The behaviour of long-extinct animals remains an area of major public and scientific interest – the great perennial problem being that we’re always massively constrained, if not crippled, by a frustrating lack of data. Think of all the things we want to know, versus the things that we actually do know. In a paper recently published in Journal of Zoology, I aimed to review what we know about the behaviour of fossil birds (Naish 2014).

Simply knowing about the behaviour of fossil animals is neat and interesting. It allows us to better imagine the daily lives and lifestyles of given species, and thus firm-up hypotheses about their ecology, the timetables of their lives, and the selection pressures that might have acted on their evolution. But it can also give us hard data on the antiquity or novelty of given bits of behaviour, and indicate which behaviours were inherited from ancestral groups, and which were or are specific to certain groups. It is often astonishing how little we know. Conversely, there are areas where key pieces of evidence, or key analyses, can allow us to make fairly detailed statements.

‘Form leads to function’… except where it doesn’t

Where do we start in reviewing a subject as broad as the behaviour of fossil birds? (and keeping in mind that I’m keen not to re-write the paper I’m talking about). To start with, basic indications as to a fossil bird’s ecology and lifestyle can of course be determined by looking at its anatomy, the basic tenet ‘form leads to function’ being an important guiding principle. So, if we look at a fossil bird that combines long limbs with a long, slender bill (both sets of features resembling those of modern wading birds), we can reasonably assume that the bird was an aquatic forager; if it has both a reinforced, strongly curved rostrum and enlarged, strongly curved foot claws, it was almost certain raptorial, and so on.

Compared to many animal groups, birds are relatively well studied, and a huge amount of work has been done on how their proportions and skeletal and soft tissue anatomy correlate with specific lifestyles and ecologies. This whole field is termed ecomorphology and enough is known about ecomorphological correlations in living birds (e.g., Baker 1979, Leisler 1980, Leisler & Winkler 1984, Miles & Ricklefs 1984, Winkler & Leisler 1985, Bairlein et al. 1986, Miles et al. 1987, Carrascal et al. 1990, Hertel 1995, Piersma et al. 1998, Barbosa & Moreno 1999, Nebel et al. 2005) for us to say very specific things about ecology and lifestyle when enough is known about mass, proportions, wing-loading, and about bill, wing and hindlimb anatomy, and so on. Consider that ornithologists can say, for example, that a given bird is not just an insectivore, but that it’s a forest-adapted insectivore that forages for small insects at and underneath branch tips (Carrascal et al. 1990).

Alas, when it comes to fossil taxa, a number of problems erode our confidence. Firstly, some fossil taxa possess anatomical features that don’t precisely resemble those of modern ones, or possess unusual combinations of features not present within living species. The large-bodied, flightless gastornithids of the Eurasian and North American Paleogene and the Australian dromornithids or mihirungs of the Paleogene and Neogene, for example, differ from extant birds in combining robust hindlimb bones with deep, massive crania. As you’ll know if you’ve followed the literature on these birds, there has consequently been much disagreement as to whether they were arch-predators, bone-cracking ‘hyaena-birds’, dedicated herbivores, or omnivores that did a bit of everything.

Secondly, we’re often missing key bits of information when it comes to the anatomy of fossil species. We might be looking at the shapes of the skull bones, for example, while lacking the all-important rhamphotheca; likewise for claws where the keratinous sheath is absent or incomplete.

Thirdly, the ‘form leads to function’ tenet is not always as reliable as we might like. Some living animals do things that we almost certainly wouldn’t predict if we only knew of them from their bones. My favourite examples come from the world of armadillos (Smith & Redford 1990) and fruit-eating crocodylians (Brito et al. 2002). And there are quite a few examples of this sort of thing within birds. The Palm-nut vulture Gypohierax has a predatory bauplan but is almost exclusively frugivorous [adjacent image by Hans Hillewaert]; dippers (Cinclidae) routinely practice aquatic behaviour yet apparently reveal no trace of this in the skeleton [adjacent image by Mark Medcalf]; and several auks nest in trees yet appear anatomically ill-suited for perching or indeed frequenting trees at all.

Despite these problems, people have of course used data on proportions and inferred wing-loading and so on to make a huge number of inferences about the behaviour of fossil birds (e.g., Brathwaite 1992, Hertel 1995, Hopson 2001, Noriega 2001, Elzanowski 2002, Worthy & Holdaway 2002, Tambussi & Hospitaleche 2008, Hinic-Frlog & Motani 2010, Wang et al. 2011; Nudds et al. 2013).

Feeding behaviour and dietary preferences

How might we confirm the predictions about lifestyle and ecology made based on anatomy? Stomach and gut contents provide pretty good answers: I tabulated all the examples I was aware of at the time of submission (Naish 2014) (needless to say, new specimens have been published since, including Piscivoravis from the Jiufotang Formation of China).

Many fossil birds with stomach or gut contents confirm the ideas already arrived at on the basis of anatomy: there are hesperornithines and fossil loons with fish remains in their guts, and numerous moa with leaves and twigs in their stomachs, for example. Mesozoic birds – conventionally assumed to be faunivorous animals that preyed on arthropods and fish – have proved more diverse in diet and ecology than previously thought (Naish 2014), however, with evidence for seed-eating and sap-eating known in addition to faunivory.

Among Paleogene crown-birds, early members of some living lineages seem to be very different from the modern species: Eocene rollers were seemingly omnivorous or herbivorous whereas living ones are mostly faunivorous, for example, while Strigogyps – apparently a Middle Eocene member of the seriema-phorusrhacid clade – preserves evidence of a plant diet, a surprise in view of its affinities.

Additional support for behaviour comes from the bones of prey species damaged by vultures and eagles, and shells transported in the bodies of seabirds. We can also say things about the behaviour of fossil birds based on the functional studies that have been done: work on limb-bone strength in moa and phorusrhacids provides insight into locomotory abilities and perhaps (in the phorusrhacids) predatory behaviour. Finite element analysis of the phorusrhacid Andalgalornis allowed Degrange et al. (2010) to devise hypotheses about how this animal tackled prey.

There are also fossil feeding traces, some of which indicate modern-style dabbling and sediment probing occurring as far back as the Paleogene and even Early Cretaceous. The idea that we might detect the ecology and feeding behaviour of certain extinct birds by looking at the plants and animals they co-evolved with – the idea that we are sometimes seeing the ‘ghosts of predators past’ – was covered in the recent Tet Zoo article The ‘ghosts’ of extinct birds in modern ecosystems. As you might have guessed, the text that appeared in that article was excised from the submitted manuscript that became Naish (2014). The submitted manuscript was more than twice as long as allowed, so I had to axe a lot of material.

Display, combat, vocalisation

Finally, what do we know about the social and reproductive behaviour of fossil birds? Extravagant structures (like elaborate tail feathers) indicate the presence of sexual display in several fossil bird lineages, including the jeholornithids and confuciusornithids located right at the base of the avialan radiation. The long tail plumes known for confuciusornithids seem to be sexually dimorphic (Chinsamy et al. 2013) (something that some researchers had already been saying for years, and something that had been strenuously resisted by others), suggested social polygyny in these birds. Evidence for intraspecific combat in some fossil birds is strongly suggested by the presence of carpometacarpal spurs, clubs and other structures highly similar to structures used in combat in living species. Hume & Steel’s (2013) analysis of the evidence for combat in the Solitaire Pezophaps solitarius appeared too late for me to include reference to it in the paper.

Long, looping tracheae in some moa and enlarged syringeal bullae in some extinct waterfowl are almost certainly linked with unusual vocalisations in these birds but frustratingly little is known about the distribution and antiquity of the syrinx. Did it evolve within birds, or was it present in older dinosaur lineages? We still don’t know.

Considering what we know of nesting and parental behaviour in non-bird dinosaurs and other archosaurs, we might predict that birds inherited nest-building from their ancestors as well as brooding (perhaps involving substantial male involvement) and precociality among hatchlings. Fossils show that colonial nesting and the creation of scrape-type terrestrial nests were present in Upper Cretaceous enantiornithines (Dyke et al. 2012, Fernández et al. 2013, Naish 2014), but that these stem-birds were unlike modern ones in some aspects of nesting behaviour (they did not turn their eggs, for example).

Where are all the nests?

Living birds are remarkable for the diverse nests they construct, many incorporating vegetation and located in trees. What does the fossil record tell us about the origin and antiquity of arboreal and vegetative nest-building behaviour? Well, pretty much nothing: only a handful of vegetative nests are known, and none (so far as I can tell) come from arboreal settings. A few Eocene and Oligocene nests suggested to be those of ducks have been mentioned or illustrated but not described in any detail. More recently, a mass of twigs and leaves, associated with five eggs, was described from the Miocene of Spain and identified as that of a stem-flamingo (Grellet-Tinner et al. 2012).

It makes some sense that vegetative nests of the sort most abundant in the modern world – masses of twigs and/or leaves, usually located high up in trees – are very rare as fossils, since they have a very low preservation potential. But the fact that they aren’t represented at all seems odd, especially when we consider how numerous nests must have been throughout the history of avian evolution, and especially when we consider how many nests there are that incorporate sediment and are actually quite durable. I mean, shouldn’t the South American Neogene fossil record be full of ovenbird nests, at least?

On that note, there are several structures made by birds that have high theoretical preservation potential and remain unknown from the fossil record: “Examples include the bowers of bowerbirds, mud nests of ovenbirds, nest cavities of hornbills, giant (sometimes colonial) nests of hammerkops, weaverbirds and monk parakeets, and cache sites of corvids and woodpeckers” (Naish 2014).

Needless to say, as always, there’s lots and lots more that could be said – there’s lots more that’s covered in the paper – but I hope that this brief discussion of some of what we know about the behaviour of fossil birds shows that, while we know something about some areas, there are other areas where we essentially know next to nothing. And that’s annoying.

Several previous Tet Zoo articles have covered some of the subjects discussed here. See…

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