Sparrow Fans Only

I’m still out of touch on holiday. So for today’s there’s a pre-scheduled blog post for all the sparrow fans out there itching to get out and check their nest boxes quite soon. I included a set of my pictures of Tree Sparrows from various locations and a variety of places.

I found some fascinating information online about the Tree Sparrow (Passer montanus), a common enough bird where I live but one hard to study as the species is extremely shy. Nesting adults leave nest boxes even as one approaches and care must be taken when ringing nestlings at the correct stage to avoid desertion by the adults. Even watching Tree Sparrows at feeding stations or in everyday field work can be difficult as the species seems to flee from inquisitive human eyes. 

Tree Sparrow

 

A number of bird species deposit eggs into other nests and it is not easy for parents to tell their own eggs from others. Such parasitism is well documented in many bird species, particularly the cuckoo and cowbird families. Despite the apparent difficulties of studying Tree Sparrows, researchers at the University of Veterinary Medicine, Vienna discovered that Tree Sparrows can recognise eggs deposited by other Tree Sparrows but do not always reject them.

From University of Veterinary Medicine -- Vienna (2013, December 11). Egg dumping -- and rearing. ScienceDaily.

Tree Sparrow

 

"Building a nest, laying and incubating eggs and taking care of a hungry brood are very demanding on birds´ energy budgets, so it is obviously in their interests to ensure that the young they are caring for are their own. Brood parasitism sometimes makes this a difficult proposition: strangers -- either of their own kind or of another species -- might want a free ride and deposit their eggs into ready-made nests. The reluctant "hosts" pay a high cost. At the very least, they waste energy on unrelated offspring, while at worst their own eggs or hatchlings are killed by parasitic chicks". 

"Strategies to avoid egg dumping vary from species to species. Birds might count eggs or recognize foreign eggs by variation in colour or size and if possible reject them. But recognizing foreign eggs is not always easy and not all species or individuals succeed. Some birds do not seem to discern even obvious differences, while some that do are physically unable to eject the strange eggs. Even when hosts manage to eject eggs from the nest, their motivation for doing so is unclear. Are they keeping the nest "sanitized" for their brood or are they attempting to prevent parasitism?"

Tree Sparrow

"The Tree Sparrow seems to have evolved strong variation in egg coloration and size between clutches, probably to enhance its ability to discriminate parasitic eggs from its own. To test for the ability to recognize conspecific eggs, Herbert Hoi and colleagues at the Konrad Lorenz Institute of Ethology of the University of Veterinary Medicine, Vienna introduced both real eggs and cardboard models into the nests of Tree Sparrows. The scientists used flat objects to ensure the birds would easily be able to remove the fake "eggs" if they wanted to (flat objects are easier to grasp with the beak than round ones). To investigate which features made ejection more likely, they included model eggs with features that are not typical of Tree Sparrow eggs". 

Tree Sparrow

The researchers also checked for the sparrows' motivation for ejecting eggs by introducing the models either at the time of egg-laying or during incubation. "If the motivation is to defend against egg-dumping, we would expect egg ejection to happen largely when the females are laying eggs," explains Hoi. "If they do it during incubation, they probably just want to keep the nest clean for their hatchlings”. In two related experiments, the researchers tested the consequences of adding eggs to 30 nest boxes during egg laying and to another 30 during incubation, first using real eggs and then using flat paper models. 

Tree Sparrow

The birds removed foreign eggs from the nest in about a third of cases in the first experiment, although the remainder accepted and continued to incubate parasitic eggs. When flat models were used, 81% of the objects were quickly thrown out, regardless of the time of introduction. The results point to an anti-parasite behaviour, albeit an imperfect one. The birds sometimes seem to have a hard time ejecting foreign eggs with their beaks, although it is also possible that they do not always correctly identify them. 

"Our tests on motivation are interesting. The sparrows threw out foreign objects of a different size more often during the egg-laying stage but they were more careful to remove unusual white objects during the incubation stage," says Hoi. "This shows that avoidance of parasitism is a motivation but nest sanitation also plays a part because the sparrows probably think the white, non-round objects are egg sacks and broken shells, which are usually removed from the nest after the young hatch." 

Tree Sparrow

Over a number of years our local Tree Sparrows have given many a cause for thought, leading one to think that not only is there more to the world of the humble Tree Sparrow than we imagine, and also that they are shrewder than we initially give them credit for. I guess that this interesting piece of research reinforces that impression. 

More news, views and birds later. Another Bird Blog which is back on home territory very soon with pictures from Lanzarote 2014.