|Rollers smell the fear of nestlings: electronic supplementary material
MATERIAL AND METHODS
In this population (37°18’N, 3°11’W), rollers were breeding in cork nestboxes recently (2003-2005) installed in the area (1).
The roller is a migratory bird that usually makes one clutch per year (2) of about 5 eggs (mean ± s.e., N = 5.04 ± 0.19, 26 nests in the study year). Incubation begins before clutch completion, which leads to a patent asynchronous hatching pattern (3). It is a socially monogamous species in which incubation takes 20 d and nestling rearing 24 d (2). Both sexes participate in incubation and feeding tasks (4;5). In the study area the most common predators of nest content are snakes (e.g. ladder snake, Rhinechis scalaris) and rodents (e.g. garden dormouse, Eliomys quercinus) (authors, unpublished data).
Nestling rollers are fed with a large share of poisonous arthropods (6) that are avoided by most of the other sympatric insectivorous birds (7).
During handling, nestling rollers regurgitate an odorous transparent orange liquid that usually include partially digested parts of ingested arthropods (authors, unpublished data). Adults, when handled, do not regurgitate.
Nests assigned to each treatment did not differ either in hatching date (Mann-Whitney U test: Z = 0.29, P = 0.77, NV = 9, NL = 6) or brood size at hatching (mean ± s.e., N = 4.2 ± 0.32, 15 nests; Mann-Whitney U test: Z = -1.30, P = 0.19, NV = 9, NL = 6). Nests were assigned to treatments the day in which the first egg hatched in each nest. Between that day and the experiment, which was done ten days later, some predation occurred. These predation events affected by chance more to nests assigned to the lemon treatment. However, this was independent of the treatment itself because it happened before treatment application.
Vomits used in the experiment were collected from about 10-days-old nestlings from other nests to those considered in this experiment to remove the possibility of individual recognition of substances produced by the own offspring. After collection, 1 ml aliquots of vomit samples were frozen at -20ºC glass vials until the day of the experiment, when they were allowed to thaw just before use. Lemon essence was obtained diluting 0.5 g scratch lemon in 1 ml distilled water. The mixture was maintained 24 hours in the fridge and then the liquid fraction collected and distributed in 1ml aliquots to be used in the experiment. Lemon essence was used as a harmless and unusual odour in nestboxes. We are sure that vomits/lemon essence smeared to the inner part of nestboxes entrance was not visible to nestlings because: 1) most nestlings in each nest had their eyes still closed when we performed the experiment (see video); 2) nestbox entrance is 20 cm above the floor of the nestbox where the nestlings are and, in addition, in all nestboxes there is a ledge below the entrance that prevents nestlings’ visual detection of the entrance hole (see video). And, 3) because spectrophotometric measures of colour of the cork smeared with scents before and after treatment application revealed that cork colour differed only slightly but in the same way for both treatments, and that this effect diluted after 15 minutes (see figure below). Reflectance data of empty nestboxes before and after the application of vomit and lemon treatments was recorded with an Ocean Optics equipment [S2000 spectrometer connected to a deuterium-halogen light (D2-W, mini) by a coaxial reflectance probe (QR-400-7-UV-vis) and the OOIBase32TM operating software (Ocean Optics, Inc. Dunedin, FL, USA)]. Reflectance was measured with the probe placed at a constant distance and reaching the nestling at 45º. Measurements were relative and referred to a standard white (WS-2) and to the dark, which we calibrated before each set of measurements (before/after vomit/lemon). All measurements were taken at dark.
Average reflectance patterns across the whole spectrum of the cork in the inner part of the nestbox entrance before and after the application of the vomit and lemon treatments.
Recordings of parental provisioning behaviour were done with a video camera Sony DCR-SR32 placed several meters away (mean ± se = 5.6 ± 0.26 m). Parental provisioning behaviour was recorded for similar durations in control odourless conditions (pre-treatment: mean ± se = 106.32 ± 8.51 min) and under experimental conditions (treatment: mean ± se = 93.82 ± 3.80 min). Moreover, recordings were done at the same time of day in both treatments groups (average time of day of beginning of recordings: vomit treatment, 10:46 h; lemon treatment, 11:14 h). Only the camera was present during recordings, but an observer visited nests once between the pre- and during-treatment periods to apply treatments.
In a pilot study in three nests (2 nests assigned to the vomit treatment and 1 assigned to the lemon treatment) we also recorded nestling behaviour before and after the application of the treatments to investigate whether the application of treatments affected in any way to nestling behaviour. This was done installing wireless microcameras (KPC-S500, black and white CCD camera, eSentia Systems Inc., Baton Rouge, LA, USA) inside nestboxes. Microcameras were installed under the roof of the nestbox camouflaged in a cork structure and were used together with a hard disk device media recorder (EMTEC, Gennevilliers, France) and a 3 inch portable monitor. The behaviour of nestlings did not differ before and after the application of scents neither in the vomit or the lemon treatment (see videos).
(1) Parejo D, Aviles JM. Predation risk determines breeding territory choice in a Mediterranean cavity-nesting bird community. Oecologia 2011;165(1):185-91.
(2) Cramp S, Simmons KEL. The birds of the western Palearctic. Oxford: Oxford University Press; 1988.
(3) Parejo D, Silva N, Avilés JM. Within-brood size differences affect innate and acquired immunity in Roller Coracias garrulus nestlings. Journal of Avian Biology 2007;38:717-25.
(4) Silva N, Aviles JM, Danchin E, Parejo D. Informative content of multiple plumage-coloured traits in female and male European Rollers. Behavioral Ecology and Sociobiology 2008;62(12):1969-79.
(5) Aviles JM, Parejo D, Rodriguez J. Parental favouritism strategies in the asynchronously hatching European Roller (Coracias garrulus). Behavioral Ecology and Sociobiology 2011;65(8):1549-57.
(6) Aviles JM, Parejo D. Dieta de los pollos de carraca (Coracias garrulus) en una zona mediterránea (Extremadura, suroeste de España). Ardeola 1997;44:234-9.
(7) Fry C. Family Coraciidae (Rollers). In: del Hoyo J, Elliott A, Sargatal J, editors. Handbook of the birds of the world. Lynx edicions; 2011. p. 342-69.