Assessment of the Abdominal Profile Index (API)

When observing individually marked birds (whether a tagged swan or simply a colour-ringed individual), it is valuable to make a standardised assessment of their fat reserves, as these reflect the general condition of the bird. A large proportion of fat reserves are stored as abdominal fat in the bird’s lower body (abdomen). The amount of this stored fat can be visually estimated in the field by examining the bird’s abdominal profile. This is recorded as the Abdominal Profile Index (API; sometimes also called the condition or fat index). Originally developed for geese (Owen 1981), this technique was later applied to the Bewick’s Swan (Bowler 1994).

Slim (left) and well-fed Bewick’s Swan (right).
© L. Vergin/H.-J. Augst

...how does it work?

The different abdominal profiles are divided into six categories (intermediate half steps are possible). These illustrations and accompanying descriptions serve as a reference for classifying the Abdominal Profile Index (API). It is recommended to observe the bird in the field for a sufficiently long period, ideally when it is positioned side-on to the observer and in a relaxed posture (not in a tense, alert stance). If the bird can also be viewed from the front or rear at times, this helps to ensure that the profile line truly reflects variations in body fat volume (and not, for example, a loose fold of skin, which can occasionally occur). It can also be helpful to sketch the profile lines yourself as part of the assessment.

Printable templates for use in the field can be downloaded below.

© NABU/F. Mohrwinkel; Fotos: L. Vergin (1,5/2,5/5), Bart van Lith/NIOO-KNAW (2/3/3,5/4), H.-J. Augst (4,5)

Downloads

Overview of the API levels “to go”. The redrawing of the silhouettes largely follows Bowler 1994, but is adapted to the shapes of profile lines that we have actually observed.

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Worksheet for field observations without pre-drawn abdominal profiles, allowing you to record them yourself.

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  • Bowler, J.M. (1994) The condition of Bewick`s Swans Cygnus columbianus bewickii in winter as assessed by their abdominal profiles. Ardea 82: 241-248.

  • Owen, M. (1981) Abdominal profile – a condition index for wild geese in the field. Journal of Wildlife Management 45: 227-230.