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Body condition and use of surplus food resources

Posted by Pascual_Lopez on 21 Nov 2013 at 09:38 GMT

I have read with great interest this paper published by García-Heras et al. (2013) about the individual use of surplus food resources (SFR) in an insular population of Egyptian vultures. Interestingly, authors show that the use of SFR is asymmetric at the individual level, mainly due to a combination of extrinsic (i.e. distance from breeding sites to SFR) and intrinsic factors (principally body condition).

In this paper authors claim that the response of scavengers to food predictability at the individual has received little attention so far. Thus, they claim that our knowledge in relation to this issue is limited. I partially agree with this statement, it’s true that there is still a long way to go with regards to the effects of predictable sources of food on the behavioral ecology of birds, and particularly with regards to scavengers’ spatial ecology. However, authors omit one reference [1] published very recently (January 2013) in PLOS ONE, in which we analyze the behavioral response of adult Egyptian vultures tracked by GPS satellite telemetry at the individual level in relation to predictability of food resources. In this paper we show that Egyptian vultures are able to shift between foraging search strategies between different periods of the annual cycle (breeding and non-breeding) and, more interestingly, within the same period of the annual cycle (i.e. within the breeding season) in successive years. It would have been interesting that authors discuss their results in the light of this evidence. I’m pretty sure that this is an unintentional omission. In this paper [1], we show how Egyptian vultures are able to perform long-distance movements (ranging from 20 km to 124 km) from their breeding sites to SFR daily and every two-three days. This provides supporting evidence that predictable sources of food are a major determinant of the spatial ecology of scavengers. In this regards, I respectfully disagree with the interpretation of García-Heras et al. (2013) in their claim about the relationship between “poor” body condition and the increase of the probability of use of SFR. In the light of some published results [1,2], it is difficult to understand how birds in supposedly “poor” condition perform long-distance travels almost daily from their breeding sites to predictable sources of food to fulfill their energy requirements. Quite the contrary, it would be necessary a “good” individual condition to make these long-distance journeys taking into account that Egyptian vultures are single-prey loaders, with the associated cost in energy terms, as authors state in their paper.

In my opinion, the relationship between body condition and individual use of SFR has a major weakness: the measure of body condition. Authors measure body condition “estimated from wing residuals (linear regression between body mass and wing chord, taking into account the sex of each individual)” and provide three interesting references [31,43,44]. However, references [4] and [5] precisely advise against using wing residuals as a good indicator of body condition given that they could give rise to spurious results (see [4] and [5] for a complete discussion about this). Finally, in my opinion, it would be not correct using a one-time, single measure of body mass, as an adequate predictor of body condition in the long-term. In fact, birds’ body mass varies during the annual cycle and thus it precludes the use of one single measure of this variable as a good indicator of body condition for a long-term period such as that reported in the paper.

I wish this scientific discussion will be useful to increase our knowledge of the ecology of this endangered species in particular and that of scavengers in general. I would like to congratulate the authors for their excellent field work.


[1] López-López, P., Benavent-Corai, J., García-Ripollés, C., Urios, V. 2013. Scavengers on the move: behavioural changes in foraging search patterns during the annual cycle. PLOS ONE 8(1): e54352 (doi: 10.1371/journal.pone.0054352)

[2] García-Ripollés, C., López-López, P., & Urios, V. 2010. First description of migration and wintering of adult Egyptian vultures Neophron percnopterus tracked by GPS satellite telemetry. Bird Study 57: 261–265.

[3] Agudo R (2010) Conservation Genetics on Islands, a case of the Canarian Egyptian Vulture. PhD thesis Universidad Complutense: Madrid. 230 p.

[4] Green AJ (2001) Mass/length residuals: measures of body condition or generators of spurious results? Ecology 82: 1473–1483. doi: 10.1890/0012-9658(2001)082[1473:MLRMOB]2.0.CO;2.

[5] Peig J, Green A (2010) The paradigm of body condition: a critical reappraisal of current methods based on mass and length. Funct Ecol 24: 1323-1332. doi:10.1111/j.1365-2435.2010.01751.x.

Pascual López-López, Ph.D.
"Juan de la Cierva" post-doctoral fellow
Vertebrate Zoology Research Group
University of Alicante, Spain
web: http://paslopez.wordpress...

No competing interests declared.