We measured the breeding performance, body condition, time budgets and foraging ranges of Kittiwakes Rissa tridactyla at Sumburgh Head, Shetland, in two years of contrasting food availability. Kittiwakes in Shetland generally feed their young almost entirely on sandeels, and fisheries data indicated that stocks of sandeels in Shetland waters were at least ten times higher in 1991 than in 1990. Fledging success of Kittiwakes was nil in 1990 and 68% of eggs laid in 1991, although clutch-size and hatching success were no different between years. Post-hatching foraging trips in 1991 were of comparable duration to those recorded at other colonies in conditions of good food supply (2–3 h), while trips recorded during incubation or post-hatching in 1990 were approximately three times longer on average than at corresponding stages of the breeding season in 1991. Radio-tracking data indicated that adults generally stayed within 5 km of the colony in 1991 but flew more than 40 km from the colony on each trip in 1990. Eggs were apparently not left unattended in either year, despite the fact that this required adults to incubate for periods in excess of 44 h in 1990. The extent to which adults were able to increase trip durations, foraging ranges and incubation shift lengths between years, while maintaining hatching success, indicates the degree to which Kittiwakes are normally buffered against adverse feeding conditions during incubation. Reduced nest attendance and lower body-condition of adults post-hatching in 1990, in conjunction with complete post-hatching breeding failure, indicate that adults were beyond the limits of their buffering capacity during chick-rearing in 1990.

The influence of food supply on the breeding ecology of Kittiwakes Rissa tridactyla in Shetland

Repeatability is the consistency with which an observer measures a set of objects. This statistic can therefore be used both to assess observer performance and to detect changes in the size of the objects between measurements. These uses are described in the context of biometric studies of birds caught by ringers. A simple account is given of how to calculate repeatability and to report the results. Several problems are discussed, with particular emphasis on those caused by the relationship between repeatability and the variability between specimens measured.

https://www.tandfonline.com/doi/pdf/10.1080/03078698.1994.9674078

Some comments on the repeatability of measurements

Biometric data were collected at three colonies of the Atlantic race of Cory's Shearwater Calonectris diomedea borealis. Significant differences were found between sexes and between study sites. For each colony, a discriminant function was derived in order to achieve external sex identification. Using three measurements, all functions correctly classified more than 95% of the birds of known sex, and the biometrical variables investigated showed no variation with age. Marked differences in plumage and bill colour, in vocalization and in size, have led some authors to suggest that C.d. edwardsii could be considered as a distinct species. Comparison of morphological data from several colonies of Cory's Shearwater confirmed that the Cape Verde subspecies C.d. edwardsii occupies a very distinct position relative to the Atlantic and the Mediterranean (nominate C.d. diomedea) races. Birds from Mediterranean and Atlantic colonies show clinal variation in size, increasing from east to west. This morphological patt...

https://www.tandfonline.com/doi/pdf/10.1080/03078698.1993.9674051

Variation in measurements of Cory's shearwater between populations and sexing by discriminant analysis

Sexing monomorphic seabirds is particularly difficult, and available methods have various disadvan- tages, such as seasonal non-applicability or stress induc- tion. The yelkouan shearwater Puffinus yelkouan is a poorly studied seabird endemic to the Mediterranean basin. Ecological and biological data are needed for this species, thus necessitating the development of appropriate field methods. Here, we tested two methods for sexing yelkouan shearwaters: the classical morphological method and a new acoustic method. Morphological analysis demonstrated sexual differences in head, leg and feather measurements, with a high degree of overlap resulting in only 87.2% accuracy. In contrast, acoustic analysis showed that the highest value of the fundamental frequency (FC) and duration of the clear note of calls did not overlap between males and females. Yelkouan shearwaters can be sexed with 100% accuracy by measuring FC. If FC 678.4 Hz, the bird is male. Other advantages of this method are its simplicity and non- invasiveness, which are particularly important for a potentially threatened species.

Morphological versus acoustic analysis: what is the most efficient method for sexing yelkouan shearwaters Puffinus yelkouan ?

We evaluate sexual adult size dimorphism and provide a sex-discriminating function for the Balearic Shearwater (Puffinus mauretanicus). Blood samples taken from adults were used to sex birds by the amplification of the CHD gene. Linear discriminant analysis was applied to eight morphometric characters to determine whether any single variable or combination of them could provide reliable sex determination. Females were significantly smaller than males for most tested variables. Analysis indicated that head plus bill length and minimum bill depth were the most accurate variables in a discriminant function model, predicting sex with about 90% accuracy.

A Discriminant Function for Predicting Sex in the Balearic Shearwater

Barrett, R. T., M. Peterz, R. W. Furness and J. Durinck. 1989. The variability of biometric measurements. Ring. & Migr. 10: 13–16. Within and between measurer consistency of measurement of auk biometrics was examined. We point out variations between individuals that may equal reported differences between populations.

R. W. Furness

Papers

The variability of biometric measurements