Does nest box design mitigate cavity loss?

Masters student, Eliza Larson tells us what she has found

The continued decline of tree cavities is increasing pressure on many hollow-utilising species. In Australia alone, around 300 species of birds, bats and arboreal marsupials which utilise tree cavities are potentially at risk. Artificial nest boxes are commonly used to mitigate cavity loss, however they are poorly insulated in comparison. The predicted continued increases in average temperature and extreme weather events such as heat waves, makes it critical to understand how species in nest boxes may be affected by these changes, and how these effects may be mitigated.

Crimson Rosella (P.e. elegans)
Crimson Rosella (P.e. elegans)

To investigate this, I selected the iconic Crimson Rosella (Platycercus elegans) as my study species. Like almost all parrot species it is an obligate cavity breeder. In addition, there are several subspecies occupying a range of habitats and climatic zones in eastern and south-eastern Australia. It also willingly breeds in the over 700 nest boxes our research group has established. I deployed temperature loggers in each nest box, and compared temperature to nest box selection, nestling growth and survival.

Thermal image showing heat lost by two nestlings
Thermal image showing heat lost by two nestlings

I began this as my Honours project at field sites at Bellbrae and Steiglitz near Geelong with the Crimson subspecies P. e. elegans, which revealed that more extreme low temperatures were most detrimental for nestling growth, and greater temperature variability had a tendency to reduce fledging success. I was lucky enough to continue my project for my Masters, and spent four wonderful (and exhausting) months in the beautiful Adelaide Hills and Fleurieu Peninsula in South Australia. These areas are home to the orange/yellow Adelaide (P. e. adelaidae) and Fleurieu (P. e. fleurieuensis) subspecies which experience a warmer and drier climate than the Crimson subspecies in southern Victoria. In addition to nest box temperature, I also recorded characteristics such as the amount of tree canopy cover and depth of nest box substrate.

Adelaide Rosella (p.e. adelaide) brooding her newly hatched chicks under her tail
Adelaide Rosella (p.e. adelaide) brooding her newly hatched chicks under her tail

Due to relatively recent logging in the 1970s, cavity bearing trees are rare at our Adelaide Hills site. Nest box uptake is therefore extremely high, and 54 out of our 60 nest boxes had at least one breeding attempt. With the assistance of Honours student Sarah Micallef, and a few enthusiastic volunteers, we scaled ladders to monitor each four metre high nest box every two days. Our efforts were rewarded with 173 rosella nestlings weighed weekly from a total of 75 breeding attempts (32 successful, 43 failed – mainly due to predation).

Galah nestlings in a nest box
Galah nestlings in a nest box

I analysed nest box preference by order of uptake using temperature variables from two weeks prior to breeding. Nest boxes with lower average temperatures and greater temperature variability were occupied earlier in the season. No tree characteristics appeared to influence nest box preference, however nest boxes with lower substrate depth also seemed to be preferred. Although no significant relationships were found between my temperature variables and clutch size or hatching success, greater average and more extreme high temperatures had strong relationships with nestling growth rates and fledging success. For every 1°C increase in average temperature, nestlings were on average two grams lighter and around two fewer chicks were expected to fledge, while a 1°C increase in high temperatures increased nestling mass by two grams and the number of fledglings by almost one. Although the finding of higher extremes being beneficial to nestlings was surprising, it is possible that they benefit from short periods of greater temperature in the form of reduced thermoregulatory costs, allowing greater energy allocation to growth and therefore survival.

A sub-adult Adelaide Rosella with her newly hatched chicks
A sub-adult Adelaide Rosella with her newly hatched chicks

I also conducted a small study testing the effects of different kinds of insulation on the internal temperature of empty nest boxes. I applied 3cm thick polystyrene, pleated foil batts, and reflective paint to the top and east sides (where solar exposure is greatest) of nine pairs of nest boxes over six weeks. My analysis thus far indicates that polystyrene and foil batts were most effective, with both reducing temperatures during the hottest part of the day by 0.5°C on average (but by up to 6 degrees), while remaining slightly warmer than untreated boxes during the night. Reflective paint appears to be quite ineffective, actually increasing internal temperature for a short period in the late afternoon.

Reflective paint study
Reflective paint study
Polystyrene study
Polystyrene study
Foil batt insulation study
Foil batt insulation study

My findings suggest that breeding rosellas may be sufficiently flexible to cope with changing climates, but only if cavities with various microclimate profiles are available. Increased nest box insulation could be beneficial to rosella nestling growth and survival by reducing average temperatures during the day, and potentially partially mitigating the effects of heat waves and cold spells. Future work should focus on testing this in the field, and I encourage those of you who make nest boxes for your gardens to run your own experiments with various thicknesses of wood!

I am extremely grateful to BirdLife for their continued support in the form of the Stuart Leslie Research and Conference Awards which assisted with fieldwork costs and helped me attend the IOC in Tokyo last year.

Eliza Larson with an adult Adelaide Rosella. Photograph by Sarah Micallef
Eliza Larson with an adult Adelaide Rosella. Photograph by Sarah Micallef

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