Blackbird.

COLLIER, B. D., COX, G. W., JOHNSON, A. W. & MILLER, P. C. 1973. Dynamic ecology. London:

GILBERT, N. 1973. Biometrical interpretation. Oxford: Clarendon Press. GRACZYK, R. 1963. (Experimental studies on the ethology of species of Genus Turdus L.). Rocz.

HAVLIN, J. 1965. (Economic importance of the Blackbird.) Zool. Listy 14: 129-142. HOLCOMB, L. C. 1972. Nest success and age-specific mortality in Traill’s Flycatchers. Auk 89:

HORN, H. S. 1968. The adaptive significance of colonial nesting in the Brewer’s Blackbird (Euphagus

in north Louisiana. Wilson Bull. 90: 261-270.

Wilson Bull. 89: 396-403.

Prentice Hall International.

Wyzsz. Szk. Roln. Pozu. 17: 21-71.

837-841.

cyanocephalus). Ecology 49 : 682-694.

Nesting success in marsh and udand habitat. Can. 1. 2001. 50: 247-263. ROBERTSON, R. J. 1972. Optimal niche space of the Red-winged Blackbird (Agelaius phoeniceus), 1.

SIMMS, E. 19%. British thrushes. Lonkon: Collins. SNOW, D. W. 1958. The breeding of the Blackbird, Turdus merula at Oxford. Ibis 100: 1-30. SNOW, D. W. & MAYER-GROSS, H. 1967. Farmland as a nesting habitat. Bird Study 14: 43-52. SOKAL, R. R. & ROHLF, F. J. 1969. Biometry. San Francisco: W. H. Freeman & Co. STRAWI~SKI, S. 1963. Studies on the synanthropism of birds in the Old Park in Ciechocinek. Acta

Ornithal. 7: 159-188.

1980 SHORT COMMUNICATIONS 517

T O M I A ~ O J ~ , L. 1978. The influence of predators on breeding Woodpigeons in London parks. Bird

T O M I A ~ O J ~ , L. & PROFUS, P. 1977. Comparative analysis of breeding bird communities in two parks

YOUNG, H. 1963. Age specific mortality in the eggs and nestlings of Blackbirds. Auk 80: 145-155.

Study 25 : 2-10.

of Wroclaw and in an adjacent Querco-Carpinetum forest. Acta Ornithol. 16: 117-177.

Department of Biological Sciences,l University of Exeter,

Exeter, Devon 14 August 1979

‘Present address : Parks Road, Oxford

Edward Grey ox1 3PS. Institute of Field Ornithology,

PATRICK OSBORNE LOUISE OSBORNE

Department of Zoology, South

SEX DIFFERENCES AND THE EFFECT OF TEMPERATURE ON THE FORAGING BEHAVIOUR OF ROBINS ERZTHACUS RUBECULA

The foraging behaviour of birds is affected by a variety of factors (e.g., Davies 1976, 1977a, Goss-Custard 1977, Greenwood & Harvey 1978). Several studies have been con- ducted on the winter foraging behavour of birds (Gibbs 1954, Brooks 1968, Wilson 1970, Austin & Smith 1972), but few have reported modifications in foraging behaviour associated with fluctuations in environmental temperature (Pulliam et al . 1974, Grubb 1975). Reduction in temperature may alter foraging activities for a variety of reasons: birds may alter their behaviour to minimize heat loss; energy requirements of birds in- crease at low temperatures (Pohl 1969), while prey abundance and availability may be reduced. Evidence exists that populations of invertebrates in leaf litter reach very low levels in January and February (Mann 1967). This paper describes sex differences and the effect of temperature on the foraging activities of the Robin Erithacus rubecula in woodland.

Between November 1977 and February 1979, the foraging behaviour of a colour-ringed population of Robins in East Sussex woodland was observed. Data were collected from five females and nine males, sexed during the 1978 and 1979 breeding seasons. Infor- mation collected from birds that died or left the wood before breeding was ignored. A total of 860 minutes of observations was used in this analysis.

Observations were made throughout the day by walking through the wood until a bird was encountered. Three minutes were allowed to elapse before recording began, to permit the bird to become accustomed to the observer’s presence. The birds were accustomed to being followed and showed no obvious change in behaviour during obser- vation periods.

Each bird was followed until it was lost from continuous view. A commentary was recorded on a dictaphone, concentrating on perch changes and foraging activities, and later transcribed against a running stopwatch. For this analysis only birds considered to be actively foraging were included. Records containing activities such as preening, resting and song were excluded. It proved difficult to distinguish a bird at rest from one that scanned for prey from a perch for a long period. Accordingly, a bird was only classified as having been at rest if, after a period of inactivity, it stretched its wings or legs. Records were split into five-minute periods. If more than one five-minute observation period was recorded for an individual in a day, an overall average value for each foraging activity was calculated.

RESULTS

Average time spent foraging on the ground In winter Robins obtain the majority of their food from the ground; in addition, small

amounts are obtained by pecking items from bark and leaves or by capturing insects in

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