Animal behavior Parental care/ Begging Birds

Animal behavior Parental care

Begging Birds

If resources are plentiful, parents may benefit most from investing equally in all offspring (although this is not always the case). However, when resources are scarce, parents often benefit from investing in some offspring more than others. Depending on the situation, this could be those who are doing the best already, or those whose need is greatest.

1. When would a parent benefit most from investing in offspring that are already doing well vs. those with a higher need?

When resources are scarce it would benefit a parent most to invest in offspring that are already doing well, and when resources are abundant it would benefit a parent to invest more in offspring that have higher needs.

2. In which case do the interests of the babies and the parents conflict? Consider both the successful and the less successful babies but keep relatedness in mind.

When resources are abundant, stronger siblings may still want to kill weaker siblings, so a greater proportion of parental care is devoted to them. Stronger siblings could also demand more care or steal resources from weaker offspring. This is against the parents’ interests, as they have the resources to keep all of the young alive.

Begging in baby birds may signal to the parents both the hunger of the chick and the quality of the chick. In many birds, begging is very loud and could draw attention to the vulnerable chicks.

• What is necessary for begging in chicks to be an honest signal?

For begging to be honest, it must accurately relay information regarding the chick. This information should not change based on how the other chicks are begging. If the chick begs more in response to other chicks begging near it, then it would not be a truly honest signal as the begging is only showing that they want food rather than how much food they need. Dishonest cues for constant feeding may overtax parents and lower their fitness, which could ultimately lower the fitness of the signaler as the parents are able to provide less care.

• Create two hypotheses for what might maintain begging as an honest signal.

One hypothesis is that dishonest signalers may overtax their parents so much that they are no longer able to provide adequate care, resulting in the dishonest signaler’s death. In this case, parents may also stop giving care to protect their own fitness.

Another hypothesis, is that a signal that was costly to produce could only be afforded by the most able signalers.

• Propose an experiment to test one of your hypotheses.

For an experiment you could manipulate the environment and set up 2 nests. You can have chicks from the same brood but have them separated by those who are well off and those that have higher needs. By controlling the environment, you can eventually combine them in one nest and have an abundance in resources and you can determine whether or not begging is an honest signal or a dishonest signal by whether or not the two types of chicks beg differently.

• What would limit the advantage that a baby bird would get by lying and begging even when it’s not that hungry?

It would result in a dishonest signal. When the baby actually needs food, the parent won’t always know if they are lying or not so it is very costly and would be a disadvantage as well.  Because the baby bird is lying and trying to get more food when it doesn’t need it can cause danger to themselves.

Methods:

To examine the risks associated with begging, we compared the frequency of predation at experimental nests with play- backs of begging calls to that at control nests with blank tapes. To make the experimental tapes, we recorded six 10-day-old tree swallow broods, each with five nestlings, during 30-min recording periods. The recordings were then placed on three, 1-min continuous tape loops with two broods on each tape. Each experimental tape consisted of 60 s of continuous begging calls followed by 60 s of silence. These calling rates mimic the maximum rates observed in our population of tree swallows (Leonard ML, unpublished data). Playback volume was set at 70 db at source, which is the natural volume of a 10-day- old brood of five begging tree swallow nestlings measured approximately 10 cm from the nest opening (Leech SM, unpublished data). The three control tapes were blank 1-min continuous tape loops.

We conducted 49 trials with nests placed on the ground and 39 trials with nests placed on platforms below nest-boxes. In the ground trials, we baited an artificial nest of dried grass with one common quail (Cottimix cotumix) egg and placed it on top of a 20 X 10 X 6 cm box, which was covered with soil and leaves. To determine which nest was taken first, we placed the egg on a trigger attached to a battery- operated clock set to 12:00. The triggers were constructed so that when the egg was removed from the nest, the circuit opened and the clock stopped.

In the nest-box trials, we placed a platform approximately 5 cm below the bottom of a tree swallow nest-box, which was mounted on a 1.3-m high metal pole. We placed an artificial nest with a quail egg on the platform, attached the egg to a clock as described above, and placed the clock inside the nest box. We placed the nest outside the box to increase the probability that a predator attracted to die nest-box would be detected. The tapes were broadcast from a speaker inside die nest-box. This experimental setup mimics the situation for a cavity nesting species more closely because calls were played from inside a raised cavity.

We considered predation to have occurred if the egg was miming or broken. If the contents of die nest were missing and the nest was torn apart, we assumed that the predator was a mammal In contrast, if the egg was miming, but the nest itself was undisturbed, we assumed that the predator was avian (HaskelL 1994; Leonard and Pieman, 1987).

1. Explain how results from this study could help address the maintenance of begging as an honest signal?
2. What concerns (ethical or methodological) do you have with this study?

Results

Predation occurred in almost half of the ground trials across all sites. Overall, experimental nests were depredated before control nests in 16 of die 19 trials in which die time to predation was recorded (binomial test, p < .002). This was not a result of several predation events at a few sites because experimental nests were depredated first at 15 of the 16 sites at which predation occurred (control nests were depredated first at 3 sites, 2 of which also had a trial with experimental nests depredated first). Seventy-five percent of die experimental nests were depredated during (55%) or within 2 h (42%) of die playback period.

Predation occurred in more than half the nest-box trials across all of the sites. Overall, experimental nests were depredated before control nests in 13 of die 18 trials in which die time to predation was recorded (binomial test, p < .04). Again, this pattern was consistent across sites, with each of the 13 trials in which experimental nests were depredated first occurring at different sites. Ninety percent of die experimental nests were depredated during (44%) or within 2 h (56%) of die playback period

1. Do these results support your hypothesis? Why or why not?
2. Could predation pressure maintain honest signaling in this species?

In many species of birds, extra-pair copulations are common. In some birds, up to 25% of the nestlings may be fathered by a male other than the mother’s social mate. In species where this is common, siblings will be less related to each other.

3. How would you expect extra-pair copulations to affect rates of costly begging?
4. Propose an experiment by which you could test this hypothesis.

Figure 1. Relation between sibling relatedness (as measured by percentage of extra-pair young) and loudness of chick begging calls (a) across 11 species of passerine birds, and independent contrasts for these variables, controlling for phylogeny by using the taxonomies of (b) Howard & Moore (1991) (y = 201.0 x, r2= 0.72) and (c) Sibley & Ahlquist (1990) (y = 206.4 x, r2= 0.46). In this analysis, increasingly negative numbers correspond to increasingly quieter calls.

1. What is the difference among the three figures shown? Why did the authors show all three?
2. What is the take-home message of these figures?
3. Do these results support your hypothesis?
4. In which of these species do you expect biased parental care?

• How would you expect these results to change if resources were extremely scarce?

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