the role of ultraviolet light in defense of brood parasitism

The Role of Ultraviolet Light in Defense of Brood Parasitism

byDawna Lee-Olsen

ECOL 484

Introduction

• Brood parasitism– host-parasite interactions are examples of co-

evolution– possibility of reduced fitness leads to

behavioral mechanisms to counteract• desertion• rejection• burial

Introduction (cont.)• Most birds have tetrachromatic vision

– 4 types of retinal cones (humans have 3)– enables them to detect near UV wavelengths in

the 320-400nm range

UV Reflectance in Plumage

Zebra Finches

Blue Throats

Methods

• Literature review consists of 16 articles based on spectra reflectance on 300-700 nm range– 12 studies on host intra-clutch variation– 1 study examined role of color characteristics in egg

recognition– 2 studies used principal components analysis (PCA)

• 3 components: brightness, brownness, and UV/greenness

– 1 study examined egg types of generalist cuckoo that parasitized several host species

Results

• Host intra-clutch variation studies– 3 studies support hypothesis that rejected host

clutches are more dissimilar than accepted ones

– 2 studies support hypothesis that rejected clutches are less dissimilar

– 7 studies found no effect of intraclutch variation on rejection

Results (cont.)

• Study on the blunt egg part found – brightness reflected was lower in rejecters than

accepters– were darker in host clutches vs. rejected eggs– were lighter in host clutches vs. accepted eggs

Results (cont.)

• 1 study showed significant UV/greenness (PC3) oppoency between host-parasite eggs

Results (cont.)

• 1 study showed common cuckoos populations match eggs of their host’s populations– able to choose nests in which eggs similar to

own

Results (cont.)

• 1 study revealed cuckoo eggs mimicked hosts’ eggs in both spectral shape and brightness

Discussion

• Compared to human vision, spectrometric measures provide good estimates of egg coloration– Gain insight into egg matching previously hidden– may need to evaluate model using artificial or painted

eggs– Disadvantage: does not assess spatial pattern of

maculation in eggs

• Ejected parasitic egg more dissimilar than accepted– Hosts recognize eggs based on discordancy or “true

recognition”– Hosts reject eggs based on direct comparisons

Discussion (cont.)

• Host egg discrimination possibly performed at certain wavelengths– explains acceptance of nonmimetic eggs

• Other mechanisms at work:– hardwiring of avian visual system– acceptance of nonmimetic eggs a function of

different light environments– spatial correlation of diet of hosts and parasites

residing in same locality

Discussion (cont.)

• Parasites such as cuckoos choose nests with eggs that closely match their own– Could explain the lower levels of rejection in naturally

parasitized nests• Blunt egg part analysis reveals rejection of parasitic

eggs in birds with lower variation in blue chroma • Additional spectrometric studies needed• Currently, use of spectrometric technology in

conjunction with human vision a promising approach for investigating– brood parasitism – egg matching

Acknowledgements

• Dr. JodyLee Estrada Duek• Dr. Robert “Bill” Mannan

Questions??

Which eggs . . . are parasitic?