Table 1.

A non-systematic inventory of publications on topics relating to feather pecking in laying hens. Research goals are classified using the type of research definitions in the Frascati Manual [17]. An expansion of this table is available as Supplementary Materials and includes short descriptions of the studies’ experiments, subjects, measured parameters, results, and conclusions.

	Research Goals
Objective	Pure Basic	Oriented Basic	Applied	Practical Application	Reference
Environment
Study the effects of BT and ground type during rearing on pecking behavior.	No	Yes	Yes	Refine husbandry conditions to influence the expression of pecking behavior.	Blokhuis and van der Haar, 1989 [9]
Investigate possible association between FP and stress.	No	Yes	Yes	Refine husbandry conditions to control FP.	El-Lethey et al., 2000 [26]
Investigate whether light exposure late in embryonic development affects early post-hatching FP behavior.	No	No	Yes	Refine husbandry practices relating to light exposure during the last week of incubation.	Riedstra and Groothuis, 2004 [28]
Investigate whether increasing dietary TRP decreases the development and performance of FP.	No	No	Yes	Refine management strategies concerning diet choices to decrease the expression of FP.	van Hierden et al., 2004 [36]
Study the effects of experience with an exploratory-rich environment during rearing on pecking behavior.	No	No	Yes	Refine husbandry practices relating to enriched environments during rearing.	Chow and Hogan, 2005 [33]
Investigate the effects of providing string devices on FP under (1) experimental and (2) commercial conditions.	No	No	Yes	Refine husbandry conditions by offering device strings to reduce FP.	McAdie and Keeling 2005 [34]
Investigate laying hen behavior under differential commercial stocking densities, flock sizes, and management practices.	No	No	Yes	Refine management strategies to reduce the risk of FP.	Zimmerman et al., 2006 [30]
Investigate the underlying motivation of GFP and SFP by comparing their motor patterns to those of dust bathing and foraging pecks.	No	Yes	Yes	Refine husbandry conditions by offering forages (straw, hay, silage) to reduce SFP.	Dixon et al., 2008 [49]
Investigate the role of environmental factors associated with the development of FP.	No	No	Yes	Refine management practices to influence the expression of pecking behavior.	Lambton et al., 2010 [11]
Study the effect of pen environment on group behavior and dynamics.	No	Yes	Yes	Refine husbandry practices to better adjust to environmental needs of laying hens.	Collins et al., 2011 [25]
(1) Investigate if dark brooders can successfully be used on commercial rearing farms. (2) Test if the subsequent FP reduction is replicable without compromising bird growth and mortality.	No	No	Yes	Offer dark brooders as an alternative to standard husbandry practices to reduce FP.	Gilani et al., 2012 [32]
(1) Investigate the effects of BT and EE during rearing on PD. (2) Study the relationship between behavior in the rearing period and PD in the laying period.	No	No	Yes	Refine management practices to influence the expression of SFP.	Hartcher et al., 2015 [8]
Study the effect of EE and a reduced stocking density on FP.	No	No	Yes	Refine management practices to influence the occurrence of FP.	Zepp et al., 2018 [31]
Genotype
Examine differences between layer hen strains with regard to diurnal rhythm of FP and tendency to FP.	No	No	Yes	Refine breeding and management strategies to reduce the risk of FP.	Kjear, 2000 [37]
(1) Identify genotypes that show LFP in a free-range environment. (2) Study the effect of a diet enriched by sulfuric amino acids on FP. (3) Investigate the impact of light intensity during rearing in an interaction with access to the range area at different ages.	No	No	Yes	Refine breeding and management strategies to reduce the risk of FP.	Kjaer and Sorensen, 2002 [29]
Estimate heritability of FP and OF response of laying hens at different ages.	No	No	Yes	Refine breeding strategies to reduce the risk of FP.	Rodenburg et al., 2003 [50]
Study the effects of selection against mortality and BT on fear-related behavior and peripheral 5-HT concentration and uptake.	No	No	Yes	Refine breeding and management strategies to reduce the risk of FP.	Bolhuis et al., 2009 [35]
Determine parameters of heart rate variability in HFP and LFP lines to elucidate ANS responses during rest and stressful situations.	No	Yes	Yes	Enable breeding efforts by mapping relationships between FP and ANS responses.	Kjaer and Jorgensen, 2011 [27]
Investigate if sub-populations of EFP birds exist in HFP and LFP lines and their F2-cross.	No	No	Yes	Refine breeding strategies to reduce the risk of FP.	Piepho et al., 2017 [38]
Investigate if PS affects the development of anxiety and SFP in their offspring.	No	No	Yes	Refine management practices to influence the expression of pecking behavior.	de Haas et al., 2014 [39]
Examine relationships between the immune system and FP by characterizing HFP and LFP lines with regard to immune characteristics.	No	Yes	Yes	Enable breeding efforts by exposing relationships between the immune system and FP.	van der Eijk et al., 2019 [40]
Propose a new model to detect EFP: (1) Introduce a new theory and statistical method for the analysis of EFP; (2) define a new trait, EFPp; (3) analyze the interrelationship of EFPp with fearfulness.	No	No	Yes	The identified new trait may refine breeding strategies to reduce EFP. An index of fear-related traits may serve as a proxy to breed indirectly for the new trait.	Iffland et al., 2019 [41]
Phenotype
Examine if tendency to avoid a NO was predictive of (1) pecking towards feather bundles or (2) FP. Analyze if pecking at feather bundles and FP were related.	No	No	Yes	Offer a less time-consuming alternative to the current method of selecting LFP that is used for breeding.	Albentosa et al., 2003 [42]
Investigate the relationship between fear responses, physiological measurements of basal plasma-CORT and whole-blood 5-HT, PD, and productivity in PS flocks.	No	Yes	Yes	Refine management strategies concerning PS flocks by taking breed differences, group size effects, and effects of human–bird interactions into account.	de Haas et al., 2013 [43]
Physiology
Investigate if lowering 5-HT turnover in the forebrain of laying hens increases expression of FP.	No	Yes	Yes	(1) Offer FP behavior as an animal model for an impulse-control disorder like trichotillomania. (2) Indicate that chronic enhancement of 5-HT neurotransmission in the chicken brain may be beneficial in reducing FP expression.	van Hierden et al., 2004 [44]
Examine if immune modulation by airborne constituents predisposes birds for harmful behavior like FP.	No	No	Yes	Refine vaccine management strategies to reduce the risk of FP.	Parmentier et al., 2009 [45]
Study the neurobiological mechanisms of SFP.	No	Yes	No	-	Kops et al., 2013 [46]
Behavior
Investigate the development of FP and related behaviors in chicks of HFP and LFP lines.	No	Yes	No	-	Van Hierden et al., 2002 [47]
Investigate if social learning is involved in the spread of cannibalism.	No	Yes	No	-	Cloutier et al., 2002 [3]
Investigate if coping theory can predict FP in laying hens.	No	Yes	Yes	Discourage use of coping theory as a tool to find the underlying mechanisms of FP.	Forkman et al., 2004 [51]
(1) Investigate HFP line preference for pecking at and/or ingestion of feathers over wood shavings and LPF line preference of wood shavings over feathers. (2) Investigate if hens housed alone in cages exhibit stronger motivation for their preferred substrate.	No	Yes	Yes	Refine management strategies to influence the expression of FP.	Harlander-Matauschek et al., 2007 [48]

Abbreviations: serotonin (5-HT); autonomic nervous system (ANS); beak trimming (BT); corticosterone (CORT); environmental enrichment (EE); extreme feather pecking (EFP); feather pecking (FP); gentle feather pecking (GFP); high feather pecking (HFP); low feather pecking (LFP); novel object (NO); open field (OF); plumage damage (PD); parent-stock (PS); severe feather pecking (SFP); tryptophan (TRP).