The Structure and Insulation of Avian Nests

Birds’ nests have evolved into many shapes and sizes, but they all function to provide a secure substrate for eggs and hatchlings, camouflage and defence from predators, as well as protect the eggs, hatchlings and incubating parent from harsh climatic conditions. My doctoral studies focused on understanding the factors influencing the structure and insulation of avian nests and hence the manner in which a nest may influence the energetic cost of incubation.

Nest materials can range from natural plant items (sticks and grass) and organic material (mud), to animal materials (hair, fur, spider silk) and man-made items (wool, wire and cloth). This nest is made completely from horse hair and a few strands of grass.

FACTORS INFLUENCING NEST SIZE AND SHAPE

Parent mass

Comparing the size and shape of nests of 36 Australian passerine species against parent mass reveals that nest surface area increases in direct proportion to the size of the parent. Nest diameter and height increase with parent mass but as nests become larger in line with increases in parent mass, the nest cup also becomes shallower and the opening becomes wider than expected, which allows for the space that the chicks will occupy.

Nest mass increases with parent mass at a rate that matches that of a supporting structure, suggesting that structural considerations of nest construction are of primary importance to nest design. The requirement for structural support is also evident for nest thickness, which increases more than proportionally expected as parent size increases. Structurally adequate nests become thicker than expected for their size in larger birds.

The clutch

Of interest is how the size or number of eggs in a clutch relates to the size of a nest. The clutch surface area and the internal surface area of the nest increase simultaneously; and the clutch volume and volume of the nest cup are also associated. Since nest design for the majority of birds in the study is in part influenced by the male and egg shape is controlled by muscles in the pelvis of the female, it is likely that one does not control the other. However, nest and egg size/shape are influenced by body size and ancestry. Therefore, it is likely that the nest and clutch are in fact independent, yet matched secondarily due to the shared influence of body mass and genetic ancestry.

The delicate nest of a Singing Honeyeater is decorated with Emu feathers and held together with spider silk and hair.

FACTORS INFLUENCING NEST INSULATION

Nest structure

As we know, the nest surface area increases in proportion to bird size, however nests become much thicker than expected as bird size increases. The thick walls provide structural support for the parent and clutch, with the consequence that structurally adequate nests achieve greater insulation than expected, as they increase in size. Nests are often viewed as objects that are designed to prevent heat loss from the clutch and incubating parent; however the requirement for adequate structural support is the primary selective influence on nest construction, not the requirement for insulation.

Nest microclimate

By assessing the insulation of Spiny-cheeked Honeyeater (Acanthagenys rufogularis) and Yellow-throated Miner (Manorina flavigula) nests under varying wind conditions, I found that wind enters the nest material and dissipates heat, resulting in a decrease in thermal insulation with greater wind speeds. The consequence of increased wind currents around and through the nests would be a near-doubling in heat production required by the parent when incubating.

While ambient temperature does not influence the structure of nests in my study, it does influence the insulation of nests and the thermal efficiency of the material. However, temperature and precipitation (henceforth referred to as climate) act in combination, indicating that the response of Australian passerines to one variable depends upon the level of the other.

 In sites with low temperatures, nest insulation may be important to maintain an appropriate microclimate for offspring and therefore birds construct nests with good insulation, irrespective of the rainfall at the site. For nests constructed in warm climates but at the two extremes of rainfall, there is a pronounced decrease in insulation for nests built in areas with high rainfall, compared to areas with low rainfall.

The effect of climate extends to the thermal efficiency of the nest materials, indicating that not only the ambient temperature, but also the precipitation of the breeding site, influences material selection during the nest construction phase. Birds breeding in warm and wet climates construct their nests with materials that have a poor thermal efficiency compared to those in dry climates. The warm temperatures may cause a relaxation in the need for insulation, and poorly insulating materials (such as sticks and grasses, rather than fur and wool) are possibly less absorptive and able to dry out faster following a rain event, to restore the insulating function of the nest.

To determine the effect of water (from rain, dew or absorption from the nest substrate) on the heat loss from the nest, I measured Tawny-crowned Honeyeater (Gliciphila melanops) nests under varying water content levels (from dry to saturated). Water penetrating the nesting material increases conductance of G. melanops nests by up to two and a half times the rate seen in a dry nest – a consequence of the decreased thermal efficiency of materials in a wet nest. As a result, additional energy is required by the incubating parent to keep clutches warm when nests become wet. Individuals should be capable of obtaining additional floral resources to deal with an energy deficit in cold and wet conditions. However, if floral resources are poor and an individual is unable to meet such energy demands, it may abandon the nest altogether.

Not all nests seem to be sufficient – this nest has large holes but is strong and doesn’t absorb water – but they have functions other than insulation of the young.

SUMMARY

My studies highlight the importance of nest design and construction for the thermal properties of nests – small variations in nest design can have significant impacts on the insulation value of a nest, which will in turn influence the energetic cost of incubation. The effect of rain and wind on nest insulation, and the consequence of this for the energetics of the incubating parent, reinforces the view that appropriate nest site selection that provides additional shelter is crucial for avian reproductive success.

Slater’s Skink – a Lesser-known Central Australian Resident

Mugshots: Spotty, Kelly and Billy (Slater) (Image C. Treilibs).

By Claire Treilibs

Without fur, feathers, or large-adorable eyes, reptiles generally draw the short straw when it comes to popular appeal of our native critters. Some (mammal-centric) commentators might argue that reptiles lack charisma, but these scaly creatures have their own je ne sais quoi.

A lesser-known central Australian resident is the endangered Slater’s skink (Liopholis slateri). With an air of nonchalance, these sly skinks laze outside their burrow entrances, peering through narrowed eyes, basking. Then – wham! At lightning speed, they pounce upon their prey – any ants or termites that might be wandering by.

I got to know a population of Slater’s skink over four years of a PhD study. I could tell who was whom from the spots and scale patterns on their faces. Once I found a way of recognising individuals by photograph, I could track them over time. Take ‘Spotty’ for instance. If I recorded where Spotty was when I snapped the photo, then I could track Spotty’s whereabouts; which burrows she uses, and for how long. I found that these skinks were surprisingly mobile within the population compared with the more sedentary habits of many of their close relatives.

Slater’s skinks are extraordinary in that they are specialist floodplain users. In fact, they only occur in the floodplains of the east and west MacDonnell Ranges. The entire global population occurs within 150 km of Alice Springs in 11 (mostly isolated) populations. Buffel grass, fire, climate change, and in some populations, cattle, are causing dramatic changes to their floodplain habitats and risking the future of this endangered skink.

Last month, indigenous ranger groups and other land managers got together to share information, discuss current monitoring and management of the skink, and how to help look after it in future. You can read more about the two-day Slater’s skink forum on the ABC post and the TNRM post.

~ Claire Treilibs

Slater’s skink, Liopholis slateri, is a floodplain specialist (Image C. Treilibs).

Significant Tree Register Goes Online

By Candice Appleby

As you may all recall earlier this year Land for Wildlife announced that we would be coordinating the rejuvenation of the National Trust NT Significant Trees Register. Overall, LFW are appointed to coordinate the maintenance of the register for the entire Northern Territory, however initially we have decided to focus our energy locally and revitalise the Central Australia Register.

A lot has changed in the region in the past 28 years since the inception of the register in 1989. Several listings have been removed from the register, as they have made way for town development or simply suffered the fate of nature (like fire, hail, old age and white ants). Likewise numerous listings were added in the 90’s when Greening Australia NT was managing the register.

Over the last few months we have made it a priority to get all this information into the digital age by GPS plotting each listing and getting all the information into an interactive database. After numerous site visits, sorting through old documents and culling expired listings, Land for Wildlife is excited to announce the Significant Trees Register (Central Australia Region) has now gone live!

Head to the project page at the Land for Wildlife website to read more about the register, see a list of the trees on the central Australian register, download PDF fact sheets about the trees and even take a ‘virtual’ tour of the register via an interactive Google Map.

Stay tuned for more updates to the register – next on the list to update is the Katherine- Daly Rivers Region. This is a region rich in Banyans, Boabs and historical blazes! Currently the Significant Tree Project is unfunded. Land for Wildlife is actively seeking funding to assist with the groundwork costs associated with reassessing trees and getting this information recorded on the database. Land for Wildlife would like to extend a great appreciation to our host Bill Low of Low Ecological Services for his ongoing support.

~ Candice Appleby

Significant Trees Register

The NT Register of Significant Trees is managed by Land for Wildlife Central Australia, on behalf of the National Trust NT.
The register was initiated by the National Trust NT, with input from Greening Australia NT, and coordination by Land for Wildlife Central Australia since 2011.

Domestic Cat Monitoring and Awareness Wrapping Up

 The Domestic Cat Monitoring and Awareness in Alice Springs program is wrapping up for another round and the cats are exhausted from all their hard work recording where they go and what they see. The tracker data has been through the wringer! Maps have been produced showing where the cats go and what their hotspots are. We will be running the timing of movements and distances through the calculator to get some statistics prepared. The final step in the process will be to collate the information and present this to the cat owners so they can see the results.

An additional monitoring round will take place in Tennant Creek in a couple of weeks if all goes to plan, with the intention of broadening the range of our community engagement. Several Tennant Creek residents with pet cats have offered to take part in the monitoring, with their data sneaking into the mix in the final couple of weeks of the funding round.

Domestic Cat Monitoring and Awareness: Possum’s roaming zone is far-reaching

As a little taster of the data to come, this is the measured home-range of Possum, a young Tabby that spends a significant amount of time outside. Possum spends much of its time outside near the house (red hotspot) but the tracking data shows that Possum also spends a good proportion of his time near the main road, roaming in the riverbed and also in nearby bushland (90% of GPS fixes are within the green shaded zone). Not only does Possum roam on the large property, but he also visits neighbouring properties. Possum’s video surveillance and tracking data will be presented to his owner in the coming weeks as the project is wrapped up for another year.

Interested in having your cat tracked but you haven’t taken part in the monitoring yet? Land for Wildlife will look at continuing the monitoring process for members of the Land for Wildlife and Garden for Wildlife programs to engage with cat owners about responsible management of their free-roaming felines.

This project is supported by Territory Natural Resource Management, through funding from the Australian Government’s National Landcare Programme.

Cat Trapping Success

Feral cats have contributed to the disappearance of many ground dwelling birds and mammals in the arid zone and continue to threaten the success of recovery programs for endangered species. It’s therefore a service to the native animals of the region to trap any feral cats you find roaming your property. Land for Wildlife loan out cat traps to members and can provide you with the information and advice needed to get you on your way to become a successful trapper.

Are you already trapping cats? Land for Wildlife would like to hear from you. We are in the process of gathering information on trapping success by Land for Wildlife members on their property. This information will be used to help the Alice Springs Town Council’s Environment Advisory Committee to assess the effectiveness of various trapping programs in the region.

We can determine trapping success by taking the ratio of the number of cats trapped to the number of trapping nights (successful and unsuccessful). If you are trapping feral cats on your property and are able to provide us with this information, we would appreciate it! Email us at lfw@lowecol.com.au with the two figures plus the suburb you are trapping in and we can collate the data from our members.