Functional traits database for North American birds

Functional diversity is based on multidimensional trait space, but there are many traits that an organism possesses, and virtually all of them can be found to be relevant. For example, using traits relevant to the Grinnellian niche might be useful for modeling the spatial distribution of a species and may concomitantly reveal habitat requirements. The Grinnellian niche, however, does not really reveal much about the functional role of particular species. For a meaningful estimation of functional role, one would need traits related to species trophic requirements and resource consumption, reproductive output, biomass and other components related to that species' effects on ecosystems.

In the dataset presented here, 23 functional traits were used that can be partitioned into categories of ecological influence and importance within habitat filtering:

• resource consumption is thought to be related to habitat choice by organisms based on the assumption that the presence of resources required by an organism is the basic prerequisite for its existence;
  • food resources needed to meet energetic demands of an organism, one of the most important resource type explaining community structure;
  • other resources needed to maintain stable population growth, such as resources needed for nesting, which may act as a limiting resource and affect biomass flows in ecosystem due to use of construction materials for nest building;
• effect intensity in terms of species traits that predict local abundance and short-term population dynamics that result in the amount of resources allocated by an individual;
  • reproductive traits affect the rate at which a population is being replenished and the amount of new individuals that are consuming resources, affecting overall fitness of species;
• life history traits affect the amount of resources consumed by one individual during its lifetime; biotic interactions related to the position of an organism within a trophic web and related to a species being keystone within a community.

The trait variables used in works related to functional diversity in birds are typically limited to a small number and are often binary or continuous, with the potential for a loss of information about functional similarity of species. To avoid it, three types of variables were used: continuous, either constrained (i.e., with a minimum possible value of 0 and maximum possible value of 1, e.g., probabilities) or not (e.g., body mass), categorical, so that species could be assigned one category among a fixed set of possible values, and discrete distributions (i.e., there was a set of variables corresponding to one trait that described a distribution of this trait among fixed categories so that the sum of the variable values for a species was always 100%).

The dataset on species traits was compiled using a set of available literature sources describing biology and life history of North American birds (Ehrlich et al. 1988, Wilman et al. 2014, Sheard et al. 2020, Billerman et al. 2022). In a case when a trait value was unknown, it was assumed based on sister taxa (e.g., a mean value for a genus or family).

Six of the traits (diet, diurnal feeding, nocturnal feeding, diet items, feeding method, feeding substrate) are related to diet and are determined by a community's effectiveness at nutrient retention.

The diet category (fruit and nectar eaters, invertebrate eaters, omnivores, plant and seed eaters, and carnivores [vertebrates, fish, and/or carrion]) reflects the trophic grouping of each species.

Diurnal and nocturnal feeding were treated as non-exclusive binary variables, indicating day/night activity; nocturnal activity is not widespread among most avian taxa, with the exception of owls and nightjars, and likely is a good predictor of functional distinctiveness.

Diet items were represented as a distribution of food types (invertebrates, endothermic vertebrates, ectothermic vertebrates, fish, unknown vertebrates, carrion, fruits, nectar, seeds, green parts of plants, and human food leftovers) consumed by a particular taxon, approximating its position within a food web.

Feeding methods reflected feeding behavior, which is related to the portion of resource space used by a species. Feeding methods included the following categories: digging (removing surface soil, rocks, and/or understory to find food), ground gleaning (picking up items from the surface), foliage gleaning (taking items from foliage and small branches), bark gleaning (excavating, drilling, and removing items from bark), gleaning while hovering (removing items from plants while in the air), hover and pouncing (hovering before swooping or dropping down on prey), hawking (short flights from a perch), aerial foliage (capturing items mid-air during continuous flights), aerial foraging (chasing and catching prey in air or snatching it from a perch), swoops (snatching prey up from ground after a gliding descent), high patrol (soaring at high altitude looking for prey), low patrol (searching flight at low altitudes, i.e., height of a tree), high dives (dropping from height into water), skimming (low flight over water, with or close to penetrating the surface with bill), surface dives (floating and diving underwater), surface dips (taking items underwater while floating), dabbing (taking items with more than half of the body being underwater), stalking and striking (hunting in water by standing above it motionless), and probing (taking items located beneath the surface of a solid substrate while standing on it).

Feeding substrate reflected the location of resources within an ecosystem and its spatial distribution determining resource partitioning and niche differentiation (MacArthur 1958). Feeding substrate consisted of following categories: mud, below water level, within the water column, tree, ground, understory, middle to high parts of trees (trunk, bark, branches), canopy (leaves and small branches), air, dumps and dumpsters.

Overall, these traits (diet, diurnal/nocturnal feeding, diet items, feeding methods, and feeding substrate) are expected to represent the position of species in a food web fully enough to assume its effect on ecosystem functioning.

Other resources that are important to birds relate to nest building because such resources presumably are limiting and predict competition for nesting sites, species occurrence, and, overall, community structure.

Nest type relates to evolutionary history, and in the work presented here it was treated as a categorical trait, so that each species could be assigned to one type: no nest (i.e., laying eggs on bare ground), parasitism (does not require building a nest, but is dependent on other species' nests presence), scrape (a simple depression with a rim to prevent eggs from rolling), crevice (a crack in a cliff or between rocks), burrow (chamber in the end of a burrowed tunnel), cavity (excavated or natural cavity in a limb or trunk of a tree), platform (structure large enough for a bird to land), saucer (shallow cup with a small rim built of small branches and other materials), cup (hemispherical nest with a large rim), spherical (i.e., enclosed with a small entrance), chamber (spherical nest hidden within a substrate), oven (a specific chamber with an explicit roof and small entrance), gourd (a chamber made of clay and saliva, typically attached to a vertical surface), and pendant (an elongate sack suspended from a branch).

Nest substrate is expected to be more variable within a species than nest shape, so that this trait was assumed to be discretely distributed among the following categories: bank (river/lake/sea banks, steep slopes near the water), ground (among rocks, roots of live or fallen trees, bare soil), grass, cliff (natural crevices and/or ledges of cliffs), artificial structures (poles, antennas, and buildings), shrubs, deciduous trees, coniferous trees, snags and dead limbs, cacti, tangles (vines, brambles, brush piles), and floating on water (e.g., anchored to emerged or submerged vegetation).

Traits related to reproduction were expected to relate intensity of effect a species has on ecosystem functioning, since species with large offspring that breed frequently might affect matter and energy fluxes due to mass effects.

Breeding system was expected to relate to local density, so that each species was assumed to be one of the following: cooperative (two or more females simultaneously breed in the same nest with or without non-breeding helpers), polygynandrous (two or more males mate with two or more females exclusively), promiscuous (males and females mate indiscriminately), lekking (females survey the males' competitive displays), polyandrous (one female mates with more than one male), polygynous (one male mates with more than one female), and monogamous (one male mates with one female). 

The mean age of first reproduction indicates the time of individual affecting ecosystem functioning directly due to resource consumption without an indirect effect through offspring.

Nest aggregation (solitary, loosely colonial, or strongly colonial) was coded as a categorical trait. A strongly colonial species would be expected to have a strong effect on ecosystem functioning due to its high local density but be more limited in nesting location choice.

The mean number of successful clutches per year is also important as it shows the fecundity of a species and its reproductive output per unit of time.

Clutch size was also considered, along with both lower and upper limits.

Breeding success was defined as the probability of a breeding pair to produce an egg that survives until fledging and successfully transitions into independent movement and feeding.

Number of clutches per year, clutch size, and breeding success combined are expected to be a good predictor of the effect of a species' reproduction on ecosystems.

Chick development time within the nest is highly related to species' phylogeny; this trait was considered categorical (precocial 2 [chicks are mobile, downy at hatching, able to follow the parents and find food], precocial 3 [mobile, downy, follow parents, but food needs to be shown by parents], precocial 4 [mobile, downy, follow parents and are fed], semiprecocial [mobile, but remain at rest and fed], semialtricial 1 [immobile, downy, eyes are open, but fed], semialtricial 2 [immobile, downy, eyes are closed, and fed], and altricial [immobile, featherless, eyes closed, and fed]).

Other life history traits are also related to the effect of species on ecosystem functioning, as well as being important for conservation efforts.

For example, mean body mass might be an obvious morphometric parameter that predicts resource consumption.

Maximum lifespan is also essential in determining the amount of resources consumed by an individual over its lifetime; although mean lifespan would be more meaningful, data on maximum lifespan is more easily obtained, especially for rare taxa.

Finally, mean adult survival is relevant for conservation purposes, since species with low survival probability might be more threatened by extinction.

The traits related to biotic interactions might be the most important ones in the context of a species' functional role, but, at the same time, the most difficult to estimate.

Among traits used in the work presented here, kleptoparasitism was related to diet and defined as the proportion of diet obtained by parasitizing other species. Nest dependency (i.e., to what extent a species relies on other species for finding a suitable nest location [e.g., owls that breed in nest holes but are dependent on large woodpeckers creating nest cavities]), and nest parasitism (i.e., relying on other individuals, either conspecifics or other species, for egg incubation) are biotic interaction traits related to reproduction. All three of these traits were quantified as ratios, representing the levels at which such a behavior is typical for a species.

Some morphological parameters were also used as a proxy for biotic interactions. For example, hand-wing index, calculated as 100×(DK/LW) where LW is the wing length (between the carpal joint and the tip of the longest primary feather) and DK is Kipp's distance (difference between LW and secondary length [i.e., from the carpal joint to the tip of the first secondary feather]), is related to diet and dispersal ability (Sheard et al. 2020). These characters play an important functional role in the ecosystems, for example, for seed dispersal.

The columns in the dataset are coded as following:

1 name [character] : scientific name of taxa {1}

2 proj_id [integer] : project ID

3 et_id [integer] : ID connector to EltonTraits database {1}

4 hbw_id [integer] : ID connector to HBW/BirdLife taxonomic list {2}

5 ebd_id [integer] : ID connector of species to eBird taxonomy (v2019) {3}

6 ebd_sub [regular sequence, i.e., a:b = (a, a+1, a+2, ..., b-2, b-1, b)] : ID connector of subspecies to eBird taxonomy (v2019) {3}

7 ebd_mix [vector, i.e., a;b;c = (a, b, c)] : ID connector of slash/hybrid taxa to eBird taxonomy (v2019) {3}

8 bbs_id [integer] : ID connector of subspecies to BBS species list {4}

9 aou_id [integer] : ID connector of subspecies to AOU taxonomy {4}

10 aou_mix [vector, i.e., a;b;c = (a, b, c)] : ID connector of slash/hybrid/genus taxa to AOU taxonomy

11 hwi [numeric] : hand-wing index {5}, 100×(DK/LW) where LW is the wing length (between the carpal joint and the tip of the longest primary feather) and DK is Kipp's distance (difference between LW and secondary length [i.e., from the carpal joint to the tip of the first secondary feather])

12 order [character] : taxonomic order {1}

13 family [character] : taxonomic family {1}

14 eng_name [character] : English taxa name {1}

15 synonyms [character, may be divided by ";"] : scientific synonyms {2, 3}

16 mlife [numeric] : maximum observed lifespan, years {6}

17 mis_mlife [binary] : is mlife data missing? (if "1", the mean of the sister taxa provided)

18 adsurvival [numeric, (0, 1)] : annual adult survival probability {6}

19 mis_adsrviv [binary] : is adsurvival data missing? (if "1", the mean of the sister taxa provided)

20 matage [numeric] : mean age of the first breeding {6}

21 parasi [numeric, (0, 1)] : nest parasitism (both intra- and interspecific) {7, 6}

22 egglow [integer] : minimum clutch size {7, 6}

23 eggupp [integer] : maximum clutch size {7, 6}

24 nsuc [numeric, (0, 1)] : mean nest success (proportion of breeding attempts that yield at least one fledgling) {6}

25 hatc [numeric, (0, 1)] : hatching probability (per one egg) {6}

26 fled [numeric, (0, 1)] : fledging probability (per one hatchling) {6}

27 ypp [numeric] : mean number of fledgling per nesting attempt {6}

28 suces [numeric, (0, 1)] : overall nesting success (i.e., probability that an egg laid in a nest will yield a fledgling)

29 mis_suces [binary] : is it possible to estimate suces based on the data available? (if "1", the mean of the sister taxa provided)

30 attem [numeric] : mean number of successful nesting attempts a year {6, 7}

31 dev [factor] : development at hatching {7, 6}:    
pre2 : precocial 2 - mobile, downy, follow parents, find food,    
pre3 : precocial 3 - mobile, downy, follow parents, are shown food by parents,    
pre4 : precocial 4 - mobile, downy, follow parents, being fed,    
semipre : semiprecocial - mobile, remain at rest, being fed,    
semialt1 : semialtricial 1 - immobile, downy, eyes open, being fed,    
semialt2 : semialtricial 2 - immobile, downy, eyes closed, being fed,    
alt : altricial - immobile, downless, eyes closed, being fed.

32 bmass [numeric] : mean biomass, g {7, 6} 

33 naggr [factor] : nesting aggregation (applies to breeding season only) {6}:    
(1) solitary,    
(2) may stay in family groups or loose colonies,    
(3) strongly colonial.

34 brsys [factor] : breeding system {6, 7}:    
coop : cooperative, two or more females simultaneously breed in the same nest with or without non-breeding helper,    
polygam : polygynandry, two or more males mate with two or more females exclusively,
promisc : promiscuity, males and females mate indiscriminately,    
lek : lekking, females survey the males' competitive displays,    
polyandr : polyandry, one female mates with more than one male,    
polygyn : polygyny, one male mates with more than one female,    
monog : monogamy, one male mates with one female.

35 mis_bree [binary] : is data on mating system missing? (if "1", the most frequent among the sister taxa provided)

36 bpstart [mm-dd] : date of the beginning of breeding season {6}

37 bpstop [mm-dd] : date of the end of breeding season {6}

38 ndepen [numeric, (0, 1)] : nest dependency - to which extent the species relies on other species in terms of nesting site availability {6}

39 nest [factor] : nest type {7, 6}:    
(1) no : laying eggs on bare ground,    
(2) parasitism : does not require building a nest, but is dependent on other species' nests presence,    
(3) scrape : a simple depression with a rim to prevent eggs from rolling,    
(4) crevice : a crack in a cliff or between rocks,    
(5) burrow : chamber in the end of a burrowed tunnel,    
(6) cavity : excavated or natural cavity in a limb or trunk of a tree,    
(7) platform : structure large enough for a bird to land,    
(8) saucer : shallow cup with a small rim built of small branches and other materials,    
(9) cup : hemispherical nest with a large rim,    
(10) spherical : enclosed with a small entrance,    
(11) chamber : spherical nest hidden within a substrate,    
(12) oven : a specific chamber with an explicit roof and small entrance,    
(13) gourd : a chamber made of clay and saliva, typically attached to a vertical surface,    
(14) pendant : an elongate sack suspended from a branch.

40...51 Nest substrate group {6, 7}
40 nsbank [%] : bank
41 nsground [%] : ground
42 nsgrass [%] : grass
43 nsclif [%] : cliff
44 nsbuild [%] : building, artificial structures (except nestboxes)
45 nsshru [%] : shrubs
46 nsdeci [%] : deciduous trees
47 nsconi [%] : coniferous trees
48 nssnag [%] : snags, dead limbs
49 sncact [%] : cacti
50 nstang [%] : tangle
51 nsfloat [%] : floating

52...61 Foraging substrate group {1, 6, 7}
52 fsmud [%] : mud
53 fswatb [%] : below water level
54 fswata [%] : around water level
55 fstimb [%] : timber
56 fsgroun [%] : ground
57 fsunder [%] : understory
58 fsmidhigh [%] : middle to high tree parts
59 fscanopy [%] : canopy
60 fsaerial [%] : air
61 fsdump [%] : dumps

62 fpnoct [binary] : is foraging at night? {7}

63 fpdiur [binary] : is foraging at day? {7} 

64 klepto [numeric, (0, 1)] : kleptoparasitism {7} 

65 fc_categ [factor] : prevalent diet category    
FruiNect : fruits and nectar,    
Invertebrate : invertebrates,    
Omnivore : omnivore,    
PlantSeed : plants and seeds,    
VertFishScav : vertebates, fish, or scavenging.

66-76 Foraging categories {1, 6, 7}
66 fcinvert [%] : invertebrates
67 fcvend [%] : endothermic (warm-blooded) vertebrates
68 fcvect [%] : cold-blooded vertebrates
69 fcfish [%] : fish
70 fcunk [%] : unknown vertebrates
71 fcscav [%] : carcasses
72 fcfruit [%] : fruits
73 fcnect [%] : nectar
74 fcseed [%] : seeds
75 fcgree [%] : green parts of plants
76 fcleft [%] : leftovers

77-95 Foraging method {6}
77 fmdig [%] : digging
78 fmgrgl [%] : ground glean
79 fmfogl [%] : foliage glean
80 fmbagl [%] : bark glean
81 fmhogl [%] : gleaning while hovering
82 fmhopo [%] : hover and pouncing
83 fmhawk [%] : hawking
84 fmaero [%] : aerial foliage
85 fmapur [%] : aerial pursuit
86 fmswoo [%] : swoops
87 fmhipa [%] : high patrol
88 fmlopa [%] : low patrol
89 fmhidi [%] : high dives
90 fmskim [%] : skimming
91 fmsdiv [%] : surface dives
92 fmsdip [%] : surface dips
93 fmdabb [%] : dabbling
94 fmstal [%] : stalking
95 fmprob [%] : probing

96 pelagic [binary] : is a pelagic taxa?

97 ebd_id2022 [integer] : ID connector of species to eBird taxonomy (v2022) {3}

98 code [character] : code ID in eBird taxonomy (v2022) {3} 

99 ebd_id2023 [integer] : ID connector of species to eBird taxonomy (v2023) {3}

{Data sources}: 

1. Wilman, H., J. Belmaker, J. Simpson, C. de la Rosa, M.M. Rivadeneira, and W. Jetz. 2014. EltonTraits 1.0: Species-level foraging attributes of the world's birds and mammals. Ecology 95:2027. https://doi.org/10.1890/13-1917.1
2. HBW, and BirdLife International. 2018. Handbook of the birds of the world and BirdLife International digital checklist of the birds of the world. Version 3. http://datazone.birdlife.org/userfiles/file/Species/Taxonomy/HBW-BirdLife_Checklist_v3_Nov18.zip
3. Clements, J. F., T. S. Schulenberg, M. J. Iliff, S. M. Billerman, T. A. Fredericks, J. A. Gerbracht, D. Lepage, B. L. Sullivan, and C. L. Wood. 2020. The eBird/Clements checklist of Birds of the World: v2020. https://www.birds.cornell.edu/clementschecklist/download/
4. Pardieck, K.L., D.J. Ziolkowski Jr., M. Lutmerding, V.I. Aponte, and M.-A.R. Hudson. 2020. North American breeding bird survey dataset 1966 - 2019: U.S. Geological Survey data release. https://doi.org/10.5066/P9J6QUF6
5. Sheard, C., M.H.C. Neate-Clegg, N. Alioravainen, S.E.I. Jones, C. Vincent, H.E.A. MacGregor, T.P. Bregman, S. Claramunt, and J.A. Tobias. 2020. Ecological drivers of global gradients in avian dispersal inferred from wing morphology. Nature Communications 11:2463. https://doi.org/10.1038/s41467-020-16313-6
6. Birds of the World (S. M. Billerman, B. K. Keeney, P. G. Rodewald, and T. S. Schulenberg, Editors). Cornell Laboratory of Ornithology, Ithaca, NY, USA. https://birdsoftheworld.org/bow/home
7. Ehrlich, P.R., D.S. Dobkin, and D. Wheye. The birder's handbook: a field guide to the natural history of North American birds. Simon & Schuster, New York, New York, USA.