Hosts are expected to evolve resistance strategies that efficiently detect and resist exposure to virulent parasites and pathogens. When recognition is not error-proof, the acceptance threshold used by hosts to recognize parasites should be context dependent and become more restrictive with increasing predictability of parasitism. Here, we demonstrate that decisions of great reed warblers Acrocephalus arundinaceus to reject parasitism by the common cuckoo Cuculus canorus vary adaptively within a single egg-laying bout. Hosts typically accept one of their own eggs with experimentally added spots and the background colour left visible. In contrast, hosts reject such spotted eggs when individuals had been previously exposed to and rejected one of their own eggs whose background colour had been entirely masked. These results support patterns of adaptive modulation of antiparasitic strategies through shifts in the acceptance threshold of hosts and suggest a critical role for experience in the discrimination decisions between inaccurate-mimic parasite eggs and hosts' own eggs.

The molecular and excimer fluorescence bands recorded for concentrated solutions of pyrene, lY2-benzanthracene, 2-methylnaphthalene, 1-fluoronaphthalene and acenaphthene exhibit an " isosbestic " point over certain limited ranges of temperature ; from the temperature coefficient of the ratio of excimer to molecular fluorescence intensities at the isosbestic wavelength the enthalpy and entropy of photoassociation can be estimated. The heat of photoassociation is found to be significantly higher for pyrene than for the other compounds examined, and for these systems the repulsion energy between unexcited molecules in the excimer configuration contributes substantially to the red-shift of 5000-6000 cm-1 of the excimer band maximum relative to the O"-O' molecular fluorescence band. Entropies of photoassociation are in the region of -20 cal/mole deg. Except at low concentrations the photoassociation of identical uncharged planar aromatic molecules A, competes with fluorescence emission,'A*-+A + hvM, 1A*+3A, and intersystem crossing, of the lowest electronically-excited singlet state 1A* in a fluid .environment. Tlic evidence for process (1) is the emission of a broad structureless band,ly 2 red-shifted by some 5000-6000 cm-1 from the structured molecular fluorescence spectrum, which originates in the radiative relaxation of the excimer 1AZ to a dissociated ground state Process (4) competes in turn with the formation of a stable dimer A2, internal conversion and possibly intersystem crossing 'A;-+2A+ Itv,. (4) 1 * &-+A,, 2A or 3A,(?j, together with dissociation relaxation, at higher temperatures.3The molecular configuration of the photoassociated state 'A; is believed to be that in which the aromatic planes are perpendicular to the axis joining the molecular centres of gravity4; this is supported by the emission of characteristic excimer * presented at the 7th European Congress on Molecular Spectroscopy,

SUMMARYMany avian hosts have evolved antiparasite defence mechanisms, including egg rejection, to reduce the costs of brood parasitism. The two main alternative cognitive mechanisms of egg discrimination are thought to be based on the perceived discordancy of eggs in a clutch or the use of recognition templates by hosts. Our experiments reveal that the great reed warbler (Acrocephalus arundinaceus), a host of the common cuckoo (Cuculus canorus), relies on both mechanisms. In support of the discordancy mechanism, hosts rejected their own eggs (13%) and manipulated ('parasitic') eggs (27%) above control levels in experiments when manipulated eggs were in the majority but when clutches also included a minority of own eggs. Hosts that had the chance to observe the manipulated eggs daily just after laying did not show stronger rejection of manipulated eggs than when the eggs were manipulated at clutch completion. When clutches contained only manipulated eggs, in 33% of the nests hosts showed rejection, also supporting a mechanism of template-based egg discrimination. Rejection using a recognition template might be more advantageous because discordancy-based egg discrimination is increasingly error prone with higher rates of multiple parasitism.

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Many hosts have evolved diverse cognitive mechanisms to recognize and reduce the cost of social parasitism. For example, great reed warblers Acrocephalus arundinaceus can accurately reject closely mimetic eggs of brood parasitic common cuckoos Cuculus canorus. Yet, these same hosts are less effective at identifying and rejecting parasitism when the clutch is parasitized by multiple cuckoo eggs, suggesting a role for discordancy (the rejection of the egg type in the minority of the clutch) and/or online self-referent phenotype matching (the simultaneous viewing of cuckoo and own eggs in the nest) to reject foreign eggs. We tested whether the presence of host's own eggs is required for the discrimination of foreign eggs by dyeing hosts' own eggs with one of several colors so that clutches contained (a) 1 dyed and 4 unmanipulated eggs, (b) 3 dyed and 2 unmanipulated eggs, or 5 eggs dyed either (c1) differently or (c2) similarly. Rejection rates of dyed eggs varied widely between different colors and were highest in treatment (a), with 1 dyed egg, compared with treatments with the majority (b) or all (c1 and c2) dyed eggs. However, relative rejection rates of dyed eggs were also consistent among specific colors across treatments, including (c1) and (c2), where no unmanipulated own eggs were available for viewing and irrespective of whether eggs were dyed all different colors (c1) or the same colors (c2). We conclude that these hosts can rely on comparisons of foreign egg colors against an internal recognition template of acceptable (own) egg phenotypes.